Learn: How Digestion Works

In this guide, you will learn about the digestion process. Learn what benefits nutrition and the role of diets.

Nutrients and energy

To sustain life, the human body must constantly take in substances, a process known as nutrition. Nutrients provide the body with:

1. Chemical energy is needed to meet its energy needs;

2. Proteins, resp. amino acids necessary for the construction of continuously exchanged proteins, as well as for the synthesis of some other nitrogen-containing biologically active metabolites;

3. Inorganic compounds (salts) and water are necessary to replace those exported from it;

4. Vitamins - are biologically active compounds that the human body is unable to synthesize.

The majority of organic compounds contained in food are oligo- and polymers, i.e. derived organic structures, for example starch, proteins, complex lipids, etc.

Many disease states of the body are associated with improper nutrition, i.e. with errors in the diet: poor ratio between individual food ingredients, over- or under-nutrition, respectively deficiency of some food ingredients. The treatment of many diseases is combined with an appropriate diet. For these reasons, special attention is paid to the diet of physiologically healthy or sick individuals.

Carbohydrates in food

The body's main energy suppliers are carbohydrates and fats. Proteins, respectively amino acids, are an additional energy resource. What part of the necessary energy the body will receive from amino acids depends on how well it covers its energy needs from carbohydrates and fats.

Glucose is the main carbohydrate source of energy for many tissues, and for some, such as the brain, it is almost the only one. The food, however, contains very little free glucose. The body gets the glucose it needs almost entirely from starch and to a lesser extent from some glucose-containing disaccharides such as simple sugar (sucrose) and milk sugar (lactose). Besides glucose, these disaccharides also contain other monosaccharides - fructose and galactose. The body also receives free glucose and fructose from honey, as well as from glucose-fructose syrup, which has been increasing in use recently. Galactose and fructose are also transformed in the liver into glucose. The body can also synthesize glucose from other sources, such as amino acids, through a metabolic process known as gluconeogenesis. The minimum daily carbohydrate requirement of a healthy person is defined as between 50 - 100 grams, but larger amounts are usually needed to avoid excessive use of fat, which can lead to ketogenesis, as well as protein (mainly muscle).

Plant food mainly contains other polysaccharides, primarily cellulose, but also hemicellulose, pectins, etc. They are polymers of either only glucose, for example cellulose, or contain other monosaccharides and their derivatives. These polysaccharides are not digested in the human digestive tract. They pass into the large intestine, where together with other indigestible food components (lignin) they form a fibrous mass known as "dietary fiber", an essential component of faeces.

Lipids in food

The other main source of energy is lipids. Food usually contains complex lipids: triacylglycerols (fats), less phospholipids and esterified cholesterol. The body receives the most energy from the oxidative breakdown of the fatty acids contained in them, and less from glycerol. Another nutritional function of fats, apart from being a source of energy, is that they support the introduction of fat-soluble vitamins into the body.

In addition, through fats, the human body supplies itself with some polyunsaturated fatty acids, which it cannot synthesize, but which it needs, because from them it builds biologically active derivatives that function as local hormones: prostaglandins, thromboxanes, leukotrienes. They are called essential (essential) fatty acids. These are linoleic (with two double bonds) and linolenic (with three double bonds). They are synthesized only in plants, but humans supply them not only through vegetable oils, but also through animal fats (usually the fats included in meat and especially in poultry and fish). To a certain extent, albeit conditionally, arachidonic acid (tetraenoic acid with 20 carbon atoms) is also accepted as indispensable, especially in the case of a dietary deficiency of linoleic acid, from which the human body synthesizes it.

Proteins in food

Proteins are the only source of nitrogen for the body's needs. They supply the amino acids necessary for building the continuously exchanged proteins, as well as for the synthesis of some other nitrogen-containing metabolites such as the purine and pyrimidine bases and the porphyrin ring of heme, respectively hemoglobin and of other iron-porphyrin compounds (cofactors of cytochromes, etc.) .

The daily protein requirement of the body depends a lot on the "quality" of the proteins contained in the food. The quality of the protein, which is more often known as its "nutritional value", is determined by the content and ratio of the essential or essential amino acids contained in it. Essential amino acids are those that the human body cannot synthesize, which is why it must be supplied with food. For humans there are eight: leucine, isoleucine, valine, methionine, phenylalanine, lysine, tryptophan and threonine. Histidine and arginine are considered essential amino acids in children and, according to some, in adults.

The closer the content of essential amino acids and their ratio is to that of human proteins, the higher the nutritional value of the proteins contained in the food and the smaller amounts of them should be contained in the diet. In general, it can be said that the nutritional value of animal proteins is higher than that of plant proteins. In the case of vegetable food, cereals contain the most proteins, and of these, soy and bean proteins are considered the most complete. Corn protein zein is very low in nutrition because it does not contain tryptophan. If a person gets proteins from plant foods, their amount in the diet should be greater than if he takes animal proteins, for example meat.

Nitrogen balance is understood as the ratio between compartments for a certain period of time (for example, 24 hours) nitrogen in the form of nitrogen-containing organic and inorganic substances from the body (with urine, sweat and feces) and the nitrogen taken during the same time with food. A healthy adult should have nitrogen balance. A positive nitrogen balance is observed in children and young people at the age of puberty, during recovery after severe and debilitating illnesses (convalescence), during pregnancy and lactation. In such conditions, the amount of proteins in the diet should be increased. A negative nitrogen balance is observed in chronic protein malnutrition, in severe debilitating diseases and in the elderly.

Vitamins in food

With food, a person supplies the necessary vitamins. Their content in various foods has long been well established. The daily needs of the body for various vitamins are also known. They change depending on the physiological state of the body (age, physical exertion, etc.).

Water-soluble vitamins pass into the portal blood, and fat-soluble vitamins move along with lipids along the lymph circulation. A large part of its necessary vitamin K is supplied by the body through the biochemical activity of the intestinal bacterial flora.

Mineral salts and water in food

The body also takes in mineral substances daily, some of which in relatively larger amounts (over 100 mg per day) are macroelements, and others in smaller amounts (below 100 mg per day) are microelements. Minerals perform both physiological and biochemical functions. Their introduction is necessary in order to replace the mineral substances constantly removed from it with urine, sweat and other excreta. Mineral substances are contained in food, but they are also partly taken with water. It is necessary to add some minerals to the food, for example table salt (sodium chloride), potassium salt, iodine (iodized salt), etc.

The body constantly expels water from itself with urine, feces, sweat and other excreta. It is necessary to replace the exported water with a new one, taken either directly as a liquid or in the composition of various foods. We are talking about water-salt balance, i.e. the ratio between the salts and water exported by the body for a certain time and the intake for the same time from the outside. There is usually a water-salt balance, but sometimes the balance is positive or negative. In water balance, the body takes in less water per day than it excretes, because water is synthesized in it as a result of aerobic oxidation processes. The body has a complex and well-organized system for regulating the water-salt balance.

PREMIUM CHAPTERS ▼

Some clinical aspects of nutrition (PREMIUM)

Errors in the diet, or the wrong way of eating, are associated with the appearance or complication of some pathological conditions of the body. Diet can improve the condition and course of some diseases.

A chronic lack of protein in the diet leads to a disease condition known as kwashiorkor. Chronic lack of energy resources (carbohydrates and fats) in food, the so-called chronic starvation, although not identified as a nosological entity, is associated with various disturbances in energy metabolism. The combined chronic lack of sufficient protein and energy sources in food eventually leads to complete exhaustion of the body, known as marasmus. A diet that reduces the amount of "dietary fiber" in the colon is believed to increase the risk of diverticula, colon cancer, and even cardiovascular disease.

The conditions that are due to the lack or reduced amount of vitamins in food (avitaminosis and hypovitaminosis) are well known. Conditions due to reduced intake of essential fatty acids and amino acids with fats, respectively with proteins, are also described.

On the other hand, many disease states are associated with excessive intake of certain nutritional components of food. Such is obesity, which (in the absence of hormonal disturbances) is associated with an excessive dietary intake of energy-providing ingredients (mainly carbohydrates, but also lipids). Excessive obesity is considered to be such a dangerous disease as insulin-independent diabetes. Excessive intake of fats, especially those containing almost only saturated fatty acids (animal fat, solid margarine), is associated with the appearance of atherosclerotic changes in the blood vessels, preceding myocardial infarction and stroke. There is no doubt that these conditions are greatly aggravated by the excessive intake of dietary cholesterol. Increased fat intake is also accepted as a risk factor for colon, breast and prostate cancer. High blood pressure and sclerosis of the cerebral arteries are also associated with increased intake of table salt. There are also known diseases that are due to excessive intake of some fat-soluble vitamins (A, D) with food - hypervitaminoses. Some diet-related illnesses are actually due to disturbances in the digestion and absorption of nutrients. These and many other facts justify the great attention paid in medicine to the diet, to the composition of food products, to the composition of diets corresponding to various diseases and to various physiological states of the body. High blood pressure and sclerosis of the cerebral arteries are also associated with increased intake of table salt. There are also known diseases that are due to excessive intake of some fat-soluble vitamins (A, D) with food - hypervitaminoses. Some diet-related illnesses are actually due to disturbances in the digestion and absorption of nutrients. These and many other facts justify the great attention paid in medicine to the diet, to the composition of food products, to the composition of diets corresponding to various diseases and to various physiological states of the body. High blood pressure and sclerosis of the cerebral arteries are also associated with increased intake of table salt. There are also known diseases that are due to excessive intake of some fat-soluble vitamins (A, D) with food - hypervitaminoses. Some diet-related illnesses are actually due to disturbances in the digestion and absorption of nutrients. These and many other facts justify the great attention paid in medicine to the diet, to the composition of food products, to the composition of diets corresponding to various diseases and to various physiological states of the body. Some diet-related illnesses are actually due to disturbances in the digestion and absorption of nutrients. These and many other facts justify the great attention paid in medicine to the diet, to the composition of food products, to the composition of diets corresponding to various diseases and to various physiological states of the body. Some diet-related illnesses are actually due to disturbances in the digestion and absorption of nutrients. These and many other facts justify the great attention paid in medicine to the diet, to the composition of food products, to the composition of diets corresponding to various diseases and to various physiological states of the body.

Digestion and absorption of nutrients (PREMIUM)

A very large part of the nutrients contained in food are derived structures, oligo- and polymers. These are di- and polysaccharides (sucrose, lactose, starch, etc.), proteins, complex lipids (triacylglycerols, phospholipids, etc.). Hydrolysis to their monomeric constituents is called digestion. In humans, it is carried out in the departments of the digestive tract with the help of enzymes from the group of hydrolases.

As pointed out earlier, only "basic organic structures", i.e. organic molecules that are no longer subject to hydrolysis, can enter cells. However, a very large part of the nutrients contained in food are derivative structures, oligo- and polymers. These are di- and polysaccharides (sucrose, lactose, starch, etc.), proteins, complex lipids (triacylglycerols, phospholipids, etc.). It is necessary before they are resorbed that they are hydrolyzed to their monomeric constituents. This process is called digestion. In humans, it is carried out in the departments of the digestive tract with the help of enzymes from the group of hydrolases. Cells can take up polymers from their environment, for example proteins, peptides, fats, etc., and even whole cells or parts of cells. This is done through a process, described as endocytosis (pino- and phagocytosis). Organic compounds absorbed in this way are included in vacuoles, where they are digested in a manner analogous to their digestion in the digestive tract, and only then the resulting monomers enter the cytosol. Therefore, the matter contained both in the digestive tract and in the food vacuoles is taken as external to the organism, respectively to the cell.

The polymeric and oligomeric components of the food are subjected to hydrolysis by specific hydrolases, which are contained in the so-called digestive juices. These are saliva - a secretion of the salivary glands and scattered cells located at the back of the tongue, gastric juice - a secretion of specific cells located in the gastric mucosa; pancreatic juice - a secretion of the exocrine pancreas, intestinal juice - a secretion of specific secretory cells scattered in the mucosa of the duodenum (duodenum) and the small intestine. Bile juice secreted by the liver helps digest complex lipids. The products of the digestion of nutrients enter the body, a process known as resorption. Resorption takes place primarily in the upper part of the small intestine (jejunum). There is quite weak resorption in the stomach as well.

Bile acid salts and other bile constituents are reabsorbed back into the blood in the lower part of the small intestine (ileum) in a process referred to as enterohepatic circulation. The undigested and indigestible remains of food and digestive juices pass into the large intestine (colon). There, they thicken by reverse resorption of part of the water and form the fecal mass. Already in the small intestine, but mainly in the large intestine, it undergoes the action of bacteria (intestinal bacterial flora), as a result of which some products are formed. Some of them are absorbed into the blood, and some are toxic to the body. Intestinal bacteria "supply" the body with some useful metabolites (e.g. vitamin K, etc.), indigestible plant ingredients such as cellulose, hemicellulose, lignin, etc. They form fibrous matter (fibrous mass called dietary fiber), which has a beneficial effect on the processes in the large intestine. Disturbances in the digestion and resorption of nutrients are usually due to reduced production of digestive enzymes, impaired transport mechanisms, or damage to the epithelial cells of the mucous membranes.

Digestion of carbohydrates (PREMIUM)

Digestion of polysaccharides begins already in saliva under the action of salivary -amylase, which separates maltose from starch components (amylose and amylopectin) and glycogen. However, its effect is very limited, because the food stays for a short time in the oral cavity, but it can continue in the stomach, although it is suppressed by the acidic gastric juice. Degradation of oligo- and polysaccharides actually takes place in the intestines under the action of enzymes secreted in the pancreatic and intestinal juices.

Pancreatic juice contains -amylase, similar to saliva (it breaks only -1,4-O-glycosidic bonds. As a result of its action, starch and the small amount of glycogen, as well as oligosaccharide residues left by the action of salivary amylase, are degraded to maltose, maltotriose, a mixture of branched oligosaccharide fragments containing -1,6-O-glycosidic linkages (the so-called "restricted dextrins", some unbranched oligosaccharides and very little glucose. Pancreatic amylase secretion is stimulated by the hormone cholecystokinin, released by cells of the duodenum and jejunum in the blood.

The hydrolysis of the remaining oligo- and disaccharide molecules is completed by enzymes of the intestinal juice, which are secreted by gland cells in the intestinal mucosa, distinguished as Bruner's and Lieberkün's glands. In their secretion, there are specific oligo- and disaccharidases such as: maltase (-glucosidase), which hydrolyzes -1,4-O-glycosidic bonds from the non-reducing end of oligosaccharides and in maltose, releasing glucose; sucrase (-fructosidase), which hydrolyzes fructose but also breaks down -1,6-linkages in "restricted dextrins"; lactase (-galactosidase), which breaks down milk sugar into galactose and glucose, but also attacks cellobiose and other di- and oligosaccharides containing -glycosidic bonds, etc. Many of these enzymes are anchored in the membranes of intestinal epithelial cells, from where they act on their substrates. As a result of the joint action of all glycosidases in the digestive tract, the oligo- and polysaccharides found in the food are broken down into the corresponding monosaccharides (mostly glucose). Cellulose and many of the complex polysaccharides cannot be hydrolyzed in the human digestive tract due to a lack of the corresponding enzymes. They pass into the large intestine, where they form, together with lignin and other indigestible substances, the so-called "food fibers" - a fibrous mass included in the feces, but which also has a certain physiological significance. due to lack of relevant enzymes. They pass into the large intestine, where they form, together with lignin and other indigestible substances, the so-called "food fibers" - a fibrous mass included in the feces, but which also has a certain physiological significance. due to lack of relevant enzymes. They pass into the large intestine, where they form, together with lignin and other indigestible substances, the so-called "food fibers" - a fibrous mass included in the feces, but which also has a certain physiological significance.

Resorption of monosaccharides (PREMIUM)

Monosaccharides obtained under the influence of digestive enzymes are mainly absorbed in the upper part of the small intestine. Their resorption follows two mechanisms:

1. Active transport against a concentration gradient;

2. Simple diffusion.

Glucose and galactose are absorbed the fastest. Fructose is absorbed more slowly. Its resorption is facilitated by a glucose transporter independent of sodium ions, i.e. glucose facilitates fructose resorption. The active transport of glucose through the cells covering the hairs of the intestinal mucosa ("brush border") is of two types:

1. With the participation of a Na+-dependent protein transporter that imports glucose molecules against a concentration gradient, in symport of sodium ions. The process is coupled to the breakdown of ATP, and energy is needed to drive a sodium/potassium pump that expels the sodium ions brought in with the glucose against the influx of potassium ions (Fig. 19-1).

2. With the participation of a sodium ion-independent glucose transporter.

 

a - by means of a Na+-dependent protein transporter; b - by means of a glucose transporter independent of sodium ions; c - by means of diffusion.

From the enterocytes, glucose passes by free diffusion into the portal circulation and is carried to the liver.

Digestion of lipids (PREMIUM)

Complex lipids such as triacylglycerols (fats), phospholipids and esterified cholesterol undergo hydrolysis in the digestive tract. Saliva contains lipase secreted by glands located in the lining of the tongue. Its action is mainly manifested in the stomach, where it is added to the lipase secreted in the gastric juice. In the stomach, triacylglycerols containing fatty acids with short and medium-long chains (up to 10 - 14 carbon atoms) are attacked. Such fatty acids are mainly found in milk fat. Therefore, the hydrolysis of triacylglycerols in the stomach is of greater importance in infants. Hydrolysis is incomplete. In addition to short- and medium-chain fatty acids, 1,2-diacylglycerols are obtained. The presence of proteins in food buffers the highly acidic secretion of gastric juice and somewhat protects the lipase from acid denaturation. It is believed that gastric lipase is able to partially hydrolyze up to about 30% of the triacylglycerols ingested in the diet.

Digestion of lipids continues in the intestine. The content of the nutrient mass (chyme) entering the intestine from the stomach is acidic, but the alkaline pancreatic and intestinal juices quickly neutralize its acidity. Alkaline bile also contributes to this. So the intestinal contents have an alkaline reaction. Triacylglycerols are hydrolyzed by pancreatic lipase. The action of this enzyme, combined with the preceding action of salivary and gastric lipase, leads triacylglycerols partly to di- and monoacylglycerols, partly to free fatty acids. The action of lipases in the intestine is greatly favored by bile acid salts and is activated by a protein activator known as colipase, a protein also secreted by the exocrine pancreas. Other specific esterases cleave fatty acids from phospholipids as well. The most important of these is phospholipase A2, which hydrolyzes the ester bond at the second position in glycerophospholipids, converting them to lysophospholipids (eg, lysolecithins). Cholesterol esters are attacked by a specific cholesterolesterase to free cholesterol and fatty acids.

Role of bile acids, respectively salts in digestion of lipids (PREMIUM)

The action of pancreatic lipase is highly dependent on the presence of bile in the intestinal tract. Bile is an excrement of the liver, but actually bile contents from the gallbladder are poured into the intestine. It contains salts of bile acids - its most quantitative component, bile pigments, cholesterol, inorganic salts, mucin, etc. Bile acid salts play an essential role in the digestion of complex lipids, especially triacylglycerols. They have the remarkable property of significantly lowering the surface tension at the lipid/water interface, which is why they accumulate as a monolayer at this interface. In this way, they contribute the most (besides the mechanical action of intestinal peristalsis and the presence of free fatty acids - products of salivary and gastric lipase) to the fine emulsification of lipids in the intestines into microscopic droplets - liposomes and micelles. In this way, they greatly increase the surface on which the lipase acts. Bile acids are typical amphipathic molecules. The action of lipase on lipid molecules covered with a monolayer of bile salts, however, becomes possible only in the presence of another specific protein called colipase, which is contained in pancreatic juice. Colipase binds strictly specifically to lipase in a 1:1 ratio and acts as a cofactor (hence the name) of this enzyme, which activates it. on which the lipase acts. Bile acids are typical amphipathic molecules. The action of lipase on lipid molecules covered with a monolayer of bile salts, however, becomes possible only in the presence of another specific protein called colipase, which is contained in pancreatic juice. Colipase binds strictly specifically to lipase in a 1:1 ratio and acts as a cofactor (hence the name) of this enzyme, which activates it. on which the lipase acts. Bile acids are typical amphipathic molecules. The action of lipase on lipid molecules covered with a monolayer of bile salts, however, becomes possible only in the presence of another specific protein called colipase, which is contained in pancreatic juice. Colipase binds strictly specifically to lipase in a 1:1 ratio and acts as a cofactor (hence the name) of this enzyme, which activates it.

Resorption of lipid breakdown products (PREMIUM)

Fats are resorbed for the most part (over 70%) as 2-monoacylglycerols and less (20 - 25%) as free fatty acids. Small amounts of 1-monoacylglycerols can also be resorbed. This resorption takes place, however, after they are incorporated into micelles and liposomes formed by the salts of bile acids, phosphatidylcholine and cholesterol and thus pass through the enterocyte membrane. In these cells, the resynthesis of triacylglycerols, which in their composition already resemble the specific human fats, takes place. Glycerol from the digested fats is reabsorbed and passes into the portal blood by simple diffusion. In addition to fatty acids, free (non-esterified) cholesterol of dietary and partly bile origin and lysophospholipids also pass through the enterocytes in a similar way. In the intestinal epithelial cells, they are converted back into esterified cholesterol and phospholipids. Together with the resynthesized triacylglycerols, they form with the help of specific proteins lipoprotein complexes called chylomicrons, which pass into the lymph and through the ductus thoracicus are taken into the blood circulation. Chylomicrons give the lymph a milky appearance (hilus). Fatty acids with shorter chains (10 - 12 carbon atoms) pass directly into the portal blood. Bile acid salts and some of the unesterified cholesterol move to the lower part of the small intestine (ileum), where they are absorbed into the bloodstream, i.e. enter the so-called "enterohepatic cycle of bile". which pass into the lymph and through the ductus thoracicus are carried into the blood circulation Chylomicrons give the lymph a milky appearance (hilus). Fatty acids with shorter chains (10 - 12 carbon atoms) pass directly into the portal blood. Bile acid salts and some of the unesterified cholesterol move to the lower part of the small intestine (ileum), where they are absorbed into the bloodstream, i.e. enter the so-called "enterohepatic cycle of bile". which pass into the lymph and through the ductus thoracicus are carried into the blood circulation Chylomicrons give the lymph a milky appearance (hilus). Fatty acids with shorter chains (10 - 12 carbon atoms) pass directly into the portal blood. Bile acid salts and some of the unesterified cholesterol move to the lower part of the small intestine (ileum), where they are absorbed into the bloodstream, i.e. enter the so-called "enterohepatic cycle of bile". where they are resorbed into the bloodstream, i.e. enter the so-called "enterohepatic cycle of bile". where they are resorbed into the bloodstream, i.e. enter the so-called "enterohepatic cycle of bile".

Digestion of proteins (PREMIUM)

The food contains significant amounts of vegetable and animal proteins. Protein digestion begins in the stomach under the action of the enzyme pepsin. Pepsin is synthesized in an inactive form as a zymogen called pepsinogen in the chief cells of the gastric mucosa. Gastric juice has a highly acidic reaction because it contains hydrochloric acid (pH about 1.0). Accordingly, pepsin also has a too low pH optimum (between 1.5 - 2.0). Hydrochloric acid is formed in the so-called parietal cells of the gastric mucosa from carbonic acid formed in them with the help of the enzyme carbonic anhydrase (Fig. 19-1). Carbonic acid dissociates into protons and bicarbonate anions. Protons (hydrogen cations) are exported into the lumen of the stomach by an antiport mechanism, against the import of K+, coupled with the breakdown of ATP (H+-K+ pump).

Pepsin is a typical endopeptidase. It selectively hydrolyzes peptide bonds within the polypeptide chain formed by aromatic or dicarboxylic amino acids, eg tyrosine or glutamate. A mixture of smaller or larger peptides is obtained. In addition to pepsin, another proteinase - rennin - is released in the gastric juice. Rennin attacks the milk protein casein, turning it into paracasein. In the presence of Ca2+, paracasein precipitates (coagulates) and thus keeps the milk longer in the stomach. This action is physiologically beneficial, especially for infants who receive almost exclusively milk proteins. Perhaps that is why this enzyme is almost absent in the stomach of adults for whom milk is not a staple food. Protein digestion continues in the intestines under the action of enzymes contained in the pancreatic juice. These are the endopeptidases trypsin and chymotrypsin and the exopeptidase carboxypeptidase. The reaction of pancreatic juice is alkaline (pH between 7.5 and 8.0). The acidic stomach contents that have entered the duodenum stimulate the production of a hormone - secretin, which causes the release of bicarbonate in the pancreatic juice. Thus, the content of the food mass becomes alkaline and favorable for the action of the enzymes found in the pancreatic and intestinal juices. All pancreatic proteinases are synthesized in an inactive state as precursors called preenzymes or enzymogens (zymogens) and are activated only when they enter the intestine (for their activation see below). stimulates the production of a hormone - secretin, which causes the release of bicarbonate in the pancreatic juice. Thus, the content of the food mass becomes alkaline and favorable for the action of the enzymes found in the pancreatic and intestinal juices. All pancreatic proteinases are synthesized in an inactive state as precursors called preenzymes or enzymogens (zymogens) and are activated only when they enter the intestine (for their activation see below). stimulates the production of a hormone - secretin, which causes the release of bicarbonate in the pancreatic juice. Thus, the content of the food mass becomes alkaline and favorable for the action of the enzymes found in the pancreatic and intestinal juices. All pancreatic proteinases are synthesized in an inactive state as precursors called preenzymes or enzymogens (zymogens) and are activated only when they enter the intestine (for their activation see below).

Trypsin attacks peptide bonds formed by basic amino acids (lysine, arginine). Chymotrypsin attacks bonds from uncharged amino acids such as phenylalanine, and elastase from low molecular weight amino acids such as glycine and alanine; this explains the effect of this enzyme mainly on such "difficult to digest" proteins as collagen and elastin. Carboxypeptidase (a zinc-containing metalloenzyme) is a typical exopeptidase. It cleaves amino acids from the carbon end of poly- and oligopeptides.

The complete digestion of proteins into amino acids is completed by enzymes secreted in the intestinal juice. These are: aminopeptidase - exopeptidase, which cleaves amino acids from the nitrogen end of polypeptide chains, as well as several dipeptidases, which degrade dipeptides in a specific way. In this way, the proteins taken in with the food are digested to their constituent amino acids.

Resorption of amino acids (PREMIUM)

Proteins are broken down in the digestive tract into amino acids. Amino acids are reabsorbed by active transport using transporters specific for either groups or individual amino acids. Some of these transporters are dependent on sodium cation symport coupled to ATP expenditure (sodium/potassium pump) by a mechanism similar to that described for glucose resorption. Other transporters are independent of sodium ion symport. From the enterocytes, the amino acids pass into the portal blood. The absorption of amino acids, as well as the other digested nutrients, takes place almost entirely (over 90%) in the upper part of the small intestine (jejunum). An insignificant part of the fatty acids (with short and medium long chains) and ethyl alcohol is resorbed through the gastric mucosa.

Degradation of nucleic acids and other organic compounds containing phosphate residues (PREMIUM)

Degradation of nucleic acids contained in food begins under the action of specific phosphodiesterases (the nucleases DNase and RNase) contained in the pancreatic juice and continues under the action of non-specific phosphodiesterases and phosphomonoesterases, as well as by nucleosidases contained in the intestinal mucosa. The nitrogenous bases of the nucleotides and to some extent the pentose are too poorly absorbed in the intestine. Intestinal phosphatases, in addition to nucleotides, also hydrolyze other organic esters of phosphoric acid: hexose phosphates, glycerophosphate, etc.

Conversion of digestive enzyme zymogens to active enzymes (PREMIUM)

Many of the digestive hydrolases are synthesized in an inactive state as enzymogens (zymogens, preenzymes). This applies mostly to proteolytic enzymes. In this way, the cells in which they are synthesized are protected from degrading their own proteins, a phenomenon known as autolysis. The activation of all zymogens follows a similar mechanism - limited (partial) proteolysis. This activation mechanism is irreversible.

Pepsinogen is converted to active pepsin by breaking one peptide bond and cleaving a 42 amino acid polypeptide from the nitrogen terminus of the polypeptide chain. The process is activated by the presence of H+ and continues at an exponentially increasing rate due to the action of the resulting active pepsin - autocatalysis. Cleavage of the polypeptide exposes the active site of the enzyme. Trypsinogen is activated in a similar way, by cleaving a hexapeptide from the nitrogen terminus of its polypeptide chain by the specific action of another endopeptidase known as enterokinase, which is secreted by duodenal cells. The active site of the enzyme is revealed. The resulting active trypsin continues to cleave the same bond in its inactive precursor, and the activation process proceeds exponentially. Active trypsin also performs a limited proteolytic attack on other zymogens secreted by the pancreas: chymotrypsinogen, preelastase and precarboxypeptidase, activating them. The activation of chymotrypsinogen is more complicated. After successive breaking of four peptide bonds (the first by trypsin, and the next three by already activated chymotrypsin - autocatalytically) and separation of two dipeptides, the final product - -chymotrypsin is obtained, consisting of three polypeptide chains covalently connected to each other by two disulfide the bridge. Phospholipase A is also secreted as a zymogen that is activated by trypsin by cleaving a pentapeptide from the nitrogen terminus of the polypeptide chain. The activation of chymotrypsinogen is more complicated. After successive breaking of four peptide bonds (the first by trypsin, and the next three by already activated chymotrypsin - autocatalytically) and separation of two dipeptides, the final product - -chymotrypsin is obtained, consisting of three polypeptide chains covalently connected to each other by two disulfide the bridge. Phospholipase A is also secreted as a zymogen that is activated by trypsin by cleaving a pentapeptide from the nitrogen terminus of the polypeptide chain. The activation of chymotrypsinogen is more complicated. After successive breaking of four peptide bonds (the first by trypsin, and the next three by already activated chymotrypsin - autocatalytically) and separation of two dipeptides, the final product - -chymotrypsin is obtained, consisting of three polypeptide chains covalently connected to each other by two disulfide the bridge. Phospholipase A is also secreted as a zymogen that is activated by trypsin by cleaving a pentapeptide from the nitrogen terminus of the polypeptide chain. linked together covalently by two disulfide bridges. Phospholipase A is also secreted as a zymogen that is activated by trypsin by cleaving a pentapeptide from the nitrogen terminus of the polypeptide chain. linked together covalently by two disulfide bridges. Phospholipase A is also secreted as a zymogen that is activated by trypsin by cleaving a pentapeptide from the nitrogen terminus of the polypeptide chain.

Processes in the colon (PREMIUM)

Fecal mass. Role of intestinal microbial flora Undigested or indigestible residues of ingested food pass into the large intestine and form the fecal mass. A large part of this mass contains plant polysaccharides (cellulose, hemicellulose, pectins, etc.) and lignin. They form the so-called "food fibers". In the colon there is also a huge mass of intestinal bacteria (mostly E. coli). They reach up to 25% of the dry weight of feces. Under their influence, the mass passed into the large intestine undergoes additional biochemical changes, affecting mainly amino acids and undigested protein residues, including those of the digestive juices - mucin. Plant polysaccharides and undigested starch residues are also partially degraded. As a result of such processes, which can generally be characterized as anaerobic fermentations, various final products such as acetic, propionic, butyric, isobutyric and other acids are obtained, as well as gases such as carbon dioxide, methane, hydrogen sulphide, etc. Bacterial decarboxylation of amino acids results in the formation of amines, some of which are toxic (ptomaines). During the breakdown of tryptophan, indole and skatole are obtained, which give the stool an unpleasant smell. Some of the products of microbial "rot" are resorbed through the lining of the large intestine and enter the bloodstream. Their disposal takes place mainly in the liver. During the breakdown of tryptophan, indole and skatole are obtained, which give the stool an unpleasant smell. Some of the products of microbial "rot" are resorbed through the lining of the large intestine and enter the bloodstream. Their disposal takes place mainly in the liver. During the breakdown of tryptophan, indole and skatole are obtained, which give the stool an unpleasant smell. Some of the products of microbial "rot" are resorbed through the lining of the large intestine and enter the bloodstream. Their disposal takes place mainly in the liver.

During the bacterial deamination of amino acids and other nitrogenous components of feces, ammonia, respectively ammonium ions, is formed. They are resorbed and taken with the blood to the liver, where they are destroyed in the urea cycle. The intestinal bacterial flora, however, is also useful because it synthesizes and supplies the body with some necessary substances, such as vitamins (vitamin K, biotin, etc.).

In the ileum and especially in the large intestine, a large part of the water contained in the digested food mass is resorbed. As a result, the fecal mass thickens and forms stool. On the other hand, "food fibers" retain part of the water and contribute to the fecal mass being "mushy".

Basal metabolism and daily energy needs (PREMIUM)

To maintain an energy balance, without gaining weight or losing weight, we must take in food corresponding to the daily energy expenditure. They include the energy to maintain:

1. basal metabolism (basal metabolism);

2. physical activity;

3. digestion.

In case of pregnancy and lactation, additional calories are needed.

The abbreviation BMR (from the English name Basal Metabolic Rate) is adopted for basal metabolism in the English literature. BMR is a measure of the energy needed to sustain life: functioning of various organs, maintaining ion gradients across membranes, carrying out biochemical reactions, etc. BMR can be determined by measuring the oxygen consumption of a person at rest, recently after waking up and without to have taken food for at least 12 hours.

BMR is usually expressed in kcal/day. It depends on:
      1) gender (the value is higher in men;
      2) body temperature (e.g. increases in fever);
      3) the temperature of the environment (the value is lower in a warm climate);
      4) the activity of the thyroid gland (the value is higher in hyperthyroidism);
      5) age (the value is higher in childhood);
      6) increases during pregnancy and lactation.

Although there are special and complex formulas for calculating BMR, a simple, albeit approximate (rough) calculation of BMR is often resorted to. For this purpose, it is assumed that 24 kcal (100 kJ in SI) or 1 kcal/kg/hour (4.85 kJ/kg/hour in SI) are needed for 1 kg per day.

BMR = 24 (kcal/kg) x weight (kg)
To convert to SI, given that 1 kcal = 4.185 kJ,
BMR = 100 (kJ/kg) x weight (kg)

 

 

The energy required for digestion (the term "food-induced thermogenesis" is also used) is about 10% of BMR and is usually often neglected in calculations. Therefore, daily energy expenditure is calculated as the sum of BMR (kcal/day) and the energy required for physical activity For example, for a physical worker weighing 100 kg during a working day

BMR = 100 kg x 24 kcal/kg = 2400 kcal or 10044 kJ
High activity according to [1] = 2400 x 0.5 = 1200 kcal or 5022 kJ
------------------- -------------------
Total = 2400 + 1200 = 3600 kcal or 10044 + 5022 = 15066 kJ

On a rest day, if he leads a sedentary lifestyle, energy requirements are lower.

BMR = 100 kg x 24 kcal/kg = 2400 kcal or 10044 kJ

low activity according to [1] = 2400 x 0.3 = 720 kcal or 3013 kJ
------------------- -------------------
Total = 2400 + 720 = 3120 kcal or 10044 + 3013 = 13057 kJ

The concept of "ideal weight" is difficult to define, but according to insurance companies, it is the weight at which an individual has the greatest chance of living the longest [2]. Overweight are people who have a 20% increase over their ideal weight.

Another measure of how much body weight is within desired limits is the so-called body mass index (BMI from English Body Mass Index).

BMI = weight / height2 (kg/m2)

In Bulgaria, the ones indicated in the table are used. 2 BMI values ​​proposed by WHO [3]. Individuals with an increase in BMI of up to 20% are overweight, and if the increase is more than 20%, the individual suffers from obesity (first, second or third degree).

A healthy diet (PREMIUM)

To reduce the risk of myocardial infarction, stroke, and cancer in the United States, the following guidelines are recommended for the various components of the diet.

Carbohydrates - Every day 5 or more servings of vegetables and fruits, especially green and yellow vegetables and citrus fruits. - Take starch and other polysaccharides in the form of bread, cereals and legumes 6 times a day. In addition to energy, these foods provide vitamins, minerals and fiber. These fibers, indigestible by the body, have various beneficial effects, incl. preventing constipation.
- Refined sugar consumption should be reduced below accepted American norms. This sugar has no nutritional value other than caloric content and causes tooth decay.

Fat  -  Fat should provide no more than 30% of total calories; - Saturated fatty acids should be around or below 10% of total calories; - Unsaturated fatty acids should provide about 10% of total calories (about 30% of total fat); - Cholesterol should be below 300 mg/day .

Protein  -  For adults, protein should be 0.8 g/kg of ideal weight per day.

Alcohol  -  The daily dose of alcohol should not be more than 15 g of ethanol (for example, 2 small glasses of wine). Pregnant women should not drink alcohol.

Vitamins and minerals - Salt should not exceed 3 g daily, and people prone to high blood pressure, this amount should be less. - To ensure sufficient calcium, take dairy products with reduced fat content or fat-free, as well as vegetables with dark green leaves; - to avoid excessively high doses of vitamins, especially fat-soluble ones; To protect teeth, fluoride should be present in the diet, at least during the teeth-forming years. Keep in mind that overdose is dangerous.