It is now a well established fact. Various chronic pathologies are associated with poor nutrition, especially through force ultra-processed products, or to a dysfunction of the microorganism community installed in our digestive tract - that is to say our intestinal microbiota: theobesity, Type 2 diabetes, cardiovascular illnesses, c, psychic diseases, digestive diseases, etc. However, these associations, observed by scientists from different but complementary disciplines, naturally lead to questioning the links that have been forged, during evolution, between abiotic (the characteristics of our food) and biotic ( microorganisms) of the human intestinal ecosystem.
Sf our diet has an impact on our health, it is undoubtedly because it affects the diversity and the activities of the intestinal microbiota, which we are going to look at in this article. And we can only emphasize the interest of a diet not only varied, but also rich in fruits, vegetables and other plant products whose nutritional content must be as close as possible to what it is before they are picked.
What are the nutrients from plants? Their wall forms a complex network comprising mainly polymers carbohydrates (up to 90% cellulose, hemicelluloses, pectins), but also non-carbohydrate polymers (lignin, proteins, lipids), minerals and various substances which vary according to the species. And it is in the former that a large part of the energy resulting from photosynthesis is concentrated.
The cohesion between the different constituents of the wall is ensured by chemical bonds, the complexity of which is not yet fully understood. We know, however, that it is constructed by a tangle of microfibrils of cellulose, which are inserted into a loose matrix ofhemicellulose and pectin. But the wall is also encrusted with lignin : this increases its rigidity and gives it great mechanical and chemical resistance. Finally, depending on the species, one can observe a mineralization (silica, limestone) or a gelation (gums, mucilages) of the walls, by accumulation of encrustation and encrustation substances (waxes, suberins, cutin, sporopollenin, etc.) .
The plant matrix, food for the microbiota
Cellulose is a homopolymer linear consisting of D-glucose units. The degree of polymerization is between 250 and 15, and the parallel assembly of 000 to 16 cellulose chains constitutes a microfibril. Each microfibril has highly arranged crystallized zones, and other so-called amorphous regions where the chains are poorly ordered. As for the fibers, each is an arrangement of macrofibrils themselves composed by the association of microfibrils.
Hemicellulose, on the other hand, is a heteropolymer. It binds to cellulose microfibrils, in which it can be trapped, by hydrogen bonds. Its structure is characterized by a linear main chain of glucose, mannose and / or xylose. Most of the main chains are branched and contain pentoses, hexoses and uronic acids. According to their primary structure, they are classified into four groups: xyloglycans (xylans and arabinoxylans), mannoglycans (glucomannans and galactomannans), β-glucans, and xyloglucans, the proportion of which varies according to the plant species and the tissue. organic.
Pectin, another essential compound of the plant wall, is rich in galacturonic acid (70% of the molecule). Up to 17 different monosaccharides can be found there, with more than 20 types of bonds. And it can also be strongly esterified by methyl and acetyl groups: one thus distinguishes highly methylated pectins (MD> 50%), and others weakly methylated (MD <5%). Finally, pectic polymers are classified into three groups, namely homogalacturonans (HG), rhamnogalacturonans-I (RG-I) and rhamnogalacturonans-II (RG-II) - although other galacturonans can more rarely be found. substitute therein (xylogalacturonans and apiogalacturonans).
More treatments, less fiber
Technological treatments - for example, refining cereals or pressing fruits - reduce the fiber content of plant products. And in ultra-processed foods, which often consist of recombinations purified ingredients and additives, they are considerably reduced.
In fact, it is estimated that each French person ingests around 20 g of fiber per day, when it is recommended to consume about 30 g / day. And on this subject, it should be noted that taken in the form of food supplements, fibers do not have the structural complexity and therefore the richness of natural products. In practice, the contributions are insufficient with regard to the needs of the intestinal microbiota of each one. However, this microbiota performs a number of essential functions ...
In humans as in animals, this microbiota is characterized by its extreme specific diversity (several hundred species belonging to Archaea et B, but also bacteria viruses or bacteriophages). It is also responsible for essential functions that only microorganisms can perform: digestion of the main constituents of plants, synthesis of vitamins, but also production of various metabolites such as butyrate, which is a source of energy for the cells of the intestinal epithelium and whose protective role against colon cancer is well established.
Assembly line work to digest fibers
The intestinal microbiota is not distributed evenly throughout the digestive tract. Its concentration and diversity are indeed maximum (10 billion cells per ml of content) in the colon, and it is there that the plant fibers provided by fruits and vegetables are digested. In the small intestine, the microbial community is much less abundant, and is mainly involved in the development and stimulation of our immune system.
Let us insist on the fact that the digestion of vegetable fibers is only ensured by microorganisms. Because except for them, all living beings including man are devoid of enzymes ensuring the degradation of the various polymers that constitute them - namely cellulose, hemicellulose and pectins. The process is complex. It involves several microbial groups, organized in a trophic chain to counteract the great specialization of microorganisms. And this chain ensures the circulation of matter (therefore biochemical energy) between the various constituents of the ecosystem, ensuring its cohesion and stability. It takes place in several stages.
In the first, hydrolytic (fibrolytic) microorganisms degrade the polyholosides of the plant walls and release simple fermentable molecules: in particular numerous osidic fragments. With the help of other bacteria constituting the second link in the trophic chain, these hydrolytic species then use the soluble compounds as energy sources for fermentation. The processes involved then generate short chain fatty acids (SCFA), intermediate metabolites (lactic, succinic, formic acids) and gases (H2 and CO2).
If the former are sources of energy known in humans, the latter are quickly consumed by bacteria. As for hydrogen, it is used by hydrogenotrophic microorganisms which represent the third link in the chain. Finally, depending on the individual, we find Archaea that reduce CO2 in methane, bacteria that form acetate, and others that produce hydrogen sulfide.
Why is microbial diversity important?
The answer lies above all in the multitude of functions that the microbiota must perform to degrade and ferment food. But we must also keep in mind the other roles that it must play, roles equally essential for our nutrition as for our health: stimulation of the immune system, production of vitamins, production of metabolites essential to our cardiovascular system and our health. brain, etc. Ultimately, the different microbial communities are very specialized in their functions.
Within each functional group, species occupy ecological niches that vary according to several parameters. First, as a function of physiological characteristics: affinity for the substrate, energy yield, maintenance energy, capacity of adhesion to the substrate, resistance to acidity, response to substrate concentration gradients, etc. Then, depending on the nature of the many interactions that the species have woven between themselves: competition, synergy, hydrogen transfer, nutritional complementarity, etc.
The complexity of plant structures, their very great heterogeneity, their properties, the diversity of chemical bonds within polymers as well as between them, or that of the compounds of plant tissues require from microorganisms a wide variety of strategies and strategies. perfectly adapted mechanisms: adhesion to fibers, embrittlement and destructuring of tissues, multiplicity and complementarity of hydrolytic enzymes.
These mechanisms, which differ according to the species, are complementary. And they are based on a spatial, functional and metabolic organization of microbial species, with innumerable ecological niches each corresponding to a specific microbial biotype.
What is the impact of ultra-processed foods?
By its nature, its presentation, its quantities, the frequency with which it is ingested, a food necessarily has an impact on the balance of the intestinal microbiota. In fact, it depends on the supply of nutrients and energy for the community of microorganisms. And by being degraded and fermented, it acts on the physicochemical parameters of the environment, which in turn condition the balance of the populations of microbes.
From this point of view, a food ultra-transformed does not have the same properties as a natural product. While it can have an equivalent macronutrient composition, it often lacks the "matrix" effect. In addition, the interactions between its compounds and its fiber content are not the same, it does not offer such a large number of ecological niches. The circulation of energy in the ecosystem will be altered. And therefore, the nature and the concentration of the metabolites formed by the bacteria will be different (with for example less AGCC). Finally, like the sweeteners and emulsifiers, some additives can disrupt the microbial balance.
We know that the microbiota of great apes (gorillas, bonobos, chimpanzees) is much more varied than that of humans. It is also rich in Fibrobacter, a bacterial genus strongly involved in the degradation of plant polyholosides. While the human microbiota can contain a lot of Bacteroides, bacteria involved in the digestion of proteins and fats. These differences are explained by changes in diet, with great apes eating mostly natural plant foods. As for the decline in diversity, it is more marked in highly industrialized countries, especially in the United States, where the share of ultra-processed products in the diet is greater than in Europe or Asia.
Generally speaking, during evolution, humans have reduced their ability to digest plant compounds. However, the resulting loss of diversity in the intestinal microbiota reduces its capacity for resilience in the face of food or environmental disturbances, which makes it more vulnerable.
As we have just mentioned in this article, there is therefore a close link between the characteristics of our food and our gut microbiota. Through the metabolites that it produces by breaking down what we eat, these people of microbes act directly on our nutrition. But its functioning depends on the nature and properties of the food, and in particular on their fibrous matrix.
Such an observation, which is a consensus among microbiologists, gives weight to the discourse of nutritionists: the latter recommend favoring minimally processed and varied plant products (3V rule), so as to provide a quantity of different fibers to the microbiota, in a preserved matrix form. But naturally, other parameters can alter the functioning of the microbiota: antibiotic therapy, stress, etc. And it is therefore necessary to avoid that their effects add up or, worse, act in synergy. A holistic view of food issues (“matrix” effect of food, production and consumption patterns), hygiene of life and health is more than ever essential.
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