Digestive processes: digestion of fats, carbohydrates, proteins. Biochemistry of nutrition and digestion

Some people believe that carbohydrates, fats and proteins are always completely absorbed by the body. Many people think that absolutely all the calories present on their plate (and, of course, counted) will enter the bloodstream and leave their mark on our body. In reality, everything is different. Let's look at the absorption of each macronutrient separately.

Digestion (assimilation)- this is a set of mechanical and biochemical processes through which the food absorbed by a person is converted into substances necessary for the functioning of the body.

The digestion process usually begins in the mouth, after which the chewed food enters the stomach, where it is subjected to various biochemical treatments (mainly protein is processed at this stage). The process continues in the small intestine, where, under the influence of various food enzymes, carbohydrates are converted into glucose, lipids are broken down into fatty acids and monoglycerides, and proteins into amino acids. All these substances, absorbed through the intestinal walls, enter the blood and are distributed throughout the body.

Macronutrient absorption does not last for hours and does not stretch over the entire 6.5 meters small intestine. The absorption of carbohydrates and lipids by 80%, and proteins by 50% is carried out during the first 70 centimeters small intestine.

Absorption of carbohydrates

Assimilation various types carbohydrates happens differently because they have different chemical structure, and therefore different speed assimilation. Under the action of various enzymes, complex carbohydrates are broken down into simple and less complex sugars, which have several types.

Glycemic index (GI) is a system for classifying the glycemic potential of carbohydrates in various products. Essentially, this system looks at how a particular food affects blood glucose levels.

Visually: if we eat 50 g of sugar (50% glucose / 50% fructose) (see picture below) and 50 g of glucose and check the blood glucose level after 2 hours, the GI of sugar will be lower than that of pure glucose, since its amount in sugar is lower.

What if we eat an equal amount of glucose, for example, 50 g of glucose and 50 g of starch? Starch is a long chain consisting of a large number of glucose units, but in order for these “units” to be detected in the blood, the chain must be processed: each compound is broken down and released into the blood one at a time. Therefore, starch has a lower GI, because the level of glucose in the blood after eating starch will be lower than after eating glucose. Imagine, if you throw a spoonful of sugar or a cube of refined sugar into tea, which will dissolve faster?

Glycemic response to foods:

• left - slow absorption of starchy foods with low GI;


• right - rapid absorption of glucose from sharp drop blood glucose levels as a result of the rapid release of insulin into the blood.

GI is relative value, and it is measured relative to the effect of glucose on glycemia. Above is an example of the glycemic response to pure glucose eaten and to starch. In the same experimental way, GI has been measured for more than a thousand foods.

When we see the number “10” next to cabbage, this means that the strength of its effect on glycemia will be equal to 10% of how glucose would affect it, for a pear 50%, etc.

We can influence our glucose levels by choosing foods that are not only low GI, but also low in carbohydrates, which is called the glycemic load (GL).

GN takes into account both the GI of the product and the amount of glucose that enters the blood when it is consumed. So, often foods with a high GI will have a small GI. It is clear from the table that it makes no sense to look only at one parameter - it is necessary to consider the picture comprehensively.

(1) Although buckwheat and condensed milk have almost the same carbohydrate content, these products have different GI values ​​because the type of carbohydrates in them is different. Therefore, if buckwheat leads to the gradual release of carbohydrates into the blood, then condensed milk will cause sharp jump. (2) Despite the identical GI of mango and condensed milk, their effect on blood glucose levels will be different, this time not because the type of carbohydrates is different, but because the amount of these carbohydrates is significantly different.

Glycemic index of foods and weight loss

Let's start with something simple: there is huge amount scientific and medical research, which indicate that low GI foods have a positive effect on weight loss. Biochemical mechanisms There are many who are involved in this, but let’s name the most relevant for us:

1. Low GI foods make you feel more full than high GI foods.


2. After eating foods with a high GI, insulin levels rise, which stimulates the absorption of glucose and lipids into the muscles, fat cells and liver, simultaneously stopping the breakdown of fats. As a result, the level of glucose and fatty acids in the blood falls, and this stimulates hunger and new food intake.


3. Foods with different GIs have different effects on fat breakdown during rest and during sports training. Glucose from low GI foods is not so actively deposited into glycogen, but during exercise, glycogen is not so actively burned, which indicates increased use fats for this purpose.

Why do we eat wheat but not wheat flour?

• The more crushed the product (mainly refers to grains), the higher the GI of the product.

Differences between wheat flour(GI 85) and wheat grain (GI 15) fall under both of these criteria. This means that the process of breaking down starch from grain is longer and the resulting glucose enters the blood more slowly than from flour, thereby providing the body with the necessary energy for longer.

• The more fiber a product contains, the lower its GI.


• The amount of carbohydrates in a product is no less important than GI.

Beetroot is a vegetable with more high content fiber than flour. Even though she is tall glycemic index, she has low content carbohydrates, i.e. lower glycemic load. In this case, despite the fact that its GI is the same as that of a grain product, the amount of glucose entering the blood will be much less.

• GI raw vegetables and fruits are lower than boiled ones.

This rule applies not only to carrots, but also to all vegetables with a high starch content, such as sweet potatoes, potatoes, beets, etc. During cooking, a significant part of the starch is converted into maltose (a disaccharide), which is very quickly absorbed.

Therefore, even boiled vegetables It’s better not to boil them, but to make sure that they remain whole and firm. However, if you have diseases such as gastritis or stomach ulcers, it is still better to eat cooked vegetables.

• Combining proteins with carbohydrates reduces the GI of a serving.

Proteins, on the one hand, slow down the absorption of simple sugars into the blood, on the other hand, the very presence of carbohydrates contributes to the best digestibility of proteins. In addition, vegetables also contain fiber that is beneficial for the body.

Natural products, unlike juices, contain fiber and thereby lower the GI. Moreover, it is advisable to eat fruits and vegetables with the skin, not only because the skin contains fiber, but also because most of the vitamins are located directly on the skin.

Protein absorption

Digestion process proteins requires increased acidity in the stomach. Gastric juice with increased acidity necessary for activating enzymes responsible for the breakdown of proteins into peptides, as well as for the primary dissolution of food proteins in the stomach. From the stomach, peptides and amino acids enter the small intestine, where some of them are absorbed through the intestinal walls into the blood, and some are further broken down into individual amino acids.

To optimize this process, it is necessary to neutralize the acidity of the gastric solution, and this is the responsibility of the pancreas, as well as the bile produced by the liver and necessary for the absorption of fatty acids.
Proteins from food are divided into two categories: complete and incomplete.

Complete proteins- these are proteins that contain all the amino acids necessary (essential) for our body. The source of these proteins is mainly animal proteins, i.e. meat, dairy products, fish and eggs. There are also plant sources of complete protein: soy and quinoa.

Incomplete proteins contain only a portion of the essential amino acids. It is believed that legumes and cereals themselves contain incomplete proteins, but their combination allows us to get all the essential amino acids.

In many national cuisines correct combinations, leading to adequate protein consumption, have emerged naturally. Thus, in the Middle East, pita with hummus or falafel (wheat with chickpeas) or rice with lentils is common, in Mexico and South America They often combine rice with beans or corn.

One of the parameters that determines protein quality is presence of essential amino acids. In accordance with this parameter, there is a product indexing system.

For example, the amino acid lysine is found in small quantities in cereals, and therefore they receive low rating(cereals - 59; whole wheat - 42), and legumes contain no large number essential methionine and cysteine ​​(chickpeas - 78; beans - 74; legumes - 70). Animal proteins and soybeans receive a high rating on this scale, as they contain the necessary proportions of all essential amino acids (casein (milk) - 100; egg white- 100; soy protein - 100; beef - 92).

In addition, it is necessary to take into account protein composition, their digestibility from of this product, as well as the nutritional value of the entire product (the presence of vitamins, fats, minerals and calorie content). For example, a hamburger will contain a lot of protein, but also a lot of saturated fatty acids, so its nutritional value will be lower than that of a chicken breast.

Proteins from different sources and even different proteins from the same source (casein and whey protein) are utilized by the body at different rates.

Nutrients from food are not 100% digestible. The degree of their absorption can vary significantly depending on the physicochemical composition of the product itself and the products absorbed simultaneously with it, the characteristics of the body and the composition of the intestinal microflora.

The main goal for detox is to get out of your comfort zone and try new nutritional systems.

Moreover, very often, like “cookies for tea,” eating meat and dairy products is a habit. We have never had the opportunity to research their importance in our diet and understand how much we need them.

In addition to the above, most nutritional organizations recommend that the basis healthy diet a large amount of plant food was laid down. This step out of your comfort zone will send you in search of new tastes and recipes and diversify your daily diet afterwards.

In particular, research results indicate an increased risk cardiovascular diseases, osteoporosis, kidney disease, obesity and diabetes.

At the same time, low-carbohydrate, but high-protein diets based on plant sources of protein lead to lower concentrations of fatty acids in the blood and to a reduced risk of heart disease.

But even with a great desire to relieve our body, we should not forget about the characteristics of each of us. Such a relatively sudden change in diet may cause discomfort or side effects, such as bloating (a consequence of a large amount of vegetable protein and the characteristics of the intestinal microflora), weakness, dizziness. These symptoms may indicate that this strict diet is not entirely suitable for you.

When a person consumes a large amount of protein, especially in combination with a low amount of carbohydrates, the breakdown of fats occurs, during which substances called ketones are created. Ketones may have negative impact to the kidneys, which secrete acid to neutralize it.

There are claims that to restore acid-base balance Skeletal bones secrete calcium, and therefore increased calcium leaching is associated with high animal protein intake. Also, a protein diet leads to dehydration and weakness, headaches, dizziness, and bad breath.

Digestion of fats

Fat entering the body passes through the stomach almost intact and enters the small intestine, where there are a large number of enzymes that convert fats into fatty acids. These enzymes are called lipases. They function in the presence of water, but this is problematic for fat processing, since fats do not dissolve in water.

In order to be able to recycle fats, our body produces bile. Bile breaks up fat clumps and allows enzymes on the surface of the small intestine to break down triglycerides into glycerol and fatty acids.

Transporters for fatty acids in the body are called lipoproteins. These are special proteins that are capable of packaging and transporting fatty acids and cholesterol throughout the circulatory system. Next, fatty acids are packaged in fat cells in a fairly compact form, since their composition (unlike polysaccharides and proteins) does not require water.

The proportion of fatty acid absorption depends on the position it occupies relative to glycerol. It is important to know that only those fatty acids that occupy the P2 position are well absorbed. This is due to the fact that lipases have varying degrees effects on fatty acids depending on the location of the latter.

Not all fatty acids supplied with food are completely absorbed by the body, as many nutritionists mistakenly believe. They may not be partially or completely absorbed in the small intestine and may be excreted from the body.

For example, in butter, 80% of fatty acids (saturated) are in the P2 position, that is, they are completely absorbed. The same applies to fats that are part of milk and all dairy products that do not undergo the fermentation process.

The fatty acids present in mature cheeses (especially long-aged cheeses), although saturated, are still located in the P1 and P3 positions, which makes them less absorbable.

In addition, most cheeses (especially hard ones) are rich in calcium. Calcium combines with fatty acids to form “soaps” that are not absorbed and are excreted from the body. The ripening of cheese promotes the transition of its fatty acids to positions P1 and P3, which indicates their weak absorption.

High intake of saturated fat is also correlated with some types of cancer, including colon cancer, and stroke.

The absorption of fatty acids is influenced by their origin and chemical composition:

- Saturated fatty acids(meat, lard, lobster, shrimp, egg yolk, cream, milk and dairy products, cheese, chocolate, rendered fat, vegetable shortening, palm, coconut and butter), and also trans fats(hydrogenated margarine, mayonnaise) tend to be stored in fat reserves rather than immediately burned during energy metabolism.

- Monounsaturated fatty acids(poultry, olives, avocados, cashews, peanuts, peanut and olive oil) are predominantly used directly after absorption. In addition, they help reduce glycemia, which reduces insulin production and thereby limits the formation of fat reserves.

- Polyunsaturated fatty acids, especially Omega-3 (fish, sunflower, flaxseed, rapeseed, corn, cottonseed, safflower and soybean oils), are always consumed immediately after absorption, in particular, due to an increase in food thermogenesis - the body’s energy consumption for digesting food. In addition, they stimulate lipolysis (the breakdown and burning of fat deposits), thereby promoting weight loss.

IN recent years There are a number of epidemiological studies and clinical trials that challenge the assumption that low-fat dairy products are healthier than full-fat dairy products. They're not just rehabilitating dairy fats, they're increasingly finding a link between wholesome dairy products and improved health.

A recent study found that in women, the occurrence of cardiovascular disease depends entirely on the type of dairy products consumed. Cheese consumption was inversely associated with risk heart attack, while butter spread on bread increases the risk. Another study found that neither low-fat nor full-fat dairy products are associated with cardiovascular disease.

However, whole fermented milk products protect against cardiovascular diseases. Milk fat contains more than 400 “types” of fatty acids, making it the most complex naturally occurring fat. Not all of these species have been studied, but there is evidence that at least several of them have beneficial effects.

Literature:

1. Mann (2007) FAO/WHO Scientific Update on carbohydrates in human nutrition: conclusions. European Journal of Clinical Nutrition 61 (Suppl 1), S132-S137
2. FAO/WHO. (1998). Carbohydrates in human nutrition. Report of a Joint FAO/WHO Expert Consultation (Rome, 14-18 April 1997). FAO Food and Nutrition Paper 66
3. Holt, S. H., & Brand Miller, J. (1994). Particle size, satiety and the glycemic response. European Journal of Clinical Nutrition, 48(7), 496-502.
4. Jenkins DJ (1987) Starchy foods and fiber: reduced rate of digestion and improved carbohydrate metabolism Scand J Gastroenterol Suppl.129:132-41.
5. Boirie Y. (1997) Slow and fast dietary proteins modulate postprandial protein accretion differently. Proc Natl Acad Sci U S A. 94(26):14930-5.
6. Jenkins DJ (2009) The effect of a plant-based low-carbohydrate (“Eco-Atkins”) diet on body weight and blood lipid concentrations in hyperlipidemic subjects. Arch Intern Med. 169(11):1046-54.
7. Halton, T.L., et al., Low-carbohydrate-diet score and the risk of coronary heart disease in women. N Engl J Med, 2006. 355 (19): p. 1991-2002.
8. Levine ME (2014) Low protein intake is associated with a major reduction in IGF-1, cancer, and overall mortality in the 65 and younger but not older population. Cell Metabolism 19, 407-417.
9. Popkin, BM (2012) Global nutrition transition and the pandemic of obesity in developing countries. Nutrition reviews 70 (1): pp. 3 -21.
10.

Digestion in the stomach is the process by which the food we ingest changes its form to one that our body is able to absorb. After certain physical phenomena and processes, as well chemical reactions, facilitated by digestive juices, nutrients change so that the body can easily absorb them and further use them in metabolism. Digestion of food can occur while it is moving through the organs gastrointestinal tract.

Scientists consider only three main classes to be the main components of a proper and healthy diet. chemical compounds: proteins, carbohydrates (also sugar) and fats, namely lipids. Let's take a closer look at them.

Carbohydrates

These substances are present in the form of starch in plant foods. Digestion in the stomach and intestines promotes the process of converting carbohydrates into glucose, which, in turn, is stored in the form of glycogen, that is, a polymer, and is then used by the body. A single starch molecule is considered a very large polymer that is formed from many glucose molecules. It is worth noting that raw starch is formed in granules. They must be destroyed in order to enable this substance to turn into glucose. It is cooking that contributes to the destruction of the granules of starch contained in it.

It is also necessary to know that part food products contains carbohydrates in a special form of disaccharides. These are simple sugars, lactose, as well as sucrose, cane sugar. Digestion in the stomach turns these substances into even simpler compounds - monosaccharides, which do not particularly need to be digested.

Squirrels

They are represented by various polymers that are formed from twenty different types of amino acids. After digestion, free amino acids are formed as end products. Intermediate products of protein digestion are polypeptides, peptones and dipeptides.

Fats

These are fairly simple compounds that, as a result of digestion and digestion processes, are converted into fatty acids and glycerol.

Physical processes

We all know where the stomach is, but what are physical processes occur in our body - not always. The basis of digestion is the grinding of food, which occurs during chewing and rhythmic contractions of the intestines and stomach. Such effects help the food to be crushed and thoroughly mix all its particles with the digestive juices that are secreted in the intestines, stomach and mouth. Moreover, wall contractions digestive tract ensure constant movement of food through its sections. All these movements are constantly regulated and controlled by the nervous system.

Chemical reactions

Digestion in the stomach is impossible to imagine without chemical reactions inside our body. Their basis is the breakdown of carbohydrates, fats and proteins, namely hydrolysis, which is carried out by a certain set of enzymes. Nutrients are broken down during hydrolysis into small particles that are absorbed in the body. This process occurs quite quickly due to the action of enzymes contained in gastric and other digestive juices.

Food entering the human body cannot be assimilated and used for plastic purposes and the formation of vital energy, since it physical condition and chemical composition are very complex. To transform food into a state easily digestible by the body, humans have special organs that carry out digestion.

Digestion is a set of processes that provide physical change and chemical breakdown of nutrients into simple water-soluble compounds that can be easily absorbed into the blood and participate in vital functions important functions human body.

The human digestive apparatus consists of the following organs: the oral cavity (oral opening, tongue, teeth, masticatory muscles, salivary glands, glands of the oral mucosa), pharynx, esophagus, stomach, duodenum, pancreas, liver, small intestine, large intestine with rectum. The esophagus, stomach, and intestines consist of three membranes: the inner membrane, which contains glands that secrete mucus, and in a number of organs, digestive juices; middle - muscle, which ensures the movement of food by contraction; outer - serous, acting as a covering layer.

During the course of a day, a person secretes about 7 liters of digestive juices, which include: water, which dilutes food gruel, mucus, which promotes better movement of food, salts and enzyme catalysts of biochemical processes that break down food substances into simple constituent compounds. Depending on the effect on certain substances, enzymes are divided into proteases, breaking down proteins (proteins), amylase, breaking down carbohydrates, and lipases, breaking down fats (lipids). Each enzyme is active only in a certain environment (acidic, alkaline, or neutral). As a result of breakdown, amino acids are obtained from proteins, glycerol and fatty acids from fats, and glucose mainly from carbohydrates. Water, mineral salts, vitamins contained in food do not undergo changes during the digestion process.

Digestion in oral cavity

Oral cavity- This is the anterior initial section of the digestive apparatus. With the help of teeth, tongue and cheek muscles, food undergoes initial mechanical processing, and with the help of saliva - chemical processing.

Saliva - digestive juice weak alkaline reaction, produced by three pairs of salivary glands (parotid, sublingual, submandibular) and entering the oral cavity through the ducts. In addition, saliva is secreted salivary glands lips, cheeks and tongue. In just a day, about 1 liter of saliva of different consistencies is produced: thick saliva is secreted for digesting liquid food, liquid saliva is secreted for dry food. Saliva contains enzymes amylase(ptialin), which breaks down starch to maltose, an enzyme maltase, which breaks down maltose into glucose, and an enzyme lysozoum, which has an antimicrobial effect.

Food in the oral cavity is relatively short time(10-25 s). Digestion in the mouth consists mainly of the formation of a bolus of food prepared for swallowing. The chemical effect of saliva on food substances in the oral cavity is negligible due to the short residence of food. Its action continues in the stomach until the bolus of food is completely saturated with acidic gastric juice. However, processing food in the mouth is of great importance for the further progress of the digestive process, since the act of eating is a powerful reflex stimulator of the activity of all digestive organs. The bolus of food, with the help of coordinated movements of the tongue and cheeks, moves towards the pharynx, where the act of swallowing occurs. From the mouth, food enters the esophagus.

Esophagus- a muscular tube 25-30 cm long, along which, due to muscle contraction, food bolus moves to the stomach in 1-9 seconds, depending on the consistency of the food.

Digestion in the stomach. Stomach- the widest part of the digestive tract. It represents hollow organ, consisting of an inlet, a bottom, a body and an outlet. The inlet and outlet openings are closed with a muscle roller (spike). The volume of an adult's stomach is about 2 liters, but can increase to 5 liters. The inner mucous membrane of the stomach is folded, which increases its surface. In the thickness of the mucous membrane there are up to 25,000,000 glands that produce gastric juice and mucus.

Gastric juice is a colorless acidic liquid containing 0.4-0.5% hydrochloric acid, which activates enzymes gastric juice and provides bactericidal effect on microbes entering the stomach with food. The composition of gastric juice includes enzymes: pepsin,chymosin(rennet), lipase. The enzyme pepsin breaks down food proteins into more simple substances(peptones and albumoses), which undergo further digestion in the small intestine. Chymosin is found in gastric juice infants, coagulating milk protein in their ventricles. Gastric juice lipase breaks down only emulsified fats (milk, mayonnaise) into glycerol and fatty acids.

The human body secretes 1.5-2.5 liters of gastric juice per day, depending on the amount and composition of food. Food in the stomach is digested from 3 to 10 hours, depending on the composition, volume, consistency and method of processing. Fatty and dense foods stay in the stomach longer than liquid foods containing carbohydrates.

The mechanism of gastric juice secretion is a complex process consisting of two phases. The first phase of gastric secretion is conditioned and unconditioned reflex process, depending on appearance, smell and eating conditions. The great Russian scientist-physiologist I.P. Pavlov called this gastric juice “appetizing” or “incendiary”, on which the further course of digestion depends. The second phase of gastric secretion is associated with chemical pathogens of food and is called neurochemical. The mechanism of gastric juice secretion also depends on the action specific hormones digestive organs. In the stomach, partial absorption of water and mineral salts into the blood occurs.

After digestion in the stomach, the food pulp enters in small portions into the initial section of the small intestine - the duodenum, where the food mass is actively exposed to the digestive juices of the pancreas, liver and the mucous membrane of the intestine itself.

Pancreas - digestive organ, consists of cells forming lobules, which have excretory ducts connecting to common duct. Through this duct, the digestive juice of the pancreas enters the duodenum(up to 0.8 l per day). The gland produces digestive enzymes, sodium bicarbonate, which neutralizes stomach (hydrochloric) acid as well as hormones, including insulin and glycagon, that regulate blood sugar.

Digestive juice pancreas is a colorless transparent liquid of an alkaline reaction. It contains enzymes: trypsin, chymotrypsin, lipase, amylase, maltase. Trypsin and chymotrypsin break down proteins, peptones, albumoses coming from the stomach into polypeptides. Lipase With the help of bile, it breaks down food fats into glycerol and fatty acids. Amylase and maltase break down starch into glucose. In addition, the pancreas has special cells (islets of Langerhans) that produce hormone insulin entering the blood. This hormone regulates carbohydrate metabolism, facilitating the absorption of sugar by the body. In the absence of insulin, diabetes mellitus occurs.

Liver- a large gland weighing up to 1.5-2 kg, consisting of cells that produce bile up to 1 liter per day. Bile- liquid from light yellow to dark green, slightly alkaline reaction, activates the enzyme lipase of pancreatic and intestinal juice, emulsifies fats, promotes the absorption of fatty acids, enhances intestinal movement (peristalsis), suppresses putrefactive processes in the intestines.

Bile from the hepatic ducts enters the gallbladder thin-walled pear-shaped bag with a volume of 60 ml. During the digestion process, bile flows from the gallbladder through the duct into the duodenum. In addition to the digestion process, the liver is involved in metabolism and hematopoiesis, retention and neutralization of toxic substances that enter the blood during the digestion process.

Digestion in small intestine

The length of the small intestine is 5-6 m. The digestion process is completed in it thanks to pancreatic juice, bile and intestinal juice secreted by the glands of the intestinal mucosa (up to 2 liters per day).

Intestinal juice is a cloudy liquid of an alkaline reaction, which contains mucus and enzymes: polypeptidases And dipeptidases, breaking down (hydrolyzing) polypeptides into amino acids; lipase, which breaks down fats into glycerol and fatty acids; amylase And maltase, digesting starch and maltose to glucose; sucrase, breaking down sucrose into glucose and fructose; lactase, which breaks down lactose into glucose and galactose.

The main causative agent of the secret activity of the intestines is chemicals contained in food, bile and pancreatic juice.

In the small intestine, food gruel (chyme) is mixed and distributed in a thin layer along the wall, where final process digestion - absorption of digestion products of nutrients, as well as vitamins, minerals, water into the blood. Here aqueous solutions nutrients formed during the digestion process penetrate through the mucous membrane of the gastrointestinal tract into the blood and lymphatic vessels.

In the walls of the small intestine there are special absorption organs - villi, of which there are 18-40 pieces. by 1 mm 2. Nutrients are absorbed through the surface layer of villi. Amino acids, glucose, water, minerals, water-soluble vitamins enter the blood. Glycerol and fatty acids in the walls of the villi form droplets of fat characteristic of to the human body, which penetrate into the lymph and then into the blood. Next, the blood flows through the portal vein to the liver, where, having been cleared of toxic digestive substances, it supplies all tissues and organs with nutrients.

The role of the large intestine in the digestive process.

IN large intestine undigested food remains arrive. A small number of glands of the large intestine secrete inactive digestive juice, which partially continues the digestion of nutrients. The large intestines contain large numbers of bacteria that cause fermentation carbohydrate residues, rotting protein residues and partial breakdown of fiber. In this case, a number of toxic substances harmful to the body are formed (indole, skatole, phenol, cresol), which are absorbed into blood, and then are neutralized in the liver.

The composition of bacteria in the large intestine depends on the composition of the incoming food. Thus, dairy-vegetable foods create favorable conditions for the development of lactic acid bacteria, and food, rich in protein, promotes the development of putrefactive microbes. In the large intestines, the bulk of water is absorbed into the blood, as a result of which the intestinal contents become denser and move towards the outlet. Removal feces from the body is carried out through the rectum and is called defecation.

10.3.1. The main site of lipid digestion is upper section small intestine. The following conditions are necessary for the digestion of lipids:

• presence of lipolytic enzymes;
• conditions for lipid emulsification;
• optimal pH values ​​of the environment (within 5.5 – 7.5).

10.3.2. Various enzymes are involved in the breakdown of lipids. Dietary fats in an adult are broken down mainly by pancreatic lipase; Lipase is also found in intestinal juice, in saliva, in infants, lipase is active in the stomach. Lipases belong to the class of hydrolases; they hydrolyze ester bonds -O-SO- with the formation of free fatty acids, diacylglycerols, monoacylglycerols, glycerol (Figure 10.3).

Figure 10.3. Scheme of fat hydrolysis.

Glycerophospholipids supplied with food are exposed to specific hydrolases - phospholipases, which cleave ester bonds between the components of phospholipids. The specificity of the action of phospholipases is shown in Figure 10.4.

Figure 10.4. Specificity of the action of enzymes that break down phospholipids.

The products of phospholipid hydrolysis are fatty acids, glycerol, inorganic phosphate, nitrogenous bases (choline, ethanolamine, serine).

Dietary cholesterol esters are hydrolyzed by pancreatic cholesterol esterase to form cholesterol and fatty acids.

10.3.3. Understand the structure of bile acids and their role in the digestion of fats. Bile acids are the end product of cholesterol metabolism and are formed in the liver. These include: cholic (3,7,12-trioxycholanic), chenodeoxycholic (3,7-dioxycholanic) and deoxycholic (3, 12-dioxycholanic) acids (Figure 10.5, a). The first two are primary bile acids (formed directly in hepatocytes), deoxycholic acid is secondary (as it is formed from primary bile acids under the influence of intestinal microflora).

In bile, these acids are present in conjugated form, i.e. in the form of compounds with glycine H2N-CH2 -COOH or taurine H2N-CH2 -CH2 -SO3H(Figure 10.5, b).

Figure 10.5. The structure of unconjugated (a) and conjugated (b) bile acids.

15.1.4. Bile acids have amphiphilic properties: hydroxyl groups and side chain are hydrophilic, cyclic structure is hydrophobic. These properties determine the participation of bile acids in the digestion of lipids:

1) bile acids are capable emulsify fats, their molecules with their non-polar part are adsorbed on the surface of fat droplets, at the same time hydrophilic groups interact with the surrounding aqueous environment. As a result, the surface tension at the interface between the lipid and aqueous phases decreases, as a result of which large fat droplets are broken into smaller ones;

2) bile acids, along with bile colipase, are involved in activation of pancreatic lipase, shifting its pH optimum to the acidic side;

3) bile acids form water-soluble complexes with hydrophobic products of fat digestion, which contributes to their absorption into the wall of the small intestine.

Bile acids, which penetrate into the enterocytes during absorption along with hydrolysis products, enter the liver through the portal system. These acids can be re-secreted with bile into the intestines and participate in the processes of digestion and absorption. Such enterohepatic circulation bile acids can be carried out up to 10 or more times a day.

15.1.5. Features of absorption of fat hydrolysis products in the intestine are presented in Figure 10.6. During the digestion of food triacylglycerols, about 1/3 of them are completely broken down to glycerol and free fatty acids, approximately 2/3 are partially hydrolyzed to form mono- and diacylglycerols, and a small part is not broken down at all. Glycerol and free fatty acids with a chain length of up to 12 carbon atoms are soluble in water and penetrate into enterocytes, and from there through portal vein to the liver. Longer fatty acids and monoacylglycerols are absorbed with the participation of conjugated bile acids, forming micelles. Undigested fats can apparently be absorbed by the cells of the intestinal mucosa by pinocytosis. Water-insoluble cholesterol, like fatty acids, is absorbed in the intestine in the presence of bile acids.

Figure 10.6. Digestion and absorption of acylglycerols and fatty acids.

Digestion of proteins

Proteolytic enzymes involved in the digestion of proteins and peptides are synthesized and secreted into the cavity of the digestive tract in the form of proenzymes, or zymogens. Zymogens are inactive and cannot digest the cells' own proteins. Proteolytic enzymes are activated in the intestinal lumen, where they act on food proteins.

In human gastric juice there are two proteolytic enzymes - pepsin and gastrixin, which are very similar in structure, which indicates their formation from a common precursor.

Pepsin is formed in the form of a proenzyme - pepsinogen - in the main cells of the gastric mucosa. Several pepsinogens with similar structures have been isolated, from which several varieties of pepsin are formed: pepsin I, II (IIa, IIb), III. Pepsinogens are activated with the help of hydrochloric acid secreted by the parietal cells of the stomach, and autocatalytically, i.e. with the help of the resulting pepsin molecules.

Pepsinogen has a molecular weight of 40,000. Its polypeptide chain includes pepsin (molecular weight 34,000); a fragment of a polypeptide chain that is a pepsin inhibitor (molecular weight 3100), and a residual (structural) polypeptide. The pepsin inhibitor has sharply basic properties, as it consists of 8 lysine residues and 4 arginine residues. Activation consists of the cleavage of 42 amino acid residues from the N-terminus of pepsinogen; First, the residual polypeptide is cleaved off, followed by the pepsin inhibitor.

Pepsin belongs to carboxyproteinases containing dicarboxylic amino acid residues in the active site with an optimum pH of 1.5-2.5.

Pepsin substrates are proteins, either native or denatured. The latter are easier to hydrolyze. Denaturation of food proteins is ensured by cooking or the action of hydrochloric acid. The following should be noted biological functions of hydrochloric acid:

1. pepsinogen activation;
2. creating an optimum pH for the action of pepsin and gastricsin in gastric juice;
3. denaturation of food proteins;
4. antimicrobial action.

The own proteins of the stomach walls are protected from the denaturing effect of hydrochloric acid and the digestive action of pepsin by a mucous secretion containing glycoproteins.

Pepsin, being an endopeptidase, quickly cleaves internal peptide bonds in proteins formed by the carboxyl groups of aromatic amino acids - phenylalanine, tyrosine and tryptophan. The enzyme hydrolyzes peptide bonds between leucine and dicarboxylic amino acids more slowly: in the polypeptide chain.

Gastricin close to pepsin in molecular weight (31,500). Its optimum pH is about 3.5. Gastricsin hydrolyzes peptide bonds formed by dicarboxylic amino acids. The pepsin/gastricsin ratio in gastric juice is 4:1. At peptic ulcer the ratio changes in favor of gastricsin.

The presence of two proteinases in the stomach, of which pepsin acts in a strongly acidic environment, and gastrixin in a moderately acidic environment, allows the body to more easily adapt to dietary patterns. For example, vegetable and dairy nutrition partially neutralizes the acidic environment of gastric juice, and the pH favors the digestive action of gastricsin rather than pepsin. The latter breaks down the bonds in food protein.

Pepsin and gastrixin hydrolyze proteins into a mixture of polypeptides (also called albumoses and peptones). The depth of protein digestion in the stomach depends on the length of time food is in it. Usually this is a short period, so the bulk of the proteins are broken down in the intestines.

Intestinal proteolytic enzymes. Proteolytic enzymes enter the intestine from the pancreas in the form of proenzymes: trypsinogen, chymotrypsinogen, procarboxypeptidases A and B, proelastase. Activation of these enzymes occurs through partial proteolysis of their polypeptide chain, i.e., the fragment that masks the active center of proteinases. Key process activation of all proenzymes is the formation of trypsin (Fig. 1).

Trypsinogen coming from the pancreas is activated by enterokinase, or enteropeptidase, which is produced by the intestinal mucosa. Enteropeptidase is also secreted as a kinase gene precursor, which is activated by bile protease. Activated enteropeptidase quickly converts trypsinogen into trypsin, trypsin carries out slow autocatalysis and quickly activates all other inactive precursors of pancreatic juice proteases.

The mechanism of trypsinogen activation is the hydrolysis of one peptide bond, resulting in the release of an N-terminal hexapeptide called trypsin inhibitor. Next, trypsin, breaking peptide bonds in other proenzymes, causes the formation of active enzymes. In this case, three types of chymotrypsin, carboxypeptidase A and B, and elastase are formed.

Intestinal proteinases hydrolyze peptide bonds of food proteins and polypeptides formed after the action of gastric enzymes to free amino acids. Trypsin, chymotrypsins, and elastase, being endopeptidases, promote the rupture of internal peptide bonds, breaking up proteins and polypeptides into smaller fragments.

• Trypsin hydrolyzes peptide bonds formed mainly by the carboxyl groups of lysine and arginine; it is less active against peptide bonds formed by isoleucine.
• Chymotrypsins are most active against peptide bonds in the formation of which tyrosine, phenylalanine, and tryptophan take part. In terms of specificity of action, chymotrypsin is similar to pepsin.
• Elastase hydrolyzes those peptide bonds in polypeptides where proline is located.
• Carboxypeptidase A is a zinc-containing enzyme. It cleaves C-terminal aromatic and aliphatic amino acids, and carboxypeptidase B contains only C-terminal lysine and arginine residues.

Enzymes that hydrolyze peptides are also present in the intestinal mucosa, and although they can be secreted into the lumen, they function primarily intracellularly. Therefore, hydrolysis of small peptides occurs after they enter the cells. Among these enzymes are leucine aminopeptidase, which is activated by zinc or manganese, as well as cysteine, and releases N-terminal amino acids, as well as dipeptidases, which hydrolyze dipeptides into two amino acids. Dipeptidases are activated by cobalt, manganese and cysteine ​​ions.

A variety of proteolytic enzymes leads to the complete breakdown of proteins into free amino acids, even if the proteins were not previously exposed to pepsin in the stomach. Therefore, patients after surgery for partial or complete removal of the stomach retain the ability to absorb food proteins.

Mechanism of digestion of complex proteins

The protein part of complex proteins is digested in the same way as simple proteins. Their prosthetic groups are hydrolyzed depending on their structure. The carbohydrate and lipid components, after they are cleaved from the protein part, are hydrolyzed by amylolytic and lipolytic enzymes. The porphyrin group of chromoproteins is not cleaved.

Of interest is the process of breakdown of nucleoproteins, which are rich in some foods. The nucleic component is separated from the protein in the acidic environment of the stomach. In the intestine, polynucleotides are hydrolyzed by intestinal and pancreatic nucleases.

RNA and DNA are hydrolyzed under the action of pancreatic enzymes - ribonuclease (RNase) and deoxyribonuclease (DNase). Pancreatic RNase has an optimum pH of about 7.5. It cleaves internal internucleotide bonds in RNA. In this case, shorter polynucleotide fragments and cyclic 2,3-nucleotides are formed. Cyclic phosphodiester bonds are hydrolyzed by the same RNase or intestinal phosphodiesterase. Pancreatic DNase hydrolyzes internucleotide bonds in DNA supplied with food.

Products of hydrolysis of polynucleotides - mononucleotides are exposed to enzymes intestinal wall: nucleotidases and nucleosidases:

These enzymes have relative group specificity and hydrolyze both ribonucleotides and ribonucleosides and deoxyribonucleotides and deoxyribonucleosides. Nucleosides, nitrogenous bases, ribose or deoxyribose, H 3 PO 4 are absorbed.