Prebiotics: decoding the facts

Introduction

The collection of trillions of microbes which include bacteria, fungus, viruses, archaea and eukaryotes are called human microbiota. These tiny organisms maintain a symbiotic relationship with the human body. The gut harbors the largest collection of microbes in the body that are collectively called as the gut microbiota. These microbes continuously evolve throughout the life span of an individual adapting themselves to the external and internal changes. A number of factors like the mode of birth, type of feeding, genetics, lifestyle, race, health status, exposure to medicines, environmental factors such as pollutants, noise, weather conditions, hygiene and aging influence the microbial framework in an individual.

The gut microbial ecosystem is dominated by bacteria and it is estimated that about 500-1000 species harbor the gut. A healthy gut is colonized by beneficial microbes which includes the bacteria that come under the phylum Firmicutes, Bacteroidetes, Actinobacteria, Proteobacteria, Fusobacteria and Verrucomicrobia. Some of the dominant members of the Firmicutes phylum include Lactobacillus, Enterococcus, Clostridium, Veillonella, Roseburia, Faecalibacterium and Ruminococcus. The predominant members of the Bacteroidetes phylum include Bacteroides and Prevotella. The phylum Actinobacteria is dominated by Bifidobacterium whereas Akkermansia muciniphila is the main bacteria of the phylum Verrucomicrobia.

These numerous microbes that are part of the gut microbial ecosystem play a vital role in maintaining the overall health and well being of an individual. Hence the gut is also regarded as a metabolic organ and the second brain. The bi-directional communication which the gut bacteria have established with all the organ and organ systems greatly influences their normal functioning. The production of metabolites, nutrients, chemical messengers as well as strengthening the gut and the overall immunity carried out by the gut microbes is crucial in maintaining the homeostasis i.e., body’s internal balance.

Any imbalance in the gut microbial ecosystem causes disturbed homeostasis resulting in disease process. Hence promoting a healthy and diversified gut bacteria is critical in terms of the health status of an individual. One of the dietary strategies to encourage the growth and development of the healthy gut bacteria is to increase the consumption of a specific nutrient called prebiotics.

Prebiotics concept

The habit of consumption of the fermentable dietary fibers has been known since thousands of years. There have been reports about the excessive consumption of lactose to enrich the intestinal bacteria. The notion that specific types of nutrients like carbohydrates can improve the gut bacteria has been known for a long time, well before the definitions of these nutrients were put forward. The scientific research conducted in the 1950s bought into light a type of factor called ‘bifidus factor’ present in human milk. This compound promoted the growth of Bifidobacterium in infants. Further scientific studies on this compound revealed the presence of oligosaccharides and glycans (also called polysaccharides) which are different types of carbohydrates. The oligosaccharides were also detected in cow’s milk. But there was no clarity regarding their physiological function.

In the later years, compounds such as inulin and other oligosaccharides produced from sucrose, galactose and xylose were identified as having bifidogenic properties i.e., those having the ability to enhance the growth of Bifidobacterium. But it was only in the year 1995 the concept of prebiotics came into existence which was put forth by Glenn Gibson and Marcel Roberfroid in their publication named “Dietary modulation of the human colonic microbiota: Introducing the concept of prebiotics “. According to them ‘prebiotics are a specific type of food ingredient that are non-digestible, but are capable of encouraging the growth and colonization of a particular or a certain number of colonic bacteria that are beneficial for the health’. Though this concept is scientifically acknowledged, some confusions on this term still exist. However, the evolution of the prebiotics concept has greatly influenced the scientific knowledge about the gut health.

With the growing list of compounds being recognized as having prebiotic potential, the research continued into identifying the specific microbes in the gut whose growth was enhanced by the use of prebiotics. It was consistently observed that prebiotics selectively increased the beneficial bacteria like Lactobacillus and Bifidobacterium. In the later studies that followed, it was interesting to note that the beneficial effects of prebiotics extended to increasing the numbers of Faecalibacterium and Akkermansia muciniplila. While it is presumed that more beneficial bacteria may also be enhanced by prebiotics, more studies with regards to the bacterial sequencing are warranted.

In the light of these observations, an expanded definition for prebiotics was put forward by researchers. According to this, prebiotics were regarded as ‘undigested dietary carbohydrates’ which are fermented by the gut bacteria producing short chain fatty acids. The recent definition of prebiotics was put forward in 2016 by International Scientific Association for Probiotics and Prebiotics (ISAPP) by an expert committee in the fields of microbiology, nutrition and clinical research. Their definition of prebiotics is “a substrate that is selectively utilized by the host microorganisms conferring a health benefit”. So, the overall picture of the prebiotic concept is the presence of three components i.e., the compound, its health beneficial effects and gut microbial enhancement.

With evidence supporting the presence of a direct link between the presence of a diversified gut bacteria and overall health, and with the increasing number of compounds being labelled as prebiotics, it is obvious that consuming a wide range of foods having prebiotic properties are needed to support the growth of healthy gut bacteria.

Categories explained

A compound can be considered as a prebiotic if it satisfies the following criteria

*Tolerant to the stomach acid.

*Does not undergo chemical breakdown due to the action of digestive enzymes.

*Is not absorbed during digestion.

*Undergoes fermentation by the gut microbes.

*Enhances the growth of the healthy gut bacteria.

*Has favorable effects on the overall health and well-being.

Among the several types of prebiotics that exist, most of them fall under the carbohydrate group and are called oligosaccharide carbohydrates. Initially the prebiotics included fermentable carbohydrates such as soluble fibers, fructooligosaccharides (FOS), galactooligosaccharides (GOS), human milk oligosaccharides (HMOs), inulin, galactans, fructans and lactulose. However, owing to the advancements in the concept and scientific research, other substances such as resistant starch (RS), pectin, polyphenol compounds like those present in curcumin (turmeric), quercetin (plant pigment present in apples, berries, citrus fruits, onions) and polyunsaturated fatty acids (PUFA) have been proposed to have prebiotic potential.

Carbohydrates

Oligosaccharides

Oligosaccharides and polysaccharides are a group of complex carbohydrates that have a prebiotic potential. They are made up of simple sugars that are bonded together. The simple sugar in question may be either glucose, fructose or galactose. The main difference between the two lies in the number of sugar units they are made of. While the oligosaccharides are made up of two or more sugar units, polysaccharides contain more than ten sugar units. Some of the examples of oligosaccharides include inulin type fructans, galactooligosaccharides, arabinoxylans and human milk oligosaccharides. The compounds such as beta-glucans, isometodextrin and resistant starch come under polysaccharides.

Fructans

Fructans are the general term used to describe the naturally occurring non-digestible carbohydrates present in foods that are derived from plants. They are mainly made up of fructose polymers i.e., many fructose molecules that are bonded together. However, they are different from fructose. The main difference lies in the number of fructose molecules. While the fructose can be digested and absorbed by the body, fructans on the other hand are non-digestible but are fermented by the gut microbes mainly colonic bacteria.

Fructans form an important part of everyday diet as they are contained in many plant foods. The plants that contain high amount of fructans have found many uses such as food, animal feed and folk medicines since ancient times. Apart from being a food component, fructans also exhibit antioxidant, immunomodulatory (a substance that modifies the immune system either by enhancing or suppressing it), anti-inflammatory, anticancer, antiallergic, antihyperglycemic (an agent that reduces the high blood sugar levels) and prebiotic properties. They also increase the absorption of minerals like calcium, iron and magnesium from the intestine.

Classification

There are five classes of fructans depending on the differences in the chemical structure. They are inulin-type fructans, levan-type fructans, graminans, inulin neoseries and mixed-type levans. The inulin-type fructans regarded as oligo or polysaccharides are the most extensively researched among the fructans. They include inulin, fructooligosaccharides and oligofructose. The main difference between these fructans lies in the number of the fructose molecules with inulin regarded as a longer chain molecule and the other two as short-chain molecules. In order to obtain the long chain inulin, they need to be physically separated while the other two can be either extracted from the plants or produced from glucose and sucrose by a chemical reaction called hydrolysis.

Applications

The longer chain inulin is less soluble and has a creamy texture whereas the short chain fructooligosaccharide is easily soluble having similar qualities to sugar. Due to these properties fructans have found application in pharmaceuticals and food industries like the production of baked foods, beverages, dairy products and food preservatives. They have been used as a sugar substitute in the production of low-calorie foods. They can be used as a replacement for fats in food production due to its ability to impart a creamy texture and gelling (property of a substance to become solid). Their applications have also been extended to producing aerated desserts, ice creams, spreads and sauces due to their stabilizing properties. They also exhibit humectant effects (retain moisture) and hence considered to offer protection against harmful microbes.

Sources

Inulin is present in a number of food plants and is stored in stems and underground tissues. Some of the plants that are good sources of fructans include onions, shallots, scallions (green or spring onions) barley, Brussels sprouts (baby cabbage), cabbage, broccoli, pistachio, garlic, wheat, chicory, asparagus (thin long green spear shaped vegetable), leek (long white vegetable with green leaves at the top that has the smell of an onion) Jerusalem artichoke (a crunchy root vegetable that resembles a ginger root), agave (a plant with large succulent leaves) and bananas.

Biological actions

Intestinal health

The inulin-type fructans have been regarded as functional foods due to their health benefits. A functional food means “a natural conventional food consumed everyday that is beneficial to the health in addition to their basic nutritive value, enhancing the overall health and well-being of an individual in addition to reducing the risk of disease and improving the quality of life”. They are considered as an important dietary fiber that positively influences the intestinal and overall health.

The colon or the large intestine has been increasingly identified as an organ that is crucial for maintaining the overall health and protection against diseases. The inulin-type fructans have been shown to positively influence the functioning of the colon and reduce the risk of diarrhoea, constipation, infections, inflammatory bowel diseases (IBD), irritable bowel syndrome (IBS) and cancer.

The inflammatory bowel disease (IBD) is a collective term used to describe the chronic inflammatory disorder of the gastrointestinal tract affecting both the small as well as the large intestine. IBD is classified into types namely ulcerative colitis (UC) and Crohn’s disease (CD). Though symptomatically both these conditions have common features such as abdominal pain, diarrhoea, rectal bleeding and weight loss, there are some differences between these two conditions. While ulcerative colitis predominantly affects the mucosal (inner) lining of the colon, Crohn’s disease can affect any part of the gastrointestinal tract though commonly affects the ileum, the last part of the small intestine.

In one of the studies conducted in individuals with ulcerative colitis, it was observed that the administration of inulin-type fructans brought about reductions in the colonic inflammation. This effect was due to the modifications of the gut bacteria by the inulin-type fructans wherein an increased population of butyrate producing bacteria were noted. The short chain fatty acid butyrate produced by the gut bacteria has anti-inflammatory effects on the colonic cells. On the other hand, evidence from the human studies conducted on the effects of inulin-type fructans in individuals with Crohn’s disease is not convincing though recent studies have pointed to their potential in preventing the Crohn’s disease in those at risk. However, larger studies are needed to ascertain the benefits of inulin- type fructans in individuals with UC and CD.

The above-mentioned effects of inulin-type fructans on the gut bacteria and butyrate production have also been recognized to protect against colon cancer especially sourced from chicory root and Mexican agave. However recent studies have revealed that consumption of fibers from whole grain foods was more favorable in reducing the cancer risk compared to fibers from fruits.

Gut microbial balance

It is a well-known fact that a balanced gut microbial ecosystem is crucial for maintaining the body’s internal balance. The inulin and oligofructose have been acknowledged as having pre-biotic properties in line with the numerous studies conducted. They reach the colon (large intestine) in an undigested state wherein they are fermented and exert their prebiotic effects. Reports from the studies conducted on the influence of inulin and oligofructose on the gut microbes have revealed increase in the numbers of health promoting bacteria such as Bifidobacterium and Lactobacillus. Also, increased population of butyrate producing bacteria namely Clostridium, Eubacterium and Faecalibacterium have been reported. Butyrate is a type of short chain fatty acid, a metabolite produced by the gut bacteria which is a major source of energy to the colonic cells.

Recent studies conducted on the effects of inulin type fructans in individuals with diabetic nephropathy have revealed their effectiveness in increasing the population of Akkermansia muciniplila. The increased production of short chain fatty acids especially acetate by this bacterium has been found to regulate the blood glucose levels and protect the kidneys against damage. Thus, inulin type fructans safeguard the kidneys against damage by modifying the gut microbial composition.

Bowel habit regulation

One of the basic functions of the colon is stool production and excretion. Inulin type fructans have been shown to regulate the bowel habits in addition to improving the stool mass and consistency. These effects were noted in several studies conducted on individuals with chronic constipation. This is regarded as one of the typical effects of the dietary fiber and recent studies conducted on children between the ages of 2-5 years with functional constipation (a condition commonly seen in children characterized by irregular bowel movements and hardening of stools), have acknowledged improvement in the bowel habits with the addition of inulin-type fructans.

Irritable bowel syndrome (IBS) is one of the most frequently encountered functional gastrointestinal disorder worldwide causing much distress and discomfort. Abdominal pain, bloating, altered bowel habits (constipation/diarrhoea/or both) without any obvious underlying causes are some of the features of IBS. Though the exact mechanisms behind IBS is incompletely understood, it is thought to be due to the disturbances in the gut-brain axis i.e., the reciprocal connection between the gut and the brain. In one of the recent studies that explored the effect of inulin-type fructans in individuals with IBS having constipation, it was found that there was an improvement in the overall bowel function as well as the quality of life. Despite the clearly evident benefits, more studies are needed for stronger evidence.

Strengthens gut barrier

One of the most crucial defense systems in the body is the intestinal or the gut barrier. It constitutes the largest surfaces that forms an intersection between the human body and the external environment. The gut barrier system comprises of mucus layer which is the inner slimy lining of the intestines, friendly gut microbes and a single layer of specialized epithelial cells that are tightly packed. The gut barrier selectively allows the passage of nutrients while protecting the gut from coming into contact with the harmful substances. The inulin-type fructans have been shown to strengthen the mucosal lining in addition to modifying the mucin, a special protein which is present in the mucus. This activity enhances the ability of the gut to prevent the growth of harmful microbes and the dislocation of the gut bacteria into the systemic circulation.

Balances the gut hormones

The enteroendocrine system consists of a group of specialized cells called enteroendocrine cells that are scattered throughout the gastrointestinal tract. These cells produce hormones that regulate the functioning of the gut, food intake, absorption of the nutrients, metabolism and post-prandial glucose levels. The GLP-1 mainly stimulates the production of insulin and controls the blood sugar fluctuations following a meal. The hormone ghrelin also called as the ‘hunger hormone’ regulates the appetite and food intake in addition to maintaining the blood glucose levels, protecting the heart, maintaining the muscle and bone mass.

Preliminary studies exploring the effect of inulin-type fructans on the production of hormones especially glucagon-like-peptide-1 (GLP-1) and ghrelin have revealed the effectiveness of inulin-type fructans in regulating these hormones thus balancing the appetite and blood glucose levels. In line with this observation, recent studies have put forth the potential of inulin-type fructans supplements in the management of prediabetes, type 2 diabetes, overweight and obesity. However, more studies are needed to ascertain this.

Improves metabolic health

The inulin-type fructans have been shown to bring about improvements in the blood lipid profile. Studies have demonstrated the effectiveness of inulin in bringing about reductions in the triglyceride levels. But the reports on the cholesterol levels have been inconsistent. Recent studies comparing the effects of inulin in different groups of individuals have revealed consistent reductions in the low-density lipoprotein (LDL, bad cholesterol)) levels whereas increases in the high-density lipoprotein (HDL, good cholesterol) and the total cholesterol levels were observed in individuals with prediabetes and diabetes. These studies also reported similar observations with regards to the blood glucose levels also. Though more long-term studies are needed, these findings provide insights to the potential of inulin type fructans in improving the metabolic health.

Galacto-oligosaccharides (GOS)

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These are an important group of foods that include the non-digestible carbohydrates having prebiotic properties. They are made up of a short-chain of galactose molecules linked together. There are two types of GOS. They are alpha-galactooligosaccharides (alpha-GOS) and beta-galactooligosaccharides (beta-GOS). While the alpha-GOS is present in plant foods, the beta-GOS has been widely used both for research and commercial purposes.

Apart from the many health effects of GOS, they also exhibit certain favorable properties such as easy solubility, low viscosity (meaning they are less thick), temperature stability and an agreeable flavor and texture. Since they come close to the prebiotic properties of the oligosaccharides present in breast milk, they are commercially acclaimed in the production of infant foods. The Yakult pharmaceutical industry holds the foremost position in producing the first commercial GOS more than three decades back and has seen significant growth till the current times. In addition to this their prebiotic effects they have seen their extended use in dairy products, beverages, snacks and nutrition bars. They have also found application in cosmetics and pharmaceutical industries.

Sources

Alpha-GOS are naturally present in legumes such as beans, chickpeas and lentils. The presence of alpha-GOS and high fiber content is associated with digestive disturbances such as flatulence (excessive gas). Though there are variations in the way an individual responds to the consumption of GOS, evidence from studies have revealed that intake of 12g or less of GOS is an acceptable level in terms of intestinal flatulence compared to 15g per day. Thorough soaking and cooking are some of the ways to reduce the GOS content. Recent studies have shown that the wastewater that is obtained as a result of cooking and soaking can be reused for GOS extraction and laboratory growth of Lactobacillus. On the other hand, the effects of germination and fermentation on the GOS and fiber content needs more evaluation.

Soybeans are nutritionally dense containing high amounts of proteins apart from the other basic nutrients. The different ways by which the soybeans are consumed are soy milk, soy flour, tofu, tempeh and miso which constitute the fermented soy products. They are also considered as one of the richest sources of GOS. However, the GOS content depends upon the level of maturity of the soyabeans. Comparative analysis of yellow (mature) and green (immature) soyabeans have shown a higher concentration of GOS in yellow soyabean seeds. Despite being nutritionally rich, the main drawback of soybeans and their products is their tendency to cause gastrointestinal upset owing to the high concentration of alpha-GOS. Studies conducted on soymilk have revealed that fermentation with Lactic acid bacteria is one of the effective strategies to reduce the alpha-GOS content and their side effects.

In addition to the above-mentioned sources, alpha-GOS are also present in vegetables such as peas and butternut squash and in nuts like cashew nuts, almonds and pistachios.

A special mention about the raffinose family oligosaccharides (RFO). These are a group of carbohydrates that are a part of the alpha-GOS family and include raffinose, stachyose, ciceritol and verbascose. They are one of the major components of the legumes in addition to being present in the roots, tubers (underground stem that serves as a storage organ in plants) and leaves. They offer protection to the plants against environmental stressors such as heat, cold, draught and salinity (the amount of salt present in a medium which is soil/water in this context). They also protect the plants from diseases and modulate germination and longevity.

In addition to promoting healthy gut bacteria in humans, some of the biological effects of RFO that are applicable to both humans and animals include anti-diabetic, ant-allergic, anti-obesity effects and protection against non-alcoholic fatty liver disease. They also exhibit cryoprotection which means protection to the tissues against the effects of freezing as in very cold climates. Despite their health promoting properties, flatulence caused by these compounds is a limiting factor to their consumption. The inadequacy of the enzyme called alpha-galactosidase to completely digest RFO’s results in their buildup in the large intestine. These incompletely digested RFO’s when fermented by the colonic bacteria produces gas mainly hydrogen, methane and carbondioxide resulting in abdominal discomfort.

Though certain food processing methods such as cooking, soaking, fermentation, germination, decortication (removal of the outer layer) and use of the enzyme alpha-galactosidase can lessen the RFO content from the legumes, they might affect their nutritive value. Thus, the cultivation methods involving genetic modification of the legumes so as to reduce the ROF content have been proposed.

Biological functions

Regulates bowel habits

Constipation is one of the frequent problems encountered in individuals of all ages, though its prevalence has been said to increase with advancing age. It can be defined as ‘infrequent or difficulty in the passage of stools’ adversely affecting the health and well-being of an individual. Though fiber rich diet and plenty of fluids are the suggested strategies to prevent constipation, the addition of GOS is one of the recommendations for constipation due to its laxative effect. The GOS are regarded as soluble fibers, which as the name suggests are water soluble that are converted into a thick gel and fermented by the colonic bacteria leading to an increase in the bulk as well as the water content of the stools.

Constipation in the elderly occurs due to many reasons such as use of medications, alterations in the diet, changes in the gut microbiota and reduced physical activity rather as a result of aging alone. Studies conducted on the effects of GOS in the elderly individuals with constipation have revealed improvements in the bowel movements and frequency, though the use of laxatives could not be completely avoided.

The reports from the studies conducted on adults with a tendency to constipation have supported the relationship between the GOS intake and improvements in the bowel habits. However, the studies conducted on the effects of GOS in normal healthy adults is controversial due to the varied outcomes.

Many studies conducted on infants wherein the infant formula supplemented with GOS and fructo-oligosaccharides (FOS) have put forth improved bowel habits in terms of stool consistency and frequency similar to breast fed infants. Though in these studies it was difficult to analyze the effects of GOS and FOS individually, the infant formula used for these studies contained more of GOS thus providing convincing evidence.

Gut microbial balance

The human gut harbors thousands of species of friendly bacteria which play a key role in maintaining the overall health. Studies have shown that inclusion of GOS into the dietary regimen increases the population of Bifidobacteria and Lactobacillus. These bacteria produce lactic acid which makes the intestinal environment acidic thus preventing the growth of harmful microbes. The increased colonization of these microbes has been shown to neutralize the effects of the harmful bacteria like the Clostridium, Salmonella and E. coli species thus alleviating the production of toxic bacterial fermentation products. On the other hand, enhanced counts of other beneficial bacteria like Akkermansia, Ruminococcus and Bacteroides have been reported. The increased levels of short chain fatty acids contributed by these beneficial microbes contributes to the intestinal and overall health.

Also, the oligosaccharides present in the breast milk, collectively called as human milk oligosaccharides, have been found to have a positive effect on the gut microbial colonization. Hence breastfeeding is favorable in terms of the development of healthy bacterial colonies after birth. However, the addition of GOS to the infant formula has been shown to have a similar effect to breastfeeding with respect to the development of the bacterial colonies especially Bifidobacterium.

Improves skin health

The view that the consumption of GOS improves skin health has been supported by many studies. The administration of GOS as a prebiotic has been shown to benefit both the normal as well as the diseased skin. They have been shown to safeguard the skin immune system and the skin barrier which serve as the first line of defense against the external environmental hazards. They have also been shown to reduce the incidence and severity of skin allergies and atopic dermatitis (eczema). These aspects of protection against eczema and allergies have been observed in infants fed with GOS. This explains why breastfed infants are less prone to these skin conditions. Also, enhanced skin hydration and reductions in the wrinkle areas and pigmentations have been observed in studies. These findings have provided insights into the potential of GOS application in skin care and nutrition though confirmation is needed through more studies.

Enhances intestinal calcium absorption

Evidence from many studies have recognized the favorable effects of GOS in maintaining the bone health. The possible reason behind this is the bacterial fermentation of the dietary fiber leading to reductions in the intestinal PH, increases in short chain fatty acid production and population of Bifidobacteria which in turn enhance the calcium content and absorption in the intestine. Also, studies have recognized the positive effects of GOS in maintaining the bone mineralization and structure in postmenopausal women. On the other hand, studies conducted on the adolescent girls have revealed that everyday consumption of 5g GOS promoted calcium absorption, a much-needed effect to combat the growth spurt in this stage.

Lactose intolerance is a clinical condition wherein the individuals develop digestive discomfort as a result of consumption of lactose containing products. The symptoms include abdominal pain, bloating, diarrhoea, nausea and borborygmi (the rumbling noise in the intestines that occur as a result of movement of gas and fluid). Under normal circumstances, the lactose that is present in the dairy products is converted into galactose and glucose by an enzyme called lactase. The deficiency of this enzyme either due to its reduced activity or gastrointestinal infections leads to lactose intolerance.

It is a well-known fact that the calcium derived from the dairy sources is important for bone health. Owing to the increased incidence of lactose intolerance, there has been a tendency to avoid dairy products. But most of the individuals with lactose intolerance can tolerate up to 5g of lactose which can be roughly equated to 100ml of milk. However, complete avoidance of dairy products could lead to nutritional deficiencies. Hence lactose-free alternatives have been developed by adding an enzyme called beta-galactosidase (lactase). This enzyme is present in certain bacteria, fungi and yeast. In addition to this, almonds, peaches, apples and apricots also contain this enzyme. For commercial and industrial purposes, this enzyme is sourced from the fungi called Aspergillus and yeast Kluyveromyces.

The addition of this enzyme to the dairy foods breaks down the lactose yielding a lactose-free product. Also, the production of GOS provides the added prebiotic advantage. Emerging evidence from animal studies comparing the effects of lactose-free and regular yogurt have revealed that the consumption of the lactose-free yogurt was nutritionally effective as well as enhanced the gut and bone health. The findings from this study provides insights into the nutritional and therapeutic potential of these products in individuals with lactose intolerance.

Anti-cancer effects

Colorectal cancer (CRC) is the third most common cancer in the world and is the second most common cause for cancer deaths worldwide. Various factors like genetics, environment and life-style influence the development of CRC. High body mass index, obesity, sedentary lifestyle, increased intake of processed and red meat, refined grains, sugars, alcohol combined with reduced intake of fresh fruits and vegetables are associated with increased risk for the development of CRC.

The mechanism by which the GOS exhibit their anti-cancer effect against the development of CRC has not been fully evaluated. However, it has been hypothesized that the fermentation of prebiotics by the colonic bacteria and the resultant production of metabolites inhibits the growth of cancer cells. The increased population of healthy bacteria and enhanced production of short chain fatty acids have been shown to have a positive effect on the colonic environment including PH reduction, enhanced absorption of minerals and decreased absorption of ammonia which contribute to the anti-cancer effects. Among the short chain fatty acids, the role of butyrate has been widely investigated for its anti-cancer effects as it forms the main source of nourishment to the colonic cells. Here the butyrate is sourced from lactate which is produced by the lactic acid bacteria as increased colonization of butyrate producing bacteria by GOS is not known.

The effect of GOS in regularizing the bowel movements as well as increasing the stool bulk protects the colon. Irregular bowel movements increase the duration of stool retention and contributes to its hardening thus increasing the risk for CRC.

Recent studies have put forth the positive role of raffinose and stachyose in protecting against laryngeal cancer.

Metabolic effects

GOS are less sweet and low in calories and so are not readily converted into fat and cholesterol. Hence it balances the lipid profile by reducing serum cholesterol and increasing the levels of HDL (good) cholesterol. In one of the animal studies conducted on the effect of GOS on lipid profile, it was reported that the levels of total cholesterol, LDL (bad) cholesterol and triglycerides were notably on the lesser side with the administration of GOS. Another study conducted on obese rats has revealed the effectiveness of GOS in reducing weight and protecting the liver against fat accumulation.

Only few studies have explored the role of GOS in regulating the blood glucose levels and prevention of diabetes. The studies conducted on the effects of GOS and alpha-GOS in mice and overweight women have revealed reductions in the blood glucose and insulin levels. The changes in the gut microbial composition and increased production of short chain fatty acids by the gut bacteria are the presumed mechanisms for blood glucose stabilization and diabetes prevention. However more evidence is needed to ascertain the potential of GOS as a dietary supplement in the prevention and treatment of diabetes and its complications.

Mulberries and its extracts are well known for their biological effects that positively influence the health and well-being. They possess anti-oxidant, anti-diabetic, anti-cholesterol, anti-cancer, anti-inflammatory, anti-microbial properties in addition to protecting the liver and the nervous system. However, there is no clarity on the effects of the mulberry galacto-oligosaccharide (MGO) on the gut microbiota. But recent evidence from studies conducted on mice with type 2 diabetes, have revealed the effectiveness of MGO in reducing the blood glucose levels and improving insulin sensitivity. This study also observed the at the cellular level MGO was able to modulate the chemical processes involved in the glucose metabolism. Also, balance in the gut microbes with an increase in the healthy bacteria like Lactobacillus was noted. Hence this study provides insights into the anti-diabetic potential of MGO though more studies are warranted.

Arabinoxylans (AX)

These are complex carbohydrates that are present in the cell wall that constitute various parts of the plant. Regarded as a functional food, these are composed of xylose and arabinose units that are linked together by bonds. Xylose is a type of sugar present in plants and ranks second to glucose in being the commonly available sugar on earth. Arabinose is an important plant sugar which together with xylose and other sugars constitute hemicellulose which is a complex plant sugar that gives stability to the cell wall. This is present in agricultural waste such as rice straw, corn cobs and certain parts of hard woods.

Arabinoxylans exhibit viscous texture and water retaining properties due to which they are regarded as a dietary and functional dietary fiber conferring health benefits. Due to these properties, AX has found wide application in the cereal-based food industry like making breads, separating the gluten and starch as well as animal feeds. The presence of phenolic acid namely ferulic acid in AX is responsible for their anti-oxidant properties.

Sources

They are mainly present in cereal grains such as rice, ragi, wheat, barley, oats, rye, sorghum, maize, millet and also in plants like psyllium, flax seed, pangola grass, bamboo shoots and rye grass. The composition of arabinoxylans is highest in cereals such as rye, wheat, barley, oats, rice and sorghum in the descending order. Depending upon the type of plant, arabinoxylans are present in the cell wall or the husk. The fact that the AX content is maximum in the husk makes them an important source for their extraction though they can be separated from other parts of the grains.

Apart from the plant sources, arabinoxylans are also present in foods and beverages that are cereal based such as bread, pasta, biscuits, crackers, pastries, pancakes, waffles, cereal bars, ready to eat breakfast cereals, cereal bars and beer. These also have been added to foods such as bread, porridge, chapati and biscuits.

Biological functions

Though the biological effects of AX go in line with the beneficial effects of fiber rich diet, some of the specific health effects are

Gut microbial balance

Studies have reported the prebiotic effects of the AX in promoting the healthy bacteria with notable increases in the population of Bifidobacterium and Lactobacillus. This observation was put forth in a study conducted on brewer’s spent grain (this is the main by-product obtained from the brewing industry like beer brewing) wherein AX was extracted and studied for their effects on the gut microbes. It has also been hypothesized that the changes in the gut microbial balance brought about by the AX relates to their chemical structure which in turn is influenced by factors such as the type of grain, source, germination and extraction process.

So far, most of the studies conducted on AX are centered around those present in wheat. Apart from promoting healthy gut bacteria, AX has been shown to protect the gut against dysbiosis (bacterial imbalance), growth of harmful microbes and preventing gastrointestinal disorders. The preliminary data emerging from the recent experimental and animal studies have pointed to the beneficial effects offered by the wheat AX in protecting the gut against inflammatory bowel diseases. Though this study provides an insight into the anti-inflammatory effects of wheat AX, more studies are warranted to clarify these findings. Recent studies investigating the effects of wheat AX whose composition is modified by bio-engineering techniques have reported increased population of Bacteroides, Bifidobacterium and Eubacterium with increases in the levels of short chain fatty acids. However, more studies are needed to ascertain these observations.

Though not many studies have been conducted on rice bran, the available data from animal studies have revealed the potential of the AX extracted from rice bran towards increasing the alpha-diversity (a measure that indicates the gut health i.e., a more diverse bacterial colonies means better gut health) of the gut microbes in addition to enhancing the colonization of Bifidobacterium and Akkermansia.

It has been observed that many gut bacteria including Lactobacilli, Bacteroides, Roseburia, Faecalibacterium and Clostridium (non-pathogenic) are capable of breaking down the AX contained in the food sources. The resultant increase in the production of short chain fatty acids, that is attributed to the effects of AX has been documented in some studies. Despite the variations exhibited with the absorption capacity of dietary AX by the individuals, it has been justified that AX increases the production of short chain fatty acids. The evidence for this comes from the studies conducted on individuals who ingested bread whose AX content was boosted by its external addition.

Among the short chain fatty acids, increases in the levels of acetate, propionate and butyrate is seen. However, the production of butyrate takes an edge over the other two due to increased population of butyrate producing bacteria especially Roseburia and Eubacterium. The modification of gut bacteria with an increase in the production of the short chain fatty acids offer innumerable health benefits, protection against gastrointestinal infections, inflammations and colon cancer in addition to enhanced intestinal immunity and mineral absorption.

Anti-oxidant effects

The anti-oxidant effects of AX are mainly attributed to the presence of phenolic acids, in particular ferulic acid (FA). Various lines of evidence have supported the unique role of ferulic acid as anti-oxidant. Apart from whole grains and cereals, ferulic acid is present in spinach, parsley, grapes and rhubarb. It is considered as an exceptional anti-oxidant as it is easily absorbed by the body and stays in the circulation longer. Its multiple anti-oxidant role include protection against inflammation, infection, cancer (skin, lung, breast, colon), thrombosis (blood clots), diabetes and aging. It also protects the nerve cells and repairs the damaged cells.

As any other anti-oxidant, ferulic acid acts as a scavenger and eliminates free radicles. These are highly unstable molecules the production of which is driven by many factors such as metabolic activity, diet and environmental factors. Their ability to cause cellular damage is linked to many diseases and aging. The scavenging ability of ferulic acid has found its application as a sports supplement (to combat muscle fatigue), pharmaceuticals, food and skin care products especially sun screens, anti-aging creams and brightening agents.

Anti-cancer effects

The protective effect of AX on colon cancer is due to their combined prebiotic and anti-oxidant effects. Also, it has been hypothesized that the ability of the AX to strengthen the immune system contributes to its anti-cancer effects. The reports from one of the studies has documented the ability of the wheat AX in preventing the growth of colon cancer cells. Though the cancer prevention of AX also applies to other types of cancers such as stomach, neuroblastoma and liver, most of the studies have focused on their chemopreventive (a chemical agent that slows down or prevents the development of cancer) effects against colon cancer due to their prebiotic effects.

Recent animal studies which explored the chemopreventive effect of biobran (a natural compound made from rice bran to which enzymes from Shitake mushrooms have been added) on the development of adenocarcinoma of the stomach (a type of cancer arising from the mucus producing cells) have revealed the potential of biobran to contain the growth of adenocarcinoma.

Blood glucose regulation

Several lines of evidence have pointed to the beneficial effects of AX on the blood glucose levels. Studies have reported an overall reduction in the blood glucose and the post prandial glucose levels in individuals with prediabetes and type 2 diabetes. The effect of AX on the post prandial glucose levels is attributed to their high viscosity. Also, it has been hypothesized that the reductions in the post prandial glucose levels by AX is due to their effect on an enzyme called alpha-amylase.

The enzyme alpha amylase which is mainly present in the saliva and pancreas and helps in the breakdown of complex carbohydrates to simple sugars. Reducing the enzyme activity could prevent post prandial sugar spike and this could be a promising treatment strategy in individuals with type 2 diabetes. Due to the above-mentioned effects on the blood glucose levels, AX has been increasingly incorporated into various types of foods and beverages though there is a paucity of evidence in this regard.

Xylooligosaccharides (XOS)

These are a type of functional oligosaccharides that exhibit prebiotic potential. They are made up of xylose units that are bonded together and are produced from the xylan present in the plant fiber. Xylan is nothing but a type of carbohydrate that is a part of the cell wall (the outer thick covering of the cell) giving shape, structure and support to the cells. They exhibit favorable properties such as resistance to heat and acid, low sweetness and calories, water solubility, water retaining capacity, hygroscopic ability (absorption of moisture from the surroundings), anti-freezing (protecting the tissues from cold shock) and good organoleptic qualities (related to taste, feel, texture, color).

XOS is extensively used in food, agricultural, pharmaceutical and cosmetic industries. They can be used as a replacement for fat and sugar, augment taste and texture, weight management, specialized diabetic foods and immune enhancer. Since the XOS syrup imparts only half the sweetness of sucrose, it can be used as a sweetener. It can also be used to retain moisture and improve the fiber content of beverages, bakery and dairy products, chocolates and fruit jellies. With regards to the food processing, XOS is advantageous compared to the other oligosaccharides like FOS, inulin and GOS.

Sources

XOS are naturally present in wheat bran (the hard outer layer of the wheat kernel), barley hulls (hull is the outer thick covering of a fruit or seed)), gram husk (husk is the thin outer covering of a seed), almond shells, bamboo, corn cob, straw, brewery spent grains, honey and milk. On an industrial scale, the raw materials used for the isolation of XOS are sugarcane bagasse (a dry, pulpy and fibrous residue obtained after crushing of sugarcane), hulls, corn cobs, straw from the crops such as wheat and sugarcane, rice husk, malt cakes, bran, hardwood and cotton stalk (an agricultural waste obtained during cotton production).

Biological functions

Gut microbial balance

The fact that XOS encourages the colonization of healthy bacteria in the gut is supported by studies. The studies investigating the XOS derived from different plant sources have shown promising results with respect to the gut bacteria. It has been observed that the XOS derived from giant awn (awn is a slender bristle like part seen in plants like barley, oats and certain grasses) increased the population of Bifidobacteria and Lactobacillus. The XOS sourced from the corn straw enhanced the colonization of Lactobacillus and Bacteroides.

Another interesting observation was that the external fermentation of the XOS obtained from birch notably enhanced the colonization of Bifidobacteria and Staphylococcus hominis, a bacterium which has the ability to prevent the growth of harmful bacteria like Staphylococcus aureus and Helicobacter pylori. In addition to these, studies have reported significant reductions in the number of harmful bacteria such as Enterobacteriaceae (Salmonella, E. coli) and Clostridium perfringens.

The increased production of the bacterial metabolites as a result of fermentation of XOS and the resultant enhancement of the intestinal immunity and the mucosal barrier with reductions in the PH of the intestine are responsible for its protective effect.

Blood sugar control

The evidence accumulated from human and animal studies have projected the positive role of XOS in reducing the blood glucose levels in individuals with type 2 diabetes and impaired glucose tolerance. Animal studies have revealed considerable reductions in the blood sugar levels on consumption of XOS sourced from cereals. On the other hand, human studies conducted on individuals with type 2 diabetes have reported significant blood sugar reductions after 8 weeks of XOS consumption.

It has been hypothesized that the reductions in the blood glucose levels are brought about by increase in the level of propionate, a short chain fatty acid which enhances the production of the hormone called glucagon like peptide (GLP-1) resulting in the production of insulin which in turn causes reductions in the blood glucose levels.

Lipid profile regulation

Evidence points to the effectiveness of XOS in reducing the blood lipid levels in obese individuals. Studies conducted on individuals with type 2 diabetes have revealed significant reductions in the lipid levels seen after consumption of xylose at a dose of 4 g/day for 8 weeks. Similarly animal studies have reported reductions in total cholesterol, low-density lipoprotein (LDL or bad cholesterol), triglycerides with concomitant increases in the high-density lipoprotein (HDL or good cholesterol).

One of the mechanisms by which the reductions in the lipid levels occurs is the increased production of short chain fatty acids by the gut bacteria secondary to the fermentation of XOS. The short chain fatty acid, propionic acid has been said to influence the cholesterol metabolism wherein it reduces the cholesterol production in the liver in addition to improving the insulin action resulting in reductions in the cholesterol levels.

Anti-oxidant action

XOS has been shown to exhibit a strong anti-oxidant activity in terms of eliminating the free radicals from the body. Also, studies have demonstrated that the ability of the anti-oxidant effect of XOS depends on the quantity consumed and is related to the release of the phenolic compounds (a group of plant substances that have health protecting effects).

Anti-cancer effects

The changes in the gut microbial balance, increased production of short chain fatty acids, enhancement of the intestinal and overall immunity and anti-inflammatory effect contribute to its protection against cancer. Studies have revealed that the XOS extracted from the algae called Caulerpa lentillifera prevented the growth of breast cancer cells. Hence this alga is regarded to have a potential role in preventing breast cancer. XOS also increases the level of enzymes called glutathione-S-transferase (a detox enzyme) and catalase (anti-oxidant enzyme) in the colon which could be protective against colon cancer. Though these studies provide insights into the anti-cancer potential of XOS, more studies are needed to ascertain its cancer preventing properties.

Dental caries prevention

Dental caries is one of the major health concerns world wide affecting all age groups though comparatively common in children. Though is believed that a bacterium called Streptococcus mutans is associated with dental caries, imbalances in the oral microbial colonies leading to the disruptions in the biofilms (they are highly organized microbial colonies that surround the tissues in the oral cavity) is also another factor behind the occurrence of dental caries.

The sugars that are consumed are fermented by the oral bacteria to produce organic acids resulting in the PH reductions giving an acidic medium. But sugars that are consumed are constantly cleared by the saliva which actually helps in neutralizing the PH of the oral cavity. However, continuous exposure to sugars results in a more consistent PH reduction affecting the tooth enamel.

The consumption of XOS is said to protect against dental caries as it is not degraded by the bacterium Streptococcus mutans into simple sugars in the mouth. In one of the studies conducted on children, wherein the effects of prebiotics, probiotics and synbiotics on dental caries were evaluated, it was found that though all three proved to be effective in preventing the dental caries, prebiotics was considered a safe, simple, natural and economical option in reducing the occurrence of dental caries. Prebiotics was also found to reduce the population of microbes causing caries. This study supports the view that prebiotics could prove to be potential agents in preventing dental caries.

Human milk oligosaccharides (HMO)

These constitute a type of complex sugar that is exclusively present in breast milk. They are the third most important component of breast milk next to lactose and lipids. The two main types of carbohydrates present in the breast milk are lactose (80%) and HMO (20%). There are more than 200 different types of HMO that have been identified, but fucosylated, sialylated and nonfucosylated are the major ones. HMOs are nearly absent in the cow milk and infant formula thus making them one of the unique components of breast milk.

HMO’s exhibit special properties such as resistance to heat and cold as well as sustenance to pasteurization and freeze-drying. They also withstand the acidic medium of the stomach and are not broken down by the digestive enzymes. Most of the HMO’s are either digested by the infant’s gut bacteria or excreted unchanged. Also, the amount and type of HMO show variations among women and depends on factors such as genetics, diet, environment and geographical location. The gestational and the lactation period also influence the amount of HMO i.e., their content in the breast milk is high post term delivery compared to pre term delivery and also in the early stages of lactation.

Sources

It is a well known fact that HMO are unique to breast milk and thus regarded as one of the key nutrients for an infants growth and development. However, due to the decline in the rates of breastfeeding and the growing reputation of nutraceuticals (functional foods) as well as reduced accessibility to the HMO, different methods have been developed to produce HMO as a food additive on a commercial scale. Though the production of HMO by chemical methods using sugars such as lactose, D-galactose and D-glucosamine was started more than four decades back, and has seen considerable progress, the main drawback with this method is a low-quality yield as a result of chemical processing.

Another method of industrial production which was developed saw the use of specific enzymes. Though this method bypassed a number of steps compared to the chemical methods, it was not cost effective for a large-scale production.

An advanced and economical production that saw promising results involved the use of microbes. The bacterium Escherichia coli is the preferred choice for producing HMO because it exhibits rapid growth and also can be cultured easily. Though it does not possess any specific enzymes for the production of HMO, the presence of alternate metabolic pathways for the production of HMO makes it a microbe of choice. However, food safety is a major concern and needs to be addressed with caution. The fact that E. coli belongs to the group of bacteria that contain lipopolysaccharide, a component of the bacterial cell, increases the risk for many diseases. Hence removal of these is crucial to maintain food safety.

Biological actions

Gut microbial establishment

The digestive system of a new born is not mature in terms of both structure and function. A competent digestive system is crucial not only for the proper absorption of nutrients, but also to protect against infections. Thus, establishing healthy bacterial colonies in infancy plays a crucial role on the health and well-being at all stages of life.

A number of factors like mode of delivery, gestational age, nutritional status of the mother and infant, antibiotic use in early life, genetics, environment and geographical location influence the early microbial colonization. In addition to these the type and amount of HMO present in the mothers milk also determines the microbial development. An infant’s gut has only few bacterial colonies and variations exists between breast and formula fed infants.

The infant’s gut is primarily colonized by the Bifidobacteria and maintaining this bacterial population is crucial due to the innumerable health benefits it offers. Many studies have supported the bifidogenic properties (promoting the growth of Bifidobacteria) of the HMO. The Bifidobacterium species dominate the infant gut in the newborn period. Also, in the initial 1000 days of an infant’s life, the gut is dominated by ‘infant type’ Bifidobacterial colonies. However not all the bacteria belonging to the Bifidobacterium species can utilize HMO. Despite the presence of many types of HMO’s, there is variation in the way it influences the gut microbial colonization and the overall health. Among the many species, Bifidobacterium infantis holds a prominent place in its efficiency of utilizing the HMO.

In addition to the Bifidobacterium, many species of Bacteroides are capable of metabolizing the HMO. The bacterial fermentation of HMO results in the production of short chain fatty acids. In addition to the multiple effects of the short chain fatty acids on the gut health, studies have shown the direct influence of the HMO in stabilizing the gut health. They have been shown to have a positive influence on the goblet cells (special cells of the digestive tract that produce mucus) resulting in the production of more mucus, a key prerequisite for a strong intestinal barrier.

In comparison to the breast milk, the colostrum has higher concentrations of HMO and this corelates to the increased colonization of Bifidobacteria. This explains the crucial role of colostrum in shaping the gut microbes in the early days of the new born period apart from its other nutrient values.

Protection against necrotizing enterocolitis

Necrosing enterocolitis is one of the life-threatening illnesses affecting the neonates wherein bacterial infection and subsequent inflammation causes severe damage to the intestines leading to intestinal perforation, sepsis and death. It usually occurs in the second or third week of life. According to the estimates, 70% of the cases are seen in premature infants born before 36 weeks of gestation though infants born full term can also get affected. Other risk factors include low birthweight and formula feeding.

The evidence accumulated from many studies have reported that this illness is more common in formula fed compared to breast fed infants. Though there is not much clarity regarding the risk promoting effects of the formula milk and protective effects of the breast milk, statistical observations have put forth higher incidence among the formula fed infants, thus giving an insight regarding the specific components of both the milk responsible for either causing or protecting the illness. However, due to the variations in the composition of the HMO between the individuals, some infants do develop this illness despite being breast fed.

Though evidence from the animal and human studies have supported the efficiency of HMO in protecting against necrotizing enterocolitis, an interesting fact that was noted related to the specific type of HMO. While it was observed that the HMO’s disialyllacto-N-tetraose (DSLNT) and 2′-fucosyllactose (2FL) protected against this illness, reduced levels of lacto-N-difucohexaose increased the risk for necrotizing enterocolitis. However, more studies are needed to explore the role of DSNLT and other HMOs as a supplement to prevent this illness.

Protection against infections

It is a well-known fact that breast milk offers protection against various infections. Recent studies have attributed this effect to the presence of HMO in the breast milk. The evidence from various studies have revealed the protective effect of the HMO against bacterial, viral, fungal and protozoal infections. Majority of microbes including virus (influenza virus, rotavirus, HIV, coronavirus), bacteria (Streptococcus pneumoniae, Hemophilus influenza, Group B streptococci) and various other parasites need to attach themselves to the surface of the cells in order to multiply. It has been proposed that HMO’s block the attachment of the microbes to the cells. Also, they keep the harmful microbes at bay by increasing the dominance of friendly microbes. Though studies have put forth the anti-infective properties of specific HMOs against specific infections, more detailed studies are warranted.

Strengthens the immune system

The development of immune system in an infant not only starts before birth, but continues in the post birth period as the infant gets exposed to microbes. HMO’s not only influence the maturation of gastrointestinal immune system, but also the systemic immune system. The effect of the HMOs on the gut microbial colonization and the metabolites derived from the gut bacteria as well as the direct influence of the HMO on the maturation of the intestinal epithelial cells contributes to the enhancement of the gut immunity. This constitutes the first line of defense and is called as the innate or the natural immunity.

As the HMO’s pass through the large intestine, they get absorbed into the circulation and thus contribute to the maturation of the systemic immune system. Evidence from the research studies have revealed the ability of the HMO to modify the immune response (the body’s ability to defend against harmful factors such as microbes, tumors, toxins and damage to the tissues) by regulating the population of the immune cells (white blood cells) and cytokine release (special proteins which are regarded as the chemical messengers that control inflammation).

Protects against allergies

The role of breast milk in protection against the development of allergies has been known for many years. Breast feeding has been shown to reduce the risk of food allergy, eczema and asthma during early childhood. Studies conducted on infants who were fed with breast milk or formula containing an HMO called 2-fucosyllactose have observed reduction in the risk of eczema at the age of 2 years. But there have been inconsistencies with the studies regarding the duration of breast feeding. However, it has been hypothesized that the effect of the HMO in bringing about a balanced gut microbial colonization as well as the increased production of short chain fatty acids contribute to the reduced risk of inflammation and allergies.

Aids brain development

The presence of sialylated milk oligosaccharides, a type of HMO in breast milk has been linked to the brain and cognition (higher functions such as reasoning, thinking) development in infants. This oligosaccharide has been shown influence the memory and learning as well as the development of the gut-brain axis (the bidirectional communication between the gut and the brain). They have also been connected to the development of language and motor skills between the ages of 12-18 months.

Some experimental studies have revealed that the presence of an HMO called 2-fucosyllactose augments the development of memory and learning. Thus, it is evident that these two types of HMOs are crucial for the development of cognitive skills in the first two years. But there is paucity of human studies with this regard. However, reports from one of the human studies revealed that exposure to 2-fucosyllactose is related to enhanced development of higher functions and skills between the ages of 12-24 months.

Though these studies give insights to the role of HMO in the infant’s brain development, more studies are needed to ascertain the same.

Prevention of diabetes

It has been hypothesized that the HMOs namely sialylated milk oligosaccharides and 2-fucosyllactose play a role in preventing type 1 diabetes. Evidence from animal studies have reported that exposure to these HMOs in the early life has a protective effect on the development of type 1 diabetes and also reduced risk for the development of insulitis (inflammation of the Islets of Langerhans, which are a specialized group of cells of the pancreas that produce insulin and glucagon).

Application in infant feeding

Since HMOs are not present in infant formulas, the babies receiving formula milk may not get the benefits from HMO present in breast milk. Hence oligosaccharides possessing bifidogenic properties such as fructooligosaccharides (FOS) and galactooligosaccharides (GOS) have been added to the infant formula. Though these two oligosaccharides have different chemical structures compared to the HMOs, they have been found to be an effective and practical alternative to enhance the quality of the infant formula. Though more clarity is needed regarding their specific effects, the available evidence points to their effectiveness in balancing the gut bacteria, enhancing the immune system and protecting from allergies and infections. In one of the studies wherein the infants fed with formula containing a mixture of GOS and FOS were compared to breast fed infants, it was found that addition of 4g/l of GOS and FOS promoted the same growth parameters as breast feeding.

Though the oligosaccharides and their properties in the human milk were recognized more than seven decades back, their production on an industrial scale was only recently attempted. Two HMOs namely, 2-fucosyllactose (2-FL) and lacto-N-neotetraose (LNnT) have been included in the infant formula. Studies comparing the effects of the infant formulas with added GOS and 2-FL with those containing a combination of FOS and 2-FL, found that both were well tolerated.

Studies comparing the effects of 2-FL or LNnT added to the infant formula found them to be safe, well-tolerated and promoted normal growth. The US Food and Drug Administration, European union and the European Food Safety Authority have labelled these two HMOs as safe. Though the addition of oligosaccharides has been found to mimic the qualities of human milk, according to the WHO, breast feeding has been considered as a gold standard, and recommends exclusive breast feeding in the first six months of life.

Polysaccharides

Beta-glucans

These constitute a group of dietary fibers or polysaccharides that are made up of D-glucose (a type of glucose that is naturally found in plants) molecules that are bonded together. Based on their functional properties they can be classified into two types i.e., soluble and insoluble beta-glucans. The insoluble fibers exhibit reduced intestinal transit time, meaning that they pass through the intestines slowly in addition to contributing to the increased stool bulk. On the other hand, the soluble fibers increase the intestinal transit time and slow the absorption of glucose from the intestines. They also have the ability to form gels. Due to their proven health benefits, they are increasingly gaining attention as food supplements and potential drugs.

Sources

Beta-glucans constitute one of the cost-effective milling products that can be sourced from the cell walls of the yeast, fungi and cereals. They are naturally present in cereal grains with the highest concentration present in barley and oats. Other cereals such as sorghum, rye, maize, triticale (a man-made cereal which is a hybrid of wheat and rye), wheat, durum wheat (also called as pasta and macaroni wheat that is high in gluten and protein content) and rice also contain beta-glucans but in smaller amounts. Other than cereals, yeasts like Saccharomyces cerevisiae, mushrooms like Maitake and Shiitake and sea weeds such as Laminaria also contain beta-glucan.

Biological functions

Strengthens the immune system

The biological role of the beta glucans sourced from fungi, sea weed, yeast and mushrooms have been doing rounds for four decades. Evidence from various animal studies have revealed the ability of beta-glycans in enhancing the clearance of harmful microbes and reduce the death rate from infections. Also, the studies on high-risk surgical patients have reported that injection of beta-glucan sourced from yeast reduced the risk of post-operative infections and the duration of ICU stay. The yeast derived beta-glucans have also been shown to enhance the activity of the neutrophils (a type of white blood cell) to fight against harmful microbes. The beta-glucan called zymosan which is derived from the yeast called Saccharomyces cerevisiae has been shown to activate the immune system.

The possible role of mushrooms as healing agents has been known since pre-historic times. In one of the studies that evaluated the effects of the beta-glucan derived from Ganoderma lucidum (a type of mushroom) it was found that they boosted immunity by promoting the maturation of dendritic cells (a type of immune cell) and increasing the production of interferons (proteins which are produced in response to infection) and interleukins (proteins that take part in the maturation of the immune cells). Also, the beta-glycans derived from mushrooms namely lentinan and sonifilan have found their application as medicines that enhance immunity. In addition to these findings, a number of animal and human studies that investigated the effects of beta-glucans from different sources reported its efficiency in preventing infections, allergy and inflammation by modulating the immune system.

Cholesterol lowering effects

The role of the beta-glucan in lowering the cholesterol levels have been known for several decades. The beta-glucan present in cereals such as oats and barley have been shown to reduce the cholesterol and are prescribed to individuals with increased cholesterol levels. They bring about reductions in the low-density lipoproteins (LDL or bad cholesterol). Hence it is regarded that the consumption of the beta-glucans from the cereal lowers the risk of heart diseases. However, the cholesterol lowering effects have been reported only for the beta-glucans from the cereals.

Blood glucose lowering effects

Several studies that have investigated the effects of beta-glucans on the blood glucose levels have reported their ability to bring about reductions in the blood glucose levels in individuals with type 2 diabetes. Recent studies conducted on the beta-glucans derived from oats have reported their ability to reduce the post-prandial blood glucose levels. In this study the outcomes were the same for both diabetic and non-diabetic individuals. Similar effects on the blood glucose control with oats beta-glucan were also observed in adolescents with type 1 diabetes.

Studies comparing the addition of 50% beta-glucans from barley to white rice versus white rice alone diet in those with type 2 diabetes reported reductions in the post-prandial glucose levels with the combination of barley beta-glucan and white rice. In addition to this, beta glucans from black yeast have been shown to normalize the fasting blood glucose levels and this effect has been proposed to improve defense against Covid-19 in diabetics.

Fermentation outcomes on health

Beta-glucans from cereals have been found to have prebiotic effects and they cause increase in the population of Bifidobacterium and Lactobacillus. The fermentation of the soluble beta-glucans by the gut bacteria results in the production of short chain fatty acids. Apart from the health benefits offered by them on the intestine and overall health, they also take part in balancing the energy metabolism and prevents extra fat from being accumulated in the adipose (fat) tissues. This activity helps in preventing obesity.

Anti-oxidant action

The beta-glucans derived from cereals such as barley and oats have been shown to possess anti-oxidant activity. They help in the elimination of reactive oxygen species (ROS) which are produced during the cell metabolism. ROS are considered as unstable molecules and have to be removed from the body as excessive accumulation leads to cell damage and death. The differences in the structure of the beta-glucan from various sources influences their anti-oxidant capacity. For example, those derived from barley have higher anti-oxidant activity compared to those present in oats and fungus. Also, the beta-glucans sourced from foods other than cereals have low ROS eliminating capacity. This indicates that their quantity in food is very important. However, some animal studies point to the anti-oxidant ability of the oral beta-glucans i.e., those prepared by separating them from various sources.

Anti-tumor action

Some of the beta-glucans that are derived from shiitake and maitake mushrooms have been used for their anti-tumor action, wherein they are given either intravenously or orally. Hence, it has been suggested that consumption of the mushroom fruiting bodies (the part of the mushroom on the ground or emerging from the tree) has potential anti-tumor effects. One of the studies has reported the anti-cancer effect of lentinan, a beta-glucan derived from mushroom, against liver and pancreatic cancers. The anti-tumor activity of the beta-glucan has been attributed to its ability to enhance the immunity against tumors as well as destroy the cancer cells.

A note on paramylon

Paramylon is a polysaccharide that is derived from Euglena gracilis which is a freshwater alga. The large-scale cultivation of this alga was established in the year 2005 and since then has provided a practical and cost-effective means for obtaining beta-glucan as a substitute or supplement. This polysaccharide is made of glucans and hence are considered to have similar biological effects as the beta-glucans from other sources as revealed by the experimental and observational studies. Also, studies have revealed that Euglena gracilis as a whole and the paramylon derived from it have the same prebiotic effect as the other beta-glucans. This alga has been given a safe status which is an important requirement for its use in the food industry.

Resistant starch (RS)

Starch is regarded as the main form of carbohydrate that is present in plants including fruits, vegetables and grains. They are considered as an integral part of a balanced diet. They are broken down by the body into simpler glucose molecules during digestion which form the main source of energy for the body. Resistant starch is the type of carbohydrate that does not undergo digestion, but are fermented by the gut bacteria in the large intestine.

Types and sources

RS type 1: These are present in the partial or whole milled cereals and pulses. Since they are tightly embedded within the cell wall, they cannot be broken down by the digestive enzymes in an unprocessed state.

RS type 2: These are present in green (unripe) bananas and raw potatoes. They are present as starch granules and undergo gelatinization in the presence of water at 60 degrees. Another type of RS type 2 starch is called high amylose corn starch. This is more resistant to gelatinization requiring a higher temperature of 120 degrees. Apart from maize, the high amylose starch is also produced from barley and potatoes and have wide application in nutrition.

RS type 3: These are present in starchy foods such as potatoes, bread, rice, pasta and oat meal that have been cooked and cooled. This type of starch is regarded as a retrograded starch which means that the molecules present in them reorganize themselves on cooling to a modified form giving rise to change in the texture, flavor and nutritional value.

RS type 4: These are chemically modified and processed and are used in making bread, pastries and cakes.

Applications

On a commercial scale, RS is used to produce high fiber products, Low glycemic index products (glycemic index is the value that is used to measure the rise in the blood glucose levels caused by specific foods. The lower the value, it is less likely to cause sugar spikes), gluten free products, sports supplement, wheat flour substitute, as prebiotic foods and low FOODMAP products (this means fermentable oligosaccharides, disaccharides, monosaccharides, polyols that are regarded as specific type of carbohydrates poorly absorbed by the intestines. It causes digestive disturbances in some people) and as a substitute for wheat flour.

Biological functions

Regulates intestinal health

In accordance with the evidence from studies, it has been proposed that the fermentation of RS by the gut bacteria leads to the production of short chain fatty acids. Among them the acetic and propionic acid lower the PH of the intestine which not only prevents the growth of the harmful bacteria, but also promotes the colonization of beneficial ones. Also, researchers revealed the distinct ability of RS in enhancing the butyrate producing bacteria. In addition to being the main energy source to the colonic cells, butyrate also protects against cancerous changes. The ability of the RS in improving the bowel habits leads to shortening of the time taken for the food to pass through the intestine. This effect also has a protective effect against colorectal cancer.

Studies conducted on the effectiveness of RS when combined with wheat bran have demonstrated increased amount of butyrate production. It was also observed that when RS was combined with psyllium, the fermentation was shifted to more distal parts of the colon. This is a significant finding because the cancers are common in the distal part of the colon. With this regard, RS could be applied as a preventive strategy against colorectal cancer.

RS also enhances the colonization of the beneficial bacteria like Lactobacillus and Bifidobacteria. It should be noted that the increase in the bacterial population depends upon the type of RS. For example, Ruminococcus and Eubacterium numbers are increased by RS2 leading to enhanced butyrate production.

Apart from the short chain fatty acids, the RS fermentation results in the production of gases like hydrogen. Methane and carbondioxide causing gaseous distention and discomfort.

Regulates blood glucose levels

Many studies that have investigated the effects of RS on the blood glucose levels have revealed its positive effects in reducing the fasting blood sugar levels by enhancing the insulin production as well as insulin sensitivity. Improved insulin sensitivity means that the body is able to utilize the sugars effectively. The opposite effect happens in case of insulin resistance. The increased production of the short chain fatty acids and their positive influence on releasing hormones like glucagon like peptide (GLP-1) is thought to be the mechanism behind this effect. Though some studies have produced inconsistent results with this regard, one of the recent studies conducted on individuals who were overweight or obese with diabetes showed the effectiveness of RS in reducing the fasting blood glucose compared to the consumption of other digestible starches. This effect was observed after eight weeks of RS consumption at a daily dose of 28 grams.

Regulates the blood lipid levels

Most of the studies and trials conducted on animals and humans have observed positive effects of RS on the blood lipid levels. In animal studies involving the consumption of 25% raw potato, bean starch, cassava starch and oat fiber, reductions in the total cholesterol were observed. On the other hand, studies conducted on the overweight and obese individuals reported reductions in the total and LDL (bad) cholesterol levels. But no significant improvement in the triglyceride levels were noted.

However, studies conducted on individuals with type 2 diabetes have produced conflicting results in terms of cholesterol and triglyceride levels. But given the fact that RS has a favorable effect on the blood lipids, it has been proposed that the increased production of short chain fatty acids and increase in the population of healthy bacteria as the possible mechanisms in bringing about the balance in the blood lipid levels. It has also been observed that RS3 possess higher cholesterol lowering properties due to its chemical structure.

Anti-obesity effects

Studies have supported the effectiveness of RS in controlling weight thus preventing overweight and obesity. The possible reasons for this effect could be increased satiety caused by RS, reduction in the calorific value of food caused by its addition and enhanced energy expenditure due to increased production of short chain fatty acids. While the animal studies reported increased satiety, reduction in body weight and fat deposition, human studies also produced encouraging results. Prolonged fullness with reduced food consumption were observed. Studies conducted on overweight and obese individuals with type 2 diabetes and prediabetes have revealed considerable reductions in the body weight attributed to consuming RS2 from bananas. Though these reports justify the effectiveness of RS in weight reduction, conflicts do exist with this regard.

Pectin

This is a type of dietary fiber mainly present in the cell walls of terrestrial plants (plants that grow on land). Apart from giving structural support and hydration to the plant, it plays a role in protecting the plant from microbes and environmental stressors like temperature changes, changes in the salinity (salt content) of the soil etc. They are regarded as complex carbohydrates made up of galacturonic acid units.

Pectin has found wide application in food, health and pharmaceutical industries. In food industry, it is used as an additive in the preparation of jams and jellies, in addition to being used as emulsifier, stabilizer and thickener. In pharmaceutical industry, it is used for the manufacture of blood cholesterol lowering medications, as a remedy for gastrointestinal disorders and treatment of cancer. It is also used in the manufacture of edible films and coatings (a thin and protective layer created around a food surface like fruits, vegetables, nuts, processed foods, meat and seafood to delay ripening and spoilage), paper substitutes and foams.

Sources

Though pectin is found in almost all plants, they are sourced from orange, lemon, grape fruit and apple for commercial production. In comparison to cherries, grapes and strawberries, fruits like quince, plums and gooseberries contain higher amounts. The pulp obtained from the apple contains 15-20% of pectin whereas dried citrus peel has 30-35% pectin. Apart from these, the other sources include cocoa, sunflower heads, sugar beet, pumpkin, watermelon, pears and potato pulp. Sugar beet yields 23% pectin depending on the method used for extraction. However, owing to their poor gelling ability, they are less used compared to the pectin obtained from apples and citrus fruits.

Biological action

Enhances gut health

It is becoming increasingly clear that pectin plays a role in improving the intestinal health. The emerging evidence from the studies have pointed to the prebiotic role of pectin. Though more confirmation is needed from the human studies, the available evidence supports the role of pectin in enhancing the population of beneficial gut bacteria. Reports suggest that the pectin derived from sources such as orange, lemon and sugar beet promote colonization of Bifidobacteria, Lactobacillus and Eubacterium. It is said that the increased production of short chain fatty acids as well as gases such as methane, carbondioxide and hydrogen have positive effects on the health. Pectin is also regarded as a dietary fiber which improves bowel movements and has a protective effect on the development of intestinal disorders and cancer.

Stabilizes blood glucose levels

There is a growing body of evidence from the animal and human studies that support the positive effects of pectin on the blood glucose levels. Studies conducted on the pectin extracted from passion fruits and unripe apples have demonstrated improvement in blood glucose levels and the overall health in diabetic individuals. On the other hand, human studies conducted on the effects of soybean seed coat, a by-product of soybean processing have revealed significant reductions in the blood glucose levels. One of the mechanisms put forth to explain the blood glucose lowering effect of pectin is its ability to decrease the absorption of glucose. The pectin derived from apple has been shown to bring about increased excretion of glucose in urine thus reducing the blood sugar levels.

Stabilizes cholesterol levels

Many studies have supported the cholesterol reducing ability of pectin, especially total cholesterol and LDL (bad) cholesterol. The increased production of short chain fatty acids, gelling ability of pectin resulting in reduced glucose absorption and enhanced removal of bile acids which helps to eliminate the excess cholesterol are the possible mechanisms that have been put forward. Studies analyzing the pectin from citrus and banana passion fruit (oval shaped fruit with yellow skin and orange pulp) have revealed that the action on the lipid profile depends on the physical properties and the chemical structure of pectin.

Removal of toxic metals

The accumulation of metals such as arsenic, cadmium, mercury and lead have negative consequences on the functioning of the body. Even though the removal of toxic metals from the body can be facilitated by the use of metal chelators (compounds that enhance the elimination of toxic metals from the body by binding), they have their own risks. Pectin has been regarded as an effective chelating agent without causing any side effects. It binds to the heavy metals in the digestive tract preventing their absorption and enhancing their elimination in the feces. Human studies have acknowledged the effectiveness of pectin as a chelating agent. The pectin derived from citrus fruits have been found to be effective in eliminating lead without any side effects.

Anti-cancer properties

A significant number of studies have reported the protective effect of pectin against various cancers such as prostate, colon, pancreas and breast. Also, its effectiveness in the prevention of cancer metastasis (spread), growth and multiplication of cancer cells and its ability to bring about apoptosis (cell death) has been reported. On the other hand, studies comparing the anti-cancer effects of pectin and modified pectin (a form of pectin produced by altering the pH and temperature of pectin) have revealed the higher efficiency of modified pectin. A limited number of human studies that have investigated the anti-cancer effects of modified citrus pectin (a type of pectin sourced from citrus fruits and modified by bringing about pH and temperature changes) have demonstrated its effectiveness in prostate cancer.

Emerging prebiotics.

Pectic oligosaccharide (POS)

These are non-digestible oligosaccharides which are regarded as a new class of prebiotics that are derived from pectin by chemical and enzyme treatment methods. Evidence from experimental studies have revealed the prebiotic potential of POS derived from citrus peel waste, apple pectin and sugar beet pulp. While the growth of Lactobacillus was observed more with sugar beet pulp, citrus peel waste demonstrated increase in the population of bacteria belonging to the Eubacterium, Faecalibacterium and Roseburia. Apart from these sources, almond seeds have also showed promising results. Recent studies have put forth the prebiotic potential of POS derived from mangoes.

Studies comparing the POS extracted from turmeric have revealed their potential to protect against gastric ulcers. Also, the inclusion of POS in infant formula have shown their effectiveness in increasing the colonization of Lactobacillus and Bifidobacterium. Similarly, POS combined with fructooligosaccharides (FOS) and galactooligosaccharides (GOS) have revealed their efficiency in preventing serious infections in pre-term infants. In addition to this, consumption of a combination of POS, FOS and GOS in individuals affected with early stages of HIV infection have shown reductions in the potential for infection. This effect was attributed to the gut bacterial balance with increase in the Lactobacillus population.

Polydextrose (PDX)

This is a nondigestible polysaccharide which is regarded as a dietary fiber. It is prepared by combining glucose (89%), sorbitol (10%) and citric acid (1%). Apart from the prebiotic properties, it is considered as a bulking agent (these are ingredients that increase the volume of food without affecting the nutritive value. They produce early satiety in addition to improving the bowel habits), stabilizer, thickener and humectant (an agent which preserves moisture). It is also low in calories and is stable at varying temperatures.

Its ability to reach the colon (large intestine) without undergoing digestion and enhance the colonization of targeted beneficial bacterial groups have supported the recognition of PDX as a prebiotic with an array of health benefits. Various studies have reported increase in the population of Bifidobacterium, Lactobacillus, Faecalibacterium in addition to enhanced counts of butyrate producing bacteria such as Ruminococcus and clostridia. Butyrate is a type of short chain fatty acid that keeps the colonic cells healthy. Evidence also supports the efficiency of PDX in improving the bowel habits, protecting against colon cancer, alleviation of allergic dermatitis, cholesterol and blood glucose lowering effects. Infant formulas with added PDX have been demonstrated to improve the stool consistency and promote Bifidobacterial colonization similar to breast fed infants.

Algae-derived oligosaccharides

Seaweeds are considered as one of the natural sources of bioactive compounds such as polysaccharides, proteins, polyphenols and polyunsaturated fatty acids. Some of the important polysaccharides that are present in the seaweeds are alginates, laminarins, fucans and cellulose (brown algae), ulvan (green algae), agars and carrageenans (red algae). The potential health benefits that have been proposed with these polysaccharides include anti-oxidant, anti-bacterial, anti-inflammatory, anti-coagulant, anti-cancerous and apoptosis (programmed cell death with elimination of damaged or unwanted cells in the body). In the light of their biological effects, several extraction methods have been developed to isolate them in the recent years.

Evidence from several studies have proposed the prebiotic potential of these polysaccharides owing to their non-digestibility in the stomach and small intestine, ability to support the growth of friendly gut bacteria, prevent the colonization of harmful bacteria and improve intestinal health by regulating the PH and short chain fatty acid production. Increases in the population of Lactobacillus and Bifidobacterium and enhanced protection against bacterial infections such as E. coli, Clostridium perfringens (bacteria that commonly causes food poisoning), Pseudomonas aeruginosa (causes infections affecting different organs especially in individuals with low immunity) and Helicobacter pylori (one of the common causes of stomach ulcer).

Studies investigating the anti-viral potential of sea weeds have revealed their effectiveness against herpes, dengue and influenza virus, though more confirmatory studies are warranted. On the other hand, studies that have explored their potential regarding strengthening of the immune system, anti-cancer, anti-diabetic and-anti-oxidant activities have shown promising results. However, a much broader analysis is needed in these areas.

Bacterial exopolysaccharides (EPS)

These are complex carbohydrates that are produced by certain bacteria. They surround the bacterial cell and serve as a protective layer against the environmental stressors. A growing number of studies have investigated the beneficial effects of these polysaccharides. The lactic acid bacteria namely Lactobacillus, Bifidobacterium, Lactococcus and Streptococcus are regarded as EPS producers.

The evidence from many studies have put forth the convincing role of EPS as an anti-oxidant, anti-inflammatory, anti-hypertensive, anti-cancer, anti-viral, anti-bacterial, anti-ulcer and anti-diabetic agent. Reports have also justified their prebiotic potential, cholesterol lowering and immune strengthening properties. In addition to these findings, their ability to prevent the formation of biofilms (aggregation of clusters of bacteria) pertaining to harmful bacteria such as E. coli and Salmonella have been supported by studies. However, most of the accumulated data is from the experimental studies. In addition to more in-depth analysis of EPS, human studies are also warranted.

Sugar alcohols

Also called as polyols, these are a type of carbohydrates that are naturally found in some fruits and vegetables. Owing to their low calorific value and sweetness, they are used as natural sugar replacers. In addition to this, they have been used in the food industry as flavor enhancer, stabilizer, thickener, emulsifier, glazing, anti-caking agent (prevents the formation of lumps when added to salt and other confectionaries), humectant (agent that preserves moisture) and bulking agent. Some of the polyols that are approved for usage in the food industry are sorbitol, mannitol, isomalt, maltitol, lactitol, xylitol and erythritol.

Some of the natural sources of sugar alcohols are as follows

Sorbitol: Apples, pears, nectarines, apricots, prunes, raisins, dates.

Mannitol: Brown Sea weeds, wine, pineapples, asparagus, sweet potatoes, carrots.

Xylitol: Berries, oats, mushrooms.

Erythritol: Melons, peaches, mushrooms, fermented foods like soy sauce, wine, beer and sake.

However, the sugar alcohols that are present in sugar free candies, chocolates, energy bars, cookies, lozenges and cough syrup are artificially produced.

Though the sugar alcohols have resemblance to sugars in their chemical structure, they do not undergo complete digestion. The undigested portion is fermented by the gut bacteria and hence they can function as prebiotics. Though preliminary data points to their ability to promote the colonization of Bifidobacteria and Lactobacillus, more studies are needed to ascertain the same. They also could prove beneficial to individuals with pre diabetes and diabetes as they do not cause sugar spikes. However, the evidence behind this is scarce. Some studies have supported the protective effects of xylitol and erythritol against dental caries. The main drawback of these sugar alcohols is gastrointestinal disturbances like abdominal pain and diarrhoea. But these effects depend on the type and amount consumed for the individual body weight.

Summary

*The concept of prebiotics has continuously evolved since the last two decades.

*Prebiotics are a ‘specific type of food ingredients that are non-digestible, but are capable of being utilized by the gut bacteria to produce short chain fatty acids that confer innumerable health benefits.

*Most of the prebiotics are regarded as complex carbohydrates, though polyphenols and PUFA have been proposed to have prebiotic potential.

*Oligosaccharides and polysaccharides belong to the group of complex carbohydrates that have prebiotic potential.

*The oligosaccharides include fructans, galacto-oligosaccharides, arabinoxylans, Xylo oligosaccharides and human milk oligosaccharides.

*Fructans are present in a wide range of plant foods

* Some of the sources of fructans include onions, garlic, shallots, Jerusalem artichoke, agave, bananas, broccoli, brussels sprouts, asparagus and chicory.

*Apart from promoting the growth of beneficial gut bacteria like Lactobacillus and Bifidobacterium, they regulate the bowel movements and balance gut hormones as well as blood sugar and lipids.

*Galacto-oligosaccharides (GOS) are present in lentils, beans, chickpeas and vegetables like peas and butternut squash as well as cashews, almonds and pistachios.

*GOS are of two types, alpha and beta.

*Raffinose family oligosaccharides (RFO) are a major group of carbohydrates present in legumes that are regarded as a part of the Alpha-GOS family.

*The specific health effects of GOS include gut microbial balance, prevention of constipation, enhancement of skin health and intestinal calcium absorption, stabilizing the blood sugar and cholesterol and protection against colorectal cancer.

*RFOs exhibit anti-diabetic, anti-obesity, anti-allergic effects in addition to offering protection against non-alcoholic fatty liver disease.

*Arabinoxylans (AX) are present in cereal grains with the highest concentration in rye, wheat, barley, oats, rice and sorghum in descending order.

*Apart from its influence on gut bacteria, metabolic health and anti-cancer properties, it exhibits anti-oxidant effects.

*The anti-oxidant effect of AX is due to the presence of ferulic acid which is regarded as a unique and exceptional anti-oxidant.

*Xylooligosaccharides (XOS) are present in wheat bran, barley hulls, gram husk, almond shells, bamboo, corn cob, straw, brewery spent grains, honey and milk.

*Apart from ensuring the gut and metabolic health, they also possess anti-oxidant and anti-cancer effects in addition to preventing dental caries.

*Human milk oligosaccharides (HMO) are one of the unique components of breast milk.

*The production of HMO on an industrial scale has seen considerable progress, starting from the use of chemical processing and enzymatic methods to the use of E. coli.

*HMO plays a crucial role in establishing healthy gut microbes in new born infants. It also offers protection against infections, allergies and necrotizing enterocolitis, in addition to strengthening the immune system and supporting brain development.

*Despite the development of infant formulas with added oligosaccharides, breast feeding remains the gold standard for an infant in the first six months of life.

*Beta-glucans are present in cereals like oats and barley. Other than cereals, yeasts like Saccharomyces cerevisiae, mushrooms like Maitake and Shiitake and sea weeds such as Laminaria also contain beta-glucan.

*Oats and barley have been widely studied for their health effects as they contain highest concentration of beta-glucans.

*Paramylon, a polysaccharide obtained from an alga called Euglena gracilis, has been regarded as a cost-effective and safe substitute for beta-glucan on a commercial basis.

*There are four types of resistant starch (RS) based on the sources and physical characteristics.

*Some of the sources of RS include partial or whole milled cereals and pulses, raw potatoes and ripe bananas.

*High amylose corn starch is a type of RS that requires higher temperatures for gelatinization.

*Retrograded starch is present in foods such as potatoes, pasta, bread, rice and oat meal that have been cooked and cooled.

*Studies have supported the effectiveness of RS in improving the intestinal and metabolic health in addition to its anti-obesity effects.

*Pectin is present in nearly all terrestrial plants in varied concentrations.

*Oranges, lemon, grapes, gooseberries, plums, dried citrus peel and apple pulp are some of the sources of pectin.

*Apart from promoting the intestinal and metabolic health, pectin has been demonstrated as an effective chelating agent.

*Modified pectin has been shown to possess higher anti-cancer effects compared to pectin.

*Though studies support the prebiotic potential of the emerging prebiotics, more confirmatory studies are warranted.