The microbiota: an organ of its own

The intestinal microbiota is a true “organ” with immense adaptive potential nested in our digestive tract (Table 1). The germs that compose it, their functions, and the relationships they maintain with the components of our body vary according to their environment. The intestinal microbiota is an essential interface between the food ingested and our intestinal barrier. Depending on the state of this interface, digestion can have a high yield or, on the contrary, trigger local acute and sometimes intense, or chronic, reactions, resulting in poor assimilation of nutrients.

A proven link between altered microbiota, overweight and pathologies

Accumulating medical evidence from research suggests that alterations in the gut microbiota are associated with weight gain, obesity, and diabetes (1). Conversely, it has been shown that healthy eating behaviour can improve the diversity and quality of the microbiota (2), reverse toxic responses, especially inflammatory ones, and reduce weight (Table 2). This has been brilliantly demonstrated by studies with transfer of “healthy” microbiota into the digestive tract of obese subjects (3).

Improving the richness and diversity of the microbiota would have several advantages: better assimilation of nutrients (3), reduction in low-grade inflammation, increased sensitivity to insulin (4), leptin, etc. The right diversity of gut microbes can therefore reduce these disorders.

Fasting & microbiota: where is the research?

Fasting is an unconventional, albeit incredibly old, dietary intervention. Serious studies combining the effects of fasting and the functions of the microbiota are still rare, are published in ultra-specialized journals too rarely read by clinical physicians despite certain recognized, documented, and promising physio pathological results. However, one should remain cautious about the clinical applications which result from this research. The findings of physiology, experience and studies point in this direction but there is not yet enough data confirming the degree of importance of the role of the microbiota in the beneficial effects of fasting.

What is the impact of fasting on the microbiota?

The microbiota needs food, and good food, to keep all its protective faculties! Removing all or part of the usual nutrients should therefore have an impact on the composition, quality, and functions of the microbiota. We have an interesting study model: bariatric surgery. Strictly speaking, this is not fasting, because surgery patients continue to eat. But the “dam” effects of this surgery very significantly reduce the volume of the bolus. Dietary restriction imposed by surgery has been shown to cause weight loss. What surprised the medical profession was the discovery that this imposed dietary restriction was responsible for major hormonal and metabolic repercussions (5-6). The postoperative fasting and the reduction in food bolus associated with the surgery (of the by-pass type) are responsible for changes in the microbiota in terms of diversity, quantity, and quality. Reversals of proportions between the different germs (improvement of the Firmicutes / Bacteroidetes ratio) were noted, associated with an improvement in glucose metabolism. Some bacterial groups, such as Feacalibacterium, abnormally represented in inflammatory phenomena and insulin resistance, disappeared if type II diabetes was cured before surgery.

Is this an “energy” crisis for the microbiota?

During a water fast, the food bolus disappears; the digestive tract continues to function thanks to the persistence of peristaltic movements and to persistent endo-digestive secretions (mainly water). Fasting could represent an “energy crisis” for microorganisms, due to the unavailability of nutrients from external sources. What is described is in fact the opposite effect. The improvement is almost constant, both physically and psychologically. At the end of any fast, weight loss is normal, chronic pain disappears, blood pressure is normalized, biological results improve with a decrease in blood sugar levels, insulin resistance, and improvement of inflammatory markers, among others. Surveys studying quality of life show significant positive effects (8). These positive experiences, for which fasting medical centres are becoming famous, suggest that the actions of the microbiota are either preserved or replaced.

Endo-nutrition: a natural selection and a source of energy

A person that fasts uses energy substrates (glucose, proteins and especially lipids) mobilized from body tissues. These changes in food metabolism make it possible to switch to “endo-nutrition” mode by “bypassing” normal digestion. This substitution method, by resting the digestive tract, would generally increase the local protective effects of the microbiota and, of butyrate (one of the protective short chains fatty acids, produced by the microbiota itself from digested fibres that provides fuel for the cells of our gut lining). This resting remains incomplete. The persistence of digestive water secretions (several litters per 24 hours, reabsorbed by the colon), the automatic motility of the intestine thanks to the migrating motor complex (continuous drainage) and the desquamation of the digested intestinal cells in the digestive lumen provides a minimum of conditions that could explain the continued functioning of the remaining microbiota.

A strengthened microbiota

A normal and healthy intestinal microbiota should generate conditions that prevent colonization by pathogenic microorganisms. This is called resistance to colonization. This resistance can be transiently attenuated in the event of disruption of the microbiota (we speak of dysbiosis, due to a reduction in the quantity and diversity of the microorganisms that compose it in favour of pathogenic germs). Pathogens can then have the ability to reach high levels. Can fasting reduce this resistance to exogenous infections of the microbiota? Studies show that the calorie restriction observed in animals does not necessarily lead to a loss of gut microbial richness and diversity. The renewal of microbes adapts to specific fasting or refeeding situations. The microbiota, when fasting, does not disappear. While being less represented in terms of volume, it becomes stronger, rid of the germs originally responsible for possible dysbiosis. The protective effect therefore persists.

Long-lasting protection?

In humans, there are few studies comparing changes in the microbiota after fasting. In 2019, Robin Mesnage (9) did a great job studying the evolution of the microbiota in 16 men before and after 10 days of fasting, then after 3 months. In this study Bacteroides (40.7%) became dominant after the fasting period, due to a sharp decrease in the relative abundance of Firmicutes (39.9%), known to degrade dietary plant polysaccharides (same results obtained during the study of the intestinal microbiota after bariatric surgery). Three months after the resumption of food, the microbiota of the subjects had returned to a basal level compared to the initial value established before the fast. In the same study, the concentrations of the main short-chain fatty acids (acetate, propionate, butyrate) were not modified by fasting. The full amount is not described, but due to the volume of faeces being logically reduced, we can extrapolate that the volume of butyrate can be reduced during a long fast. We must insist on the extrapolation of this concept, as the mystery of the evolution of endo-digestive metabolites is still unclear and very partially studied. At most, it has been shown in animals that the microbial density is greatly reduced during fasting (animal in hibernation) and that the cells of the intestinal wall tend to atrophy, with less villi. In contrast, fatty acid levels significantly increased 3 months after fasting, compared to pre-intervention levels in Robin Mesnage’s study, suggesting an optimized degree of protection in post-fasting. The lack of loss of diversity is also a positive point. Microbial diversity is a factor in the good health of the intestinal microbiota. This decrease has been described in obesity, diabetes (type II) and inflammatory bowel disease. This saved or improved diversity during a fast is another sign of a step towards good health.

Fasting limits the aggression of the microbiota from toxic substances

Another argument could explain the clinical benefits of fasting. The disappearance of all food intake eliminates any risk of absorption of toxic foods, aggressive substances, and other endocrine disruptors. The “meals” of bad bacteria would then also be greatly reduced. The good bacteria, more adaptable, would find sufficient space for a satisfactory microbial homeostasis despite the decrease in nutrients of external origin. The local toxic microbial aggression would cease inflammatory attacks from each meal thus allowing possible repairs to the intestinal barrier.

The microbiota damaged by stress

Fasting is most often practiced in a protected environment, away from the usual stressful phenomena of daily life. A growing body of evidence links stress to dysbiosis, and vice versa. The microbiota is still an underestimated mediator of stress responses and associated sequelae. Conversely, the reduction in negative stressful situations would allow a favourable local impact on the microbiota, with the possibility of amplifying the production of positive neurotransmitting metabolites.

It is important to emphasize that the practice of fasting in a supervised and medicalised  institution, is accompanied by other activities, such as the practice of adapted physical activities, dietary monitoring, and psychological support. These complementary activities have strong positive impacts on the microbiota (10) and must be continued after fasting.

Fasting a positive stress on our microbiota

Explaining clearly and precisely the effects of fasting, both on our body and on our microbiota, is a challenge. Far from representing an aggression without answers, our body responds very positively to fasting situations. It is certain that the positive effects described in this article have their limits. However, it is essential to understand why a fasting state has such positive clinical, biological, and psychological repercussions. The many functions of our microbiota in these restrictive situations have not yet been clearly and sufficiently defined and attributed. We know how to fast (even though our recent food culture has “spared us”) thanks to our genes, which for millions of years (well before Homo Sapiens) have allowed us to periodically go without food.

As we fast, far from starving our billions of gut germs, we find that they organize, adapt, and strengthen to enable us to endure food restrictions.

This strengthening is surprisingly beneficial for the whole body and supports the effects of fasting as a therapeutic tool. There is still a long way to go to convince people to change their eating habits, especially by suggesting regular periods of fasting.

Yet it is good for our bodies, and good for the planet as well.

Docteur Philippe Guérin

 “Our bodies are our gardens – our wills are our gardeners.” 

William Shakespeare


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