Defense systems of the gastrointestinal tract

The theory of adequate nutrition attaches great importance to the body's defense systems against the penetration of various harmful substances into its internal environment. The entry of nutrients into the gastrointestinal tract should be considered not only as a way to replenish energy and plastic materials, but also as an allergic and toxic aggression. Indeed, nutrition is associated with the danger of penetration of various antigens and toxic substances into the internal environment of the body. Only thanks to a complex defense system are the negative aspects of nutrition effectively neutralized.

First of all, it is necessary to note the system, which is still designated as mechanical or passive. This implies limited permeability of the mucous membrane of the gastrointestinal tract for water-soluble molecules with a relatively small molecular weight (less than 300-500) and impermeability for polymers, which include proteins, mucopolysaccharides and other substances with antigenic properties. However, for the cells of the digestive apparatus during the period of postnatal development, endocytosis is characteristic, facilitating the entry of macromolecules and foreign antigens into the internal environment of the body. There is evidence that the cells of the gastrointestinal tract of adult organisms are also capable of absorbing large molecules, including undigested ones. Such processes are designated by Mr. Volkheimer as persorption. In addition, when food passes through the gastrointestinal tract, a significant amount of volatile fatty acids is formed, some of which cause a toxic effect when absorbed, while others cause a local irritant effect. As for xenobiotics, their formation and absorption in the gastrointestinal tract vary depending on the composition of the properties and contamination of food.

There are several other mechanisms preventing the entry of toxic substances and antigens from the enteral environment into the internal environment, two of which are transformational. One of these mechanisms is associated with the glycocalyx, which is impermeable to many large molecules. The exception is molecules that are hydrolyzed by enzymes (pancreatic amylase, lipase, proteases) adsorbed in the glycocalyx structures. In this regard, contact of unsplit molecules causing allergic and toxic reactions with the cell membrane is difficult, and molecules that are hydrolyzed lose their antigenic and toxic properties.

Another transformation mechanism is determined by enzyme systems localized on the apical membrane of intestinal cells and performing the splitting of oligomers into monomers capable of absorption. Thus, the enzyme systems of the glycocalyx and lipoprotein membrane serve as a barrier preventing the entry and contact of large molecules with the membrane of intestinal cells. Intracellular peptidases, which we have considered as an additional barrier and as a mechanism of protection from physiologically active compounds, can play a significant role.

To understand the mechanisms of protection, it is important to note that the mucous membrane of the small intestine of humans contains more than 400,000 plasma cells per 1 mm. In addition, about 1 million lymphocytes per 1 cm2 of intestinal mucous membrane have been identified ^. Normally, the jejunum contains from 6 to 40 lymphocytes per 100 epithelial cells. This means that in the small intestine, in addition to the epithelial layer separating the enteral and internal environments of the body, there is also a powerful leukocyte layer.

The intestinal immune system is part of the body's immune system and consists of several different compartments. Lymphocytes in these compartments have many similarities with lymphocytes of non-intestinal origin, but also have unique features. At the same time, populations of different lymphocytes in the small intestine interact through migration of lymphocytes from one compartment to another.

The lymphatic tissue of the small intestine makes up about 25% of the entire intestinal mucosa. It is represented in the form of clusters in Peyer's patches and in the lamina propria (individual lymph nodes), as well as a population of scattered lymphocytes localized in the epithelium and in the lamina propria. The mucous membrane of the small intestine contains macrophages, T-, B- and M-lymphocytes, intraepithelial lymphocytes, target cells, etc.

Immune mechanisms can act in the small intestine cavity, on its surface and in the lamina propria. At the same time, intestinal lymphocytes can spread to other tissues and organs, including the mammary glands, female genital organs, bronchial lymphatic tissue, and participate in their immunity. Damage to the mechanisms that control the body's immunity and the immune sensitivity of the small intestine to antigens can be important in the pathogenesis of disorders of local intestinal immunity and in the development of allergic reactions.

Non-immune and immune defense mechanisms of the small intestine protect it from foreign antigens.

Although the mucous membrane of the digestive tract potentially serves as an area through which antigens and toxic substances can penetrate into the internal environment of the body, there is also an effective duplicated defense system that includes both mechanical (passive) and active defense factors. In this case, the systems that produce antibodies and the systems of cellular immunity interact in the intestine. It should be added that the protective functions of the liver barrier, which implements the absorption of toxic substances with the help of Kupffer cells, are supplemented by a system of antitoxic reactions in the epithelium of the small intestine.

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Conclusions

The discovery of general laws of assimilation of food substances, equally valid for the most primitive and for the most highly developed organisms, inevitably led to the formation of a new evolutionarily reasoned theory, suitable for interpreting the assimilation processes not only of man, but also of other groups of organisms. The theory of adequate nutrition proposed by us is not a modification of the classical one, but represents a new theory with different axiomatics. At the same time, one of the main postulates of the classical theory, according to which the intake and expenditure of food substances in the body must be balanced, is fully accepted by the new theory.

According to the theory of balanced nutrition, food, which is a complex structure and consists of nutrients, ballast substances and, in some cases, toxic products, undergoes mechanical, physicochemical and, in particular, enzymatic processing. As a result, the useful components of food are extracted and converted into compounds devoid of species specificity, which are absorbed in the small intestine and provide the body with energy and plastic needs. (Many physiologists and biochemists compare this process with the extraction of valuable components from ore.) From ballast substances, some elements of digestive juices, exfoliated cells of the epithelial layer of the gastrointestinal tract, as well as a number of waste products of bacterial flora, partially utilizing nutrients and ballast, secretions are formed that are thrown out of the body. From this scheme of food assimilation follow the principles of calculating the amount of useful substances entering the body with food, assessing its merits, etc.

According to the theory, adequate nutrition, as well as the transition from a hungry state to a satiated state, are determined not only by nutrients, but also by various vital regulatory compounds entering the internal environment of the body from the intestine. Such regulatory compounds primarily include hormones produced by numerous endocrine cells of the gastrointestinal tract, which in number and diversity exceed the entire endocrine system of the body. Regulatory compounds also include hormone-like factors such as food derivatives formed as a result of the action of enzymes of the digestive apparatus of the macroorganism and bacterial flora. In some cases, it is not possible to draw a clear line between regulatory and toxic substances, an example of which is histamine.

From the point of view of the classical theory of nutrition, the microflora of the digestive apparatus of monogastric organisms, including humans (but not ruminants), is not even a neutral, but rather a harmful attribute. From the standpoint of the theory of adequate nutrition, the bacterial flora of the gastrointestinal tract not only in ruminants, but also, apparently, in all or the vast majority of multicellular organisms is a necessary participant in the assimilation of food. It has now been established that during the feeding activity of numerous organisms, not only does the extraction of some useful part of the food - primary nutrients - occur in the digestive system, but also the transformation of various food components under the influence of microflora, as well as enrichment with the products of its vital activity. As a result, the unused part of the nutrients is converted into an active part of the enteral environment, possessing a number of important properties.

For complex organisms, it is fair to consider that in the metabolic sense they are supraorganismic systems in which the host interacts with a certain microflora. Under the influence of microflora, secondary nutrients are formed, which are extremely important, and in many cases necessary. The source of secondary nutrients are ballast food substances, which participate in the regulation of many local functions of the body.

Assimilation of food, according to the classical theory of nutrition, is reduced to enzymatic hydrolysis of its complex organic structures and extraction of simple elements - nutrients proper. From this follows a number of fundamental ideas about the advisability of food enrichment, that is, about separating components containing nutrients from ballast, as well as about using ready-made nutrients as food products - final products of splitting, suitable for absorption or even introduction into the blood, etc. In contrast, according to the theory of adequate nutrition, not only the splitting of food takes place, but also the preparation of nutrients and physiologically active substances as a result of the action of the microflora of the gastrointestinal tract, in particular on ballast substances. In this way, many vitamins, volatile fatty acids and essential amino acids are formed, which significantly affects the body's needs for food products coming from outside. The ratio between primary and secondary nutrients can vary widely depending on the species and even individual characteristics of the microflora. In addition, along with secondary nutrients, toxic substances, in particular toxic amines, are formed under the influence of bacterial flora. The activity of bacterial flora, which is an obligatory component of multicellular organisms, is closely related to a number of important characteristics of the macroorganism.

As has been noted many times, the development of the theory of adequate nutrition is based on general biological and evolutionary patterns, as well as on the achievements of a number of sciences, especially biology, chemistry, physics and medicine. Indeed, for a biologist, not only the "formula" is extremely important, but also the technology of any process, since evolution is moving in the direction of optimizing the technology of biological processes. In biological systems, a great deal depends on the technology of processes, since their high efficiency, and sometimes the very possibility, are associated with the implementation of certain intermediate links. Insufficient efficiency of their implementation or their interaction disrupts the functioning of the system as a whole. This idea explains some fundamental differences between the theories of balanced and adequate nutrition. The first theory is essentially determined by the balanced formula of nutrition, the second, in addition to such a formula, also takes into account the technology of nutrition, that is, the technology of the processes of assimilation of food by various groups of organisms.

Finally, the theory of adequate nutrition is one of the central elements of the interdisciplinary science of trophology. The unification of many sections of biological and medical sciences concerning various aspects of food assimilation by biological systems of varying complexity (from cells and organisms to ecosystems and the biosphere) into one science is necessary for understanding the fundamental unity of nature. This is also important for characterizing the processes of interaction in the biosphere based on trophic links, that is, for considering the biosphere as a trophosphere. But to no lesser, and perhaps even greater extent, the formation of trophology, including the theory of adequate nutrition, is essential for various medical sciences, since tissue trophism and its disorders, various problems of gastroenterology, theoretical and applied aspects of nutrition science are in fact irrationally divided parts of one common problem - the problem of food assimilation by organisms at different levels of the evolutionary ladder. This problem should be considered from some unitary positions based on views that are more extensive and profound than before.

Thus, the theory of adequate nutrition is, so to speak, a theory of balanced nutrition that has grown "biological wings". This means that the theory of adequate nutrition is applicable not only to humans or one specific group of animals, but also to the most diverse species of animals and, moreover, to all groups of organisms.

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