What role does the microbiota play in digestion?

At each meal, our food is broken down so that we get energy and nutrients. At the heart of this transformation: our intestinal flora.

Lunchtime has come… But also the beginning of a chain reaction involving a host of organs. Mouth, esophagus, stomach, small intestine, colon… Wherever food goes, it will be crushed, degraded, fragmented by a series of mechanical and chemical phenomena. Objective: to extract nutrients and energy necessary for the proper functioning of the body.

Its enzymes facilitate the assimilation of nutrients

In this long journey, which lasts from one to two days, unusual auxiliaries play a central role: bacteria that colonize our digestive tract. Without this microbiota, the digestion would be much less effective. First, it has specific enzymes that allow the fermentation of indigestible residues and facilitate the assimilation of nutrients. “The number of microbiota enzymes capable of degrading complex sugars is estimated at 56,000, while the human genome only has 17 genes coding for this action”, summarizes Philippe Langella, research director at Inrae (the Food and Environmental Research Institute). And their action does not stop there: bacteria participate in the synthesis of essential fatty acids and strategic vitamins, such as B12, essential for the formation of red blood cells. “They also release substances that will have beneficial effects on several metabolic pathways such as blood sugar regulation or inflammation,” says Dr. William Berrebi, gastroenterologist. The degradation of food by these micro-organisms has a counterpart: the production of gas, at the origin of flatulence. Not expensive paid to benefit from the benefits of these friendly bacteria.

Two kilos of bacteria that wish us well

Do not be fooled by their size, of the order of a few micrometers: bacteria in our digestive system are so numerous that they weigh a total of between 1.5 and 2 kilos. And it is not the only microbiota that colonizes the human body: populations of microbes nest everywhere (nostrils, skin, vagina…), with a predilection for wetlands. But their weight does not exceed a few tens or hundreds of grams. This flora is beneficial because, by occupying the land, it prevents possible pathogenic microbes from settling. The gut microbiota is by far the most important, and its function seems more complex. If it forms an organ in its own right without which certain foods would not be digested, it also plays a role in immunity and regulating our mood.

The stomach: 10 to 1,000 bacteria/ml

It is an environment that is not conducive to the microbiota: its mucous membrane produces digestive juices so acid that they would eat away at the stone. This destroys pathogens (bacteria, viruses carried with food). Only the good bacteria resist.

The small intestine: 10,000 to 10 million bacteria/ml

Here, oxygen becomes scarce and the environment becomes less acidic, providing favorable conditions for the accommodation of micro-organisms. The bacteria are located between the lumen (ie the interior of the organ) and the mucus which protects its wall.

The large intestine: 10 billion to 10,000 billion bacteria/ml

It is the great reservoir of the microbiota. The conditions there are optimal for these so-called anaerobic bacteria: constant temperature, low acid environment devoid of oxygen, slow transit… which provides abundant food conducive to bacterial development.