Intermittent fasting and gut health

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Introduction

The practice of restricting or abstaining from food and drink for a period of time is called fasting. The tradition of fasting dates back to several millennia wherein it was carried out for religious, moral and therapeutic values. With regards to the medical practice, this convention can be traced back to 5th century BCE which coincides with the time of Hippocrates. Also considered as the father of modern medicine, he said “to eat when you are sick, is to feed your illness” and recommended fasting for healing certain health conditions. Other prominent Greek figures regarded fasting as a better remedy compared to medication. In ancient Greece fasting was viewed as a strategy to ameliorate cognitive health.

However, the insights into the effects of fasting on various organs and organ systems was put forth by studies conducted on animals and humans in the 19th century. The increased knowledge regarding the nutritional aspects of the human body and the benefits of fasting saw significant changes in the approach to the methods of fasting in the 20th and the 21st centuries.

The aim of this article is to provide an overview of the different fasting regimens and their health benefits with particular reference to the gut health. Though regarded as safe, it is better to consult an expert prior to following the same as certain situations in both health and disease may pose special challenges.

The calorie constraint

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The calorie restriction with optimal nutrition (CRON-diet) is defined as the practice of restricting or reducing the energy or calorie intake that is below the recommended value making sure that the essential nutrients are taken in adequate amounts to avoid malnutrition. The practice of calorie restriction has been doing rounds since historical times. But unfortunately, most of the instances witnessed malnutrition.

One of the earliest pieces of evidence on the CRON- diet comes from the population-based studies dating back to the world war times. It was observed that imposing a planned dietary restriction accompanied by adequate intake of fruits, vegetables, whole grain and fish, saw a reduction in the death rates. Similarly, observations from the region Okinawa, Japan revealed that calorie restriction in combination with an adequate intake of fresh fruits, vegetables, soya and fish improved the life span. This region actually saw a significant number of centenarians though in the later decades’ westernization led to change in their food habits.

Though the above observations provided some insights into the benefits of calorie restriction, there was a paucity for scientific evidence. In the later decades that followed, the evidence from several studies conducted on animals and humans have consistently put forth the positive effects it has on longevity, weight reduction, metabolic health, cardiovascular health, mental health, cognition and mitochondrial function (small rod-shaped structures present inside the cell that take part in energy production). However, sustainability to calorie restriction is a major drawback as evidenced by studies conducted on obese individuals.

Intermittent fasting (IF)

In comparison to calorie restriction, intermittent fasting has gained popularity in the last few decades. The very fact that it can be independently executed, with only fixed periods of fasting allowing the intake of minimum calories and its close association with the normal eating regime have made intermittent fasting gain an upper hand over calorie restriction. Also, its health boosting effects supported by various lines of evidence has added to its wide acceptance.

Intermittent fasting is a dietary regime wherein specified periods of eating are alternated with periods of fasting with no or very little calorie intake. In this regime, the calorie consumption ranges from zero to 25% of the caloric requirement during fasting to either following a specific dietary plan or aiming at 125% of the caloric needs during periods of eating. This means that during the eating window, intermittent fasting can be applied as a period of unrestricted calorie intake or as a part of specific dietary plan.

Depending upon the duration of the fasting intervals, intermittent fasting can be classified into the following types

The 12-hour fasting

In the above pic, the half full and half empty plate represent the 12-hour eating and fasting windows. Also called 12:12 method, this is a very practical regime especially for beginners. The main advantage with this method is that the lengths of the eating and fasting window are equal and pretty much remains consistent. The rules to this method of fasting are quite simple. A specific time needs to be chosen and it is important to keep up with the same fasting time window every day. For instance, if a person decides to have the fasting window between 6 P.M and 6 A.M, the dinner should be well finished before 6 P.M and should wait till 6 A.M in the morning to break the fast. One of the best ways to include this fasting regime is to incorporate the maximum sleeping hours in the fasting window. It’s pretty usual to get the hunger pains to begin with but the body will ultimately adjust.

Alternate day fasting (ADF)

In the above picture the filled cups alternate with empty cups representing the feeding and fasting days. Also called 4:3 method or every other day diet, this regime involves fasting and eating on alternate days. Typically, on fasting days no calorific foods or beverages are consumed. However, this can also be modified wherein the individual is allowed to consume 25% of the caloric requirement on the days of fasting which means if the total caloric recommendation is 2000 calories, the individual can consume 500 calories on fast days. On the fed days there is no calorie restriction. Though calorie counting is a part of this fasting regimen, it happens only on alternate days thus allowing the individual to eat as much as desired on fed days. Also, when compared to a calorie restricted diet, the sustenance is far better.

Studies conducted on animals and humans have revealed that alternate day fasting was effective in achieving weight loss, stabilizing the blood glucose and cholesterol levels and reducing the cancer risk and inflammation. However, when compared to a calorie restricted diet, it was not considered remarkable in terms of the beneficial health effects. Though some reports justify the safety of alternate day fasting, the occurrence of light-headedness and headaches have been reported by many. There are also reports relating to the non-adaptability to hunger on fasting days even after following the fasting regimen for a considerable period of time. Hence this method of intermittent fasting may not be suitable for long term.

The 5:2 fasting

In the above picture, the five filled cups represent the eating days and the two cups that are filled only at the bottom denote the fasting days wherein either zero or low calories are consumed.

Also called as the fast diet, this modified fasting regimen consists five feeding days per week with two non-consecutive days of consuming a calorie restricted diet amounting to 500 calories per day which is 25% of the caloric requirement. The low-calorie diet on the fasting days can be either split as breakfast, lunch and dinner or as a slightly substantial breakfast and lunch adjusted to 500 calories. The duration of fasting is 36 hours. For example, If the last meal of the day i.e., dinner is consumed on Monday night, the fasting is continued through Tuesday and broken on Wednesday morning with breakfast. The choice of the days is flexible making sure that there is at least a fed day between two fasting days.

The evidence behind the health benefits of 5:2 fasting regimen is limited. However, some animal and human studies have reported significant weight loss, reduction in the fasting insulin levels, an improved lipid profile and inflammatory markers (these indicate the underlying inflammation in the body) such as C-reactive protein (CRP), tumour necrosis factor alpha (TNF-alpha), adiponectin, leptin and brain-derived neurotrophic factor. Also, mood improvements in terms of reductions in stress, anger, fatigue and enhancement of self-confidence were observed. A small proportion of individuals in these studies reported side effects such as irritability, lack of energy, hunger and cold.

Studies comparing this modified fasting regimen with that of a calorie restriction have produced conflicting results. Though some studies reported the advantages of the modified fasting regimen compared to the calorie restriction, others revealed no significant differences between the two. However, the combination of exercise and modified fasting has been shown to produce better results in terms of weight loss compared to an individual exercise or diet protocol. Despite the popularity and the health effects of this diet, adherence to this dietary regimen can be a problem due to the side effects though it is comparatively less compared to the alternate day fasting. Also, possible consumption of extra calories on non-fasting days makes sustenance difficult on a long-term basis.

Time-restricted eating (TRE)

Also called as time-restricted feeding, this fasting regimen consists of an everyday fixed eating window which runs to a period of 8-10 hours or less with fasting during the rest of the time. Ideally the fasting time is accompanied by zero calorie consumption. Water may be consumed for hydration though some prefer taking unsweetened no-calorie beverages. Some of the options for the TRE are the 16:8 or 14:10 methods.

The 16:8 method has a fasting duration of 16 hours followed by an 8-hour eating window. For example, if an individual has the first meal at 10 a.m., the eating window will be till 6 p.m. which will be the time of the last meal of the day. The fasting time will be from 6p.m till 10.a.m next day which will count to 16 hours.

The 14:10 method has a fasting duration of 14 hours followed by a 10-hour eating window. For example, if an individual has the first meal at 10 a.m., the eating window will be till 8 p.m. which will be the last meal of the day. The fasting time will be from 8 p.m. till 10 a.m. the next day which will count to 14 hours.

A number of animal studies and the newly emerging human studies have acknowledged the occurrence of weight loss with time-restricted eating. However, studies conducted in obese people so far had limited participants and had no standardized time window as the participants were permitted to choose their own timings. Due to the inconsistent results, the assessment of amount of weight loss with the TRE regimen could not be concluded. In a slightly larger study that was conducted for a duration of 12 weeks, though the achieved weight loss accounted to 5% of the body weight, there was a call for more long-term studies. Emerging evidence from the studies conducted on the overweight elderly population also produced encouraging results.

Though these studies produced promising results, an important determinant for weight loss and metabolic health was the time with regards to the carbohydrate and protein intake. It was observed that higher weight loss was achieved in those who consumed food earlier in the day compared to late eaters. Also, the consumption of a staple diet in the evenings was not beneficial for weight loss.

In a study conducted on the overweight/obese women it was observed that consuming a high calorie breakfast was favorable in terms of blood glucose and lipid levels compared to those who had a high calorie dinner. Similar findings were also seen in non-obese population. Hence a 16:8 regimen with an eating window between 6 a.m. to 2 p.m. was more advantageous health wise compared to a 11 a.m. to 7 p.m. window. Another study put forth that the eating window between 12 p.m. to 8 p.m. was not favorable for weight loss.

In comparison to the alternate day fasting and 5:2 regimen whose weight loss goes on par with the calorie restricted regimen, the weight loss with just the TRE regimen was not as much as it was seen with calorie restriction, 5:2 and alternate day fasting. Hence a combination of total calorie restriction and early eating in TRE proves to be beneficial for weight loss.

The relationship between blood cholesterol levels and TRE have been evaluated in many studies. While reports from some studies did not find significant differences in the levels of both LDL (bad) and HDL (good) cholesterol, other studies have reported reductions in the LDL cholesterol levels after 12 weeks of time restricted eating with a 10-hour eating window. Similar studies conducted on the triglyceride levels also have produced varying results. While some studies showed a significant reduction of triglyceride levels of more than 10% of the baseline value, others did not show any difference irrespective of the eating windows. One possible reason behind this finding could be the study participants having a normal triglyceride value.

The studies conducted on the effects of TRE on blood pressure have revealed reductions in both systolic and diastolic blood pressure irrespective of the eating windows. This effect was seen in individuals with higher baseline blood pressure values. While some studies opined that significant reductions in the blood pressure were seen in individuals who had lost more than 3% of the body weight, others put forth the observations wherein the reductions were seen without changes in weight.

The evidence accumulated from the studies conducted on the effect of TRE on blood glucose levels have revealed reductions in the fasting blood glucose levels with an earlier eating window which means starting the first meal at 8 a.m. and finishing the last meal by 4 or 5 p.m. These changes were observed in individuals with a higher-than-normal blood sugar levels as seen in pre-diabetes and diabetes. However, following a late eating window, for example having the first meal at 12 noon and the last at 8 p.m. was associated with increase in the fasting glucose levels in individuals with normal blood glucose levels.

On the other hand, studies conducted on the influence of TRE on other factors such as insulin resistance, fasting insulin levels and HbA1c have put forth the positive effects of TRE with early eating window in regulating these parameters. Hence intermittent fasting has been suggested as a potential non-medical management strategy for type 2 diabetes.

B2 regimen

Also called as the breakfast and lunch regimen, the feeding pattern consists of consuming two large meals per day comprising of the breakfast and lunch. The duration of fasting is for 14 hours. Generally, the breakfast is consumed between 6 a.m. and 10 a.m. and the lunch between 12 p.m. to 4 p.m. This is followed by skipping dinner and the fasting extends till next day morning. Apart from a good fasting interval, this regimen has the potential to be continued on a long-term basis.

Comparative studies between A6 (consists of three main meals which are breakfast, lunch and dinner with three small in between snacks) and B2 regimens conducted on individuals with type 2 diabetes have revealed that the B2 regimen was effective in terms of weight reduction, stabilizing the fasting blood glucose levels, reducing the fat content of the liver and improving the insulin action (sensitivity).

Whole day fasting

Also called weekly one day fasting, this method involves a fasting period of 24 hours on a once per weekly basis. The duration of fast is for 24 hours wherein either zero or low-calorie beverages are consumed. For example, if dinner is consumed on Monday night the next meal would be dinner on Tuesday night.

One of the advantages of this method of fasting is that on a fed day it is difficult to compensate for the zero or low-calorie intake on the fast day. It is practically tough to consume double the number of calories to make up for the low-calorie intake on the previous day though it is not an impossible task. On the negative side, maintaining consistency with this regimen might pose difficulties for some individuals as the side effects such as irritability, low energy and hunger might overtake. In order to overcome hunger, some may end up eating unhealthy calories when the fast is broken. The tendency towards increased intake of coffee to beat the hunger can lead to sleep difficulties and brain fog.

Studies conducted on obese individuals comparing the effects of one day per week to twice a week fasting have revealed that compared to once a week, the twice a week regimen produced better results with weight loss, waist circumference, blood sugar control, blood cholesterol, blood pressure and insulin sensitivity.

Eat stop eat

This concept of intermittent fasting was put forth by the author Brad Pilon in his popular book Eat stop Eat. He explains the fasting as taking a break from eating. This regimen involves two 24 hour fasts per week on any non-consecutive days and eating normally for the rest of the five days. This means that eating sensibly by not taking the diet to extreme levels such as binging or eating less. Though there is a tendency to marginally consume extra calories on non-fasting days, the calorie deficiency on fasting days evens it out. He emphasizes that combining this fasting regimen with strength training exercise is favorable for loosing the extra fat.

Some of the advantages of this fasting regimen are the flexibility it gives into choosing the fasting days as well as the eating times i.e., the last meal chosen could be either breakfast, lunch or dinner followed by a 24 hour fast. There is also no particular dietary restriction of what to eat on non-fasting days though healthy eating is recommended. This regimen also cuts down on the total calories consumed thus enhancing the weight loss and metabolic health. On the other hand, side effects such as irritability, hunger, lack of energy and long periods of abstinence from food might pose special challenges for some individuals.

It is a well-known fact that the body derives energy from the foods consumed as breakfast, lunch and dinner. During fasting the body uses the glucose that is stored in the body as a source of energy. When the glucose stores get exhausted, fat is used for energy and this burning of the fat is what aids in weight loss. This shift in the metabolic state is called as ketosis. When compared to a fasting duration of 12 hours, a 24-hour fast more likely produces a ketogenic state which aids in weight loss. Though this seems to be a reasonable explanation there is no convincing evidence behind this as there is variation in the ways in which individuals respond to fasting.

Intermittent very low-calorie diet (VLCD)

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Contrary to the term ‘intermittent’ incorporated in this dietary regimen, this is generally not considered as a part of intermittent fasting. This is because as the name suggests, the individual consumes a diet with a low calorie count even on fasting days. Unlike the other fasting regimens, the frequency ranges from one day per week to five days per week during which the calorie intake accounts to less than 800 calories per day

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The main aim of VLCD is to accomplish a rapid weight loss while preserving the muscle mass. Hence this is suitable for obese individuals with BMI of above 30 kg/m square in whom a low-calorie diet was ineffective in producing weight loss. In individuals with lower BMI, VLCD is likely to bring about loss of muscle mass.

Studies have shown that VLCD targeted towards obese individuals was effective in bringing about weight loss of approximately 20 kgs in 12 weeks. Although these observations were encouraging on a short-term basis, overall, the results were not good on long term as maintaining consistency with this diet was a drawback and also posed challenges with weight when the shift was done to conventional foods.

Based on the above observations, it is convincing that VLCD cuts down on the calorie count significantly despite its long-term efficiency being questionable. However, owing to its ability in reducing the food intake, studies have explored the application of intermittent VLCD for long term results. This means introducing VLCD for a period of time followed by switching over to a conventional diet and reintroducing VLCD again. But studies conducted on the effectiveness of VLCD over LCD (low calorie diet wherein the calorie intake ranges between 1000-1200calories per day), have revealed no significant differences in weight loss observed after a 50-week period.

Recent studies conducted on obese individuals with type 2 diabetes have revealed that a VLCD regimen was effective in bringing about blood sugar control, improving insulin sensitivity, cardiovascular health and quality of life. Comparative analysis of a 2 days /week and 4 days /week regimen showed the equal effectiveness of both the approaches. Though regarded as safe, VLCD should be carried out under medical supervision. Apart from the general side effects of fasting such as headaches, irritability, tiredness, extreme hunger, medical complications such as dehydration, electrolyte imbalances and gall stone formation are likely to occur.

Another type of VLCD is called very low-calorie ketogenic diet (VLCKD) has shown promising results with weight loss and long-term sustenance. This diet comprises of a total calorie intake of less than 700-800 k Cal/day, with low carbohydrate intake amounting to <30-50 g/day with a sufficient protein intake of 0.8-1.2 g/day/kg body weight. The diet is generally followed for a brief period of time with gradual transition to a low-calorie diet.

Some of the indications for VLCKD include obesity (BMI >30mg/m2), type 2 diabetes with obesity, polycystic ovary syndrome (PCOS), dyslipidaemia (imbalances in the HDL, LDL cholesterol and the triglyceride levels). Hypertension, asthma, epilepsy and obstructive sleep apnoea (breathing disorder in which there is partial or complete collapse of the airways causing sleep disturbances). Recent studies have highlighted the potential role of VLCKD in the management of food addiction and binge eating.

This dietary plan also requires strict medical supervision. A thorough assessment of the individual’s health before and during the dietary course is important in order to avoid nutritional deficiencies and unwanted side effects. However, this diet is not suitable for those with type 1 diabetes, kidney failure, cardiac issues, pregnant and lactating mothers, growing and old age.

Warrior diet

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Also called as 20:4 diet, this is a type of intermittent fasting wherein long hours of fasting or undereating are followed by short bouts of unlimited eating. This concept revolves round the dietary habits of warriors in the ancient times who ate less during the day and feasted at night. This diet was introduced by Ori Hofmekler, a former member of the Israeli Special Forces who was inclined towards fitness and diet and made his publication in the early part of 2000.

The hypothesis behind this diet is to enhance the survival instincts already present in an individual through strategies such as undereating, overeating, exercise, rest, wakefulness and sleep. It is believed that our bodies are already conditioned to these instincts. In instances such as stress or overeating, the adaptive capabilities of the body are significantly affected leading to overweight and obesity. However, there is no convincing evidence behind this.

The warrior diet consists of a dietary plan which is followed in three phases over a period of three weeks. The first phase or week one is the detox phase wherein the 20 hours fast consists of consuming foods like raw fruits and vegetables, vegetable juice and hard-boiled eggs. The eating window consists of consumption of cooked vegetables, small quantities of cheese, beans and salad. The second phase or week two eating window more or less follows the same pattern while foods such as salad, cooked vegetables, lean meat and nuts are consumed. The third phase or week three alternates between high carbohydrate and high protein low carb diet during the eating window. The second and the third phase targets the use of fats and carbohydrates by the body efficiently as energy sources. At the end of three weeks, the plan is started all over again.

While theoretically there is no restriction during the eating window, the general recommendation is to eat organic and natural foods avoiding the processed foods.

Though significant weight loss is a possibility with this diet, nutritionists believe that maintaining the weight loss might be difficult once the diet is switched over. Also, the eating window could promote unhealthy dietary habits such as overeating and binging. The two extremes of eating in this dietary pattern is also not a healthy approach to sustain the body.

There are claims that the warrior diet is as beneficial as intermittent fasting in reducing the body weight, stabilizing the blood sugar and reducing the inflammation. However, the scientific evidence behind these claims is lacking.

The gut factors

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It is a well-established fact that the gastro intestinal system is involved in the digestion and assimilation of food as well as the excretion of the waste products. Beyond the digestive functions, the role of the gut health in influencing the overall health and well-being of an individual has been extensively researched. The evidence from numerous studies have supported the existence of a connection between the gut and the other organ systems. So, considering the views that have labelled the gut as ‘second brain’ and ‘metabolic organ’ gives more clarity to its role in maintaining the health status of an individual.

The gut harbors trillions of microbes that live and communicate with the human body. The gut microbial ecosystem consisting of a collection of bacteria, fungus, virus and archaea are called gut microbiota. In line with the human evolution, the gut microbes have evolved side by side spanning over many thousand years establishing a cordial relationship with the humans. While in the initial phases of discovery, the bacteria were viewed as disease causing pathogens, continuing research has strongly supported their indispensable role in contributing to the health and well being of an individual.

There are many factors that influence the composition of the gut microbiota throughout the life span of an individual. These include the mode of birth, gestational age, type of feeding, genetics, lifestyle, diet, infections, inflammation, medications, exposure to environmental pollutants and extremes of weather, level of hygiene and aging. Hence the microbial composition is regarded as unique to each individual. However, the microbial colonization that happens in the early years determines its stability as well as the long-term health.

The contribution of the gut microbiota towards the production of metabolites, nutrients, chemical messengers and enhancing the gut as well as the overall immunity is critical for the homeostasis (body’s internal balance). Any disturbances in this ecosystem impacts the internal balance leading to the disease process. Hence Maintaining a healthy and diversified gut microbial ecosystem is crucial as it defines an individual’s health as well as the disease status. Though lifestyle and dietary interventions such as the intake of prebiotics and probiotics have been documented to improve the gut health, recent studies have explored the effects of intermittent fasting on the gut.

IF and gut bacteria

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Some of the positive effects of intermittent fasting on gut health are

Gut microbial balance

The preliminary reports about selected good gut bacteria capable of surviving on the mucus secretions and the shed intestinal cells in the absence of food have been established as early as 1915. However, recent lines of evidence have put forth the positive effects of intermittent fasting in terms of promoting the gut microbial balance. The modifications of the gut microbial composition are thought to be due to the potential of intermittent fasting to reset the gut microbial population. A healthy gut is colonized by the bacteria belonging to the phylum Firmicutes, Bacteroidetes, Actinobacteria, Proteobacteria, Fusobacteria and Verrucomicrobia. The bacteria namely Lactobacillus, enterococcus clostridium, Veillonella, Roseburia, Faecalibacterium and Ruminicoccus belong to Firmicutes. The Bacteroidetes phylum is dominated by Bacteroides and Prevotella. The main member of the Actinobacteria is the Bifidobacterium whereas Akkermansia muciniphila is an important member of Verrucomicrobia.

A growing number of animal and human studies that have explored the influence of intermittent fasting on the diversity of the gut microbes. The bacteria Akkermansia muciniphila has received considerable interest in recent years due to its health benefits. Considered as one of the dominant species of the gut microbial ecosystem, it resides in the mucus layer (inner lining) of the gut. It is also regarded as one of the key sources of propionate, a short chain fatty acid that has innumerable health benefits including its protection against gut inflammation, colon cancer, type 2 diabetes and metabolic syndrome.

Owing to the presence of a number of fasting regimes, few studies have explored the gut microbial alterations in the 16:8-time restricted feeding. The data obtained from studies conducted on obese individuals on calorie restriction have revealed increase in the population of Akkermansia muciniphila. Also, the observations from studies conducted on individuals who followed 16:8 intermittent fasting for a month have revealed significant increase in the number of Akkermansia muciniplila compared to non-fasted individuals. The increases in this bacterial population were more in young compared to the middle-aged individuals. Since this bacterium mainly depends on the mucin, a special type of protein present in the mucus layer, the bacteria can thrive even in the absence of food. Based on these findings, it has been proposed that individuals who are at increased risk for colonic cancer might benefit from intermittent fasting.

Studies analyzing the effect of time restricted feeding on bacteria producing the short chain fatty acid butyrate, have revealed increased population of Ruminococcus. Apart from being a major energy source to the colonic cells, butyrate also possesses anti-inflammatory, anti-obesity and anti-oxidant properties thus protecting against diseases such as type 2 diabetes and cardiovascular diseases. In addition to this, other studies have reported increases in Lactobacillus, Bifidobacterium and Prevotella with this regimen.

The Bacteroides are one of the important gut commensals that reside in the colon constituting a considerable part of the human gut microbial colonies. They not only guard the gut against harmful bacteria, but also provide the necessary nutrients to other gut microbes. In the absence of food, as in fasting, these bacteria have a unique ability to sustain themselves on substances called glycans present in the mucosal layer of the gut. These are nothing but the polysaccharides that are stored in the cells of the mucus layer. So, in the absence of dietary polysaccharides (complex carbohydrates) these bacteria easily switch over to the polysaccharides present in the mucus layer for sustenance. Due to its distinctive survival capabilities in the absence of food, time restricted feeding leads to predominance of Bacteroides over other microbes.

Recent studies conducted on adults with body weights ranging between average to overweight who were put on a 5:2 intermittent fasting regimen for three weeks have revealed overall increase in the population of beneficial bacteria with particular reference to the Bacteroides numbers. Also, increases in Ruminococcus numbers and enhanced production of short chain fatty acids were observed. The changes in the body weight, body mass index and improved parameters with reference to diabetes and heart disease observed in this study were attributed to the increase in beneficial bacteria and their metabolites.

A small number of studies have evaluated the effect of Buchinger fasting on the gut microbes have shown that this regimen bought about an increase in the population of Akkermansia, Bifidobacteria, Bacteroides and Proteobacteria thus stabilizing the gut. This is a century old therapeutic fasting regimen developed by Dr. Otto Buchinger. The total energy consumed per day is 250 calories with consumption of fat at 0.2 g/day, proteins at 1.8g/day, carbohydrate at 56.2g/day and fibre at 1.1g/day. The total fluid intake amounts to 2-3 liters/day. This is followed for a duration of five days or more. The concept behind this is to detox the body and when the normal eating patterns are established again the body goes through a process of rejuvenation.

Despite the fact that the composition of the gut microbial ecosystem varies with the type of fasting and the health status of the individual prior to fasting, it is worth noting the positive effects of intermittent fasting on the gut microbes which in turn influences the overall health. Also, the varied results in some studies needs more clarification.

Gut protection

The term colonization resistance refers to the ability of the normal gut microbes to prevent the invasion of harmful bacteria as well as the growth of the pathobionts (these are microbes that live in harmony inside the body but can cause diseases under certain conditions such as alterations in the immunity and gut microbial balance). This effect confers protection to the gut against infections.

On a realistic note, a diet that is lacks in fibre can have a negative impact on the intestinal mucosal layer leading to its thinning. As a result of this, the beneficial metabolites that are produced by the microbes from the dietary fibre are now sourced from the glycans (polysaccharides) present in the mucus. This mode of metabolite production together with the thinning of the mucosal (inner) lining of the intestine makes it vulnerable for infections caused by harmful microbes.

On the other hand, a diet that is deprived of fibre is not similar to a calorie restricted diet wherein there is an overall reduction in the amount of nutrients. In this scenario, the glycans present in the mucus is the source for sustaining the good bacteria while the mucus layer remains normal. This shows how the bacteria shifts its settings depending upon the dietary intake.

Preliminary data from a small number of studies have shown the favorable effect of a calorie restricted diet in terms of colonization resistance. In one of the recent studies wherein individuals were fed on a very low-calorie diet (800 Cal/day) for 8 weeks and then switched over to a low-calorie diet (1000-1200 Cal/day) for 4 weeks it was found that there was an enhanced protection against the bacterium Clostridioides difficile. Though it is regarded as a part of the normal intestinal microbial colony, its colonization is prevented by the presence of other bacteria. A weakened immune system, old age, antibiotic intake can lead to this bacterial colonization resulting in diarrhoeal illness.

Studies conducted on animal models wherein mice were fasted for 24 hours and infected with a bacterium called Salmonella typhimurium (a bacteria causing gastroenteritis) have revealed a reduction in the inflammation, intestinal damage and reduction in the bacterial numbers. It was also noted that there was an increase in the population of Akkermansia muciniphila.

Comparative analysis of individuals who were subjected to water only fasting and juice only fasting for seven days has revealed a balanced gut microbial composition in water only fasting. It was also observed that there was a reduction in the number of a bacterium called Fusobacterium which has been linked to colorectal cancer. These microbial changes were consistent even after returning to the normal diet. However no significant changes were noted in juice only fasting.

Though these studies have provided insights into the effects of fasting on harmful bacteria, more studies are warranted with regard to colonization resistance and its relation to specific diets and microbes.

Gut circadian rhythm

The term circadian rhythm has its origin from a Latin phrase “circa diem” which means “about a day”. It is referred as the 24-hour internal clock related to sleepiness and awakening, a cycle which is controlled by the brain in response to the changes in light surrounding our environment.

The fact that the functioning of the digestive system exhibits circadian rhythm has been known since the last five decades. Recent studies have put forth the hypothesis that the gut microbes also have circadian rhythm and show variations with respect to the time of the day. It is interesting to note that the gut bacteria exhibits these changes despite not directly coming into contact with the external environment. It has been observed that the gut bacteria not only show variations in different regions of the intestine, but also differences in the type of microbes residing depending on the time of the day. Hence it is important to keep the gut rhythm aligned as the beneficial bacteria are seen in higher numbers at specific times.

Emerging evidence has demonstrated the presence of higher number of bacteria in the mucous layer (inner lining) of the intestine during the night compared to the day time. It has been estimated that about 35% of the bacterial species present in the gut exhibit day-night variations. With regards to the type of bacteria, Firmicutes and Bacteroides show variations during the day and night whereas the Lactobacillus were relatively higher during the day compared to the night. The bacteria Roseburia and Ruminicoccus belonging to the Firmicutes have been regarded as dominant bacteria with regard to circadian changes. Some of the bacterial functions related to cell repair, growth and metabolism dominate in the dark compared to detox action and motility (movement) predominating in the light.

On of the important factors that influence the gut circadian rhythm is the 24-hour internal clock regulated by the brain. Disruptions to the biological clock can occur as a result of many factors like the level of nutrition, type of diet such as a high-fat and high-sucrose diet, feeding time, presence of obesity, type 2 diabetes, jet lag, shift work, artificial light at night and the surrounding environment. These disturbances lead to alterations in the gut microbial balance as well as the gut circadian rhythm which in turn promote metabolic disturbances and inflammation resulting in disease process.

Untimely consumption of food outside the normal eating window negatively impacts the circadian rhythm and the gut bacterial balance. There has been increasing number of studies in the recent years about the association between food timings, circadian rhythm and gut circadian microbes. The time of eating has been said to directly influence the gut circadian rhythm and this has been investigated in the context of intermittent fasting.

Evidence from the animal as well as the human studies have pointed to the positive effect of intermittent fasting in balancing the gut circadian rhythm. But human studies are scarce with this regard. However, one of the recent studies was conducted wherein 80 males were enrolled and 56 of them were put on a 16:8 intermittent fasting regimen for 25 days. The rest did not have any time restriction. It was observed that the fasting group demonstrated increase in the gut microbial diversity as well as the composition with an improved gut cyclical rhythm. An increase in the number of Bacteroides and Prevotella was observed. In addition to this there was increase in the level of HDL (good) cholesterol and reductions in LDL (bad) cholesterol and triglyceride levels which points to protection against obesity. The increase in the beneficial bacteria means enhanced production of bacterial metabolites such as short chain fatty acids and bile acids contributing to the gut and overall health.

Gut rest and rejuvenation

The process of digestion is vital for the body as the nutrients that are derived from the food, we consume are essential for the efficient functioning of the body. It is a complex and multistep phenomenon during which a considerable amount of energy is spent. The processing of food involves a combination of physical and chemical activities wherein the food is made suitable to be absorbed into the body for energy, growth and repair.

Gut barrier integrity is one of the key components of a healthy gut. The intestinal barrier is a layer that is made up of mucus layer, intestinal microbes and intestinal epithelial cells. The mucus layer is the first line of defence that protects the epithelial cells against harmful microbes and substances. The gut microbes attach themselves to the mucus layer by taking up extra space and offer protection to the intestinal epithelial cells. The specialized epithelial cells are tightly arranged so that the contents of the intestine do not escape. They are essential for digestion and absorption of nutrients as well as protection. Beneath the epithelium there is a loose connective tissue layer called lamina propria which carries the blood vessels as well as holds the epithelial cells in place.

One of the special features of the gut barrier is its semi-permeable nature allowing selective substances to pass through and restricting harmful microbes, toxins and food allergens.

The intestinal epithelial cells undergo wear and tear and get renewed and replaced every few days. For an efficient renewal the gut needs a break from its activities. Hence intermittent fasting provides the much-needed rest so that the gut rejuvenates itself to strengthen the barrier. This effect is of considerable significance in the presence of a leaky gut. This is a condition wherein injury to the intestinal epithelial lining causes increased intestinal permeability leading to the escape of the undigested food, bacteria and toxins into the blood stream. Any intestinal inflammation and infection, food allergies, alcohol abuse, chemotherapy and radiation therapy can damage the intestinal lining.

Data emerging from animal studies have revealed the positive effects of time restricted feeding in reducing the inflammation associated with colitis. Studies conducted on obese women who were put on very low-calorie diet for four weeks demonstrated improved gut barrier function. The reason behind this could be due to the effects of intermittent fasting in balancing the gut bacteria and increased production of short chain fatty acids. However, more studies are needed taking into account the different fasting regimens. But it is worth bearing that intermittent fasting could be one of the innovative ways to treat bowel inflammation and infections.

Summary

*Fasting refers to the practice of restricting or abstaining from food or drink for a specified period of time.

*The CRON-diet is the practice of reducing the calorie intake below the recommended value without compromising on the essential nutrients.

*Population based studies dating back to the world war times observed an increased life span and reduced death rates with the CRON-diet.

*Though scientific evidence supports the health beneficial effects of CRON-diet sustainability is a major drawback.

*The popularity of intermittent fasting is attributed not only to its health benefits, but also its flexibility and staying in line with the normal eating patterns.

*The 12-hour or 12:12 fasting is a regimen where the length of the eating and fasting window remains pretty much the same.

*The alternate day fasting or 4:3 regimen consists of fasting on alternate days with the duration of the fast running for 36 hours.

*On fed days there is no calorie restriction whereas on fasting days 25%of the calories are allowed.

*Though studies have supported health benefits, long term sustainability is a drawback.

*The fast diet or the 5:2 regimen consists of fasting for two non-consecutive days a week for a duration of 36 hours.

*Despite the available evidence claiming the metabolic and the overall health benefits, long term sustainability and the possibility of consumption of extra calories on fed days are drawbacks.

*The time restricted eating (TRE) has two methods. They are 16:8 and 14:10 fasting.

* The 16:8 fasting has an 8-hour eating window followed by a 16-hour fasting window whereas the 14:10 fasting has a 10-hour eating and a 14-hour fasting window.

*A number of studies have proposed the effect of time restricted eating in bringing about weight loss with higher weight loss observed in early compared to the late eaters.

*Studies have also put forth that consuming a high calorie breakfast compared to a high calorie dinner was more effective in stabilising the blood sugar and cholesterol levels.

*The studies conducted on TRE in relation to blood cholesterol, triglyceride, blood pressure and blood sugar levels have shown favourable results though conflicts still exist.

*The B2 or the breakfast and lunch regimen consist of consuming two large meals a day and skipping dinner.

*Studies conducted on individuals with type 2 diabetes have revealed the effectiveness of this regimen in reducing the weight, blood sugar levels and the fat content of the liver.

*The whole day fasting or weekly one day fasting involves fasting once a week basis for a duration of 24 hours.

*Though maintaining consistence is a bit challenging, it is difficult to compensate for the zero or low-calorie intake on the fasted day.

*The eat stop eat regimen involves fasting twice a week on non-consecutive days for 24 hours and eating normally for the rest of the days.

*The basic concept behind eat stop eat is taking a break and sensible eating on the fast and fed days.

*This regimen gives flexibility into choosing the eating and fasting times in addition to promoting weight loss and metabolic health.

*Intermittent very low-calorie diet (VLCD) consists of an intake of less than 800 calories per day ranging from one day to five days per week.

*VLCD is effective in bringing about weight loss while preserving the muscle mass and hence is suitable for individuals with a BMI of above 30kg/m square.

*Though VLCD is effective in reducing the calorie count, its consistency over long term is questionable.

*The term intermittent VLCD means alternating VLCD with a conventional diet.

*Evidence supports the effectiveness of VLCD in individuals with type 2 diabetes in reducing the blood sugar and improving and cardiovascular health.

*VLCD should be carried under medical supervision.

*Very low-calorie ketogenic diet (VLCKD) is another type of VLCD wherein the total calorie intake amounts to 700-800 K Cal /day with low carbohydrates and adequate proteins.

*Some of the indications for VLCD include obesity, type 2 diabetes, PCOS, hypertension, asthma, dyslipidaemia and epilepsy.

*This diet is not suitable for those with type 1 diabetes, kidney failure, cardiac issues, pregnant and lactating mothers, growing and old age.

*The warrior or the 20:4 diet involves fasting or undereating for long hours followed by short bouts of unlimited eating.

*The diet mimics the dietary habits of the ancient warriors who ate less during the day and feasted at night.

*The concept behind the warrior diet is to enhance the survival instincts to which the body is already conditioned.

*The dietary plan in warrior diet runs for three weeks in three phases with special attention to eating organic and natural foods.

*The health claims related to the warrior diet lack scientific evidence.

*The gut health influences the overall health and well-being of an individual.

*There are trillions of microbes residing in the gut which influence an individual in states of health and disease.

*The microbial colonization that happens early in life determines the long-term health.

*A diversified and healthy gut microbial ecosystem is crucial for maintaining homeostasis i.e., body’s internal balance.

*Recent lines of evidence point to the positive effects of intermittent fasting on gut health.

*Intermittent fasting has the potential to reset the gut microbial population.

*The bacteria Akkermansia muciniphila has received considerable interest in recent years due to its health benefits.

*Increases in this bacterial population have been observed in individuals put on 16:8-time restricted feeding.

*Studies also have reported increases in butyrate producing bacteria Ruminococcus, Lactobacillus, Bifidobacterium and Prevotella with this regimen.

*The Bacteroides are one of the important gut commensals that can thrive in the absence of food.

*Increases in a number of beneficial bacteria particularly Bacteroides have been observed in overweight individuals put on 5:2 fasting regimen.

*The Buchinger fasting regimen is a concept based on detox with a low-calorie diet followed by rejuvenation with normal eating patterns.

*Increase in the population of Akkermansia, Bifidobacteria, Bacteroides and Proteobacteria have been observed with Buchinger fasting.

*Colonization resistance is the ability of the normal gut microbes to prevent the invasion of harmful microbes as well as the pathobionts.

*Preliminary data from a small number of studies have put forth colonization resistance against the bacteria Clostridioides difficile and Salmonella typhimurium.

*Reductions in the number of Fusobacterium which has been linked to colorectal cancer has been observed with water only fasting.

*The gut microbes exhibit circadian rhythm showing variations with respect to the time of the day.

*It is estimated that about 35% of the gut microbial colonies show a day-night variation.

*The bacteria Firmicutes and Bacteroides dominate the picture with regards to the circadian rhythm.

*The 24-hour internal clock aligns with the gut circadian rhythm.

*Disruptions in the internal clock affects the gut circadian rhythm and the microbial balance affecting the health and well-being.

*The available evidence from the animal and human studies have pointed to the positive effect of intermittent fasting in balancing the cyclical rhythm of the gut.

*The integrity of the gut barrier is vital for the intestinal health.

*The intestinal epithelial cells get renewed every few days.

*Intermittent fasting gives the gut a break from its activities thus promoting efficient rejuvenation.

*Available evidence from the animal and human studies have pointed to the positive effects of intermittent fasting in reducing gut inflammation.

*Intermittent fasting could be a innovative approach to managing intestinal inflammation though more evidence is warranted.