Digestion and absorption of fats in the body. Digestion of fats occurs in the intestines Digestion of fats is carried out mainly

The breakdown of fat into glycerol and higher fatty acids is carried out under the influence of the enzyme lipase. For lipase to act on fat, it must be pre-emulsified, which is achieved by mixing food gruel with bile in the intestine.

Fats do not undergo chemical changes in the oral cavity. Lipase is present in the stomach, but its activity is low due to the lack of conditions necessary for fat emulsification. Only emulsified fats—milk and egg yolk fats—are hydrolyzed in the stomach. Basically, the digestion of fat occurs in the intestines and primarily in the duodenum, where bile salts, which have a powerful emulsifying effect, enter along with bile through the ducts.

Bile acids form a thin film on fat droplets , which prevents the merging of individual droplets into larger droplets. This leads to a sharp increase in the contact surface of fat with the lipase enzyme and, consequently, the rate of hydrolytic breakdown of fat. Bile acids include cholic, deoxycholic and others. In their structure they are close to cholesterol. In bile, these acids form paired compounds with glycine (glycocoll) or taurine - glyco- or taurocholic, glyco- or taurodeoxycholic and other bile acids present in the form of sodium salts.

In the cells of the intestinal epithelium, fats, or lipoids specific to a given animal species, are resynthesized from the products of hydrolysis of dietary fats. Synthesized lipids are transported to fat depots. If necessary, fats can pass from fat depots into the blood and be used by tissues as energy material.

MECHANISM OF NEUTRAL FAT OXIDATION IN TISSUE

Neutral fat entering the cells is broken down into glycerol and higher fatty acids by the action of tissue lipases. Subsequently, fatty acids and glycerol are oxidized in tissues to CO2 and H2O, while the released energy accumulates in high-energy bonds of ATP.

OXIDATION OF FATTY ACIDS IN TISSUE. The basis of modern ideas about the breakdown of fatty acids in tissues is the theory of b-oxidation, first put forward by Knoop in 1904. According to this theory, the oxidation of fatty acids occurs at the carbon atom located in the b-position relative to the carboxyl group, followed by rupture of the carbon fatty acid chains between a- and b-carbon atoms. Subsequently, this theory was refined and supplemented.

It has now been established that the oxidation of fatty acids in tissues is preceded by their activation with the participation of coenzyme A and ATP. This process is catalyzed by the enzyme thiokinase.

The activated fatty acid (acyl coenzyme A) undergoes dehydrogenation, resulting in a double bond between the a- and b-carbon atoms. This process occurs with the participation of acyl dehydrogenases, which contain FAD as a prosthetic group. Then a water molecule is added to the unsaturated acid (a, b-unsaturated derivative of acyl-CoA) and a b-hydroxyacid (b-hydroxyacyl-CoA) is formed. Next, the dehydrogenation process occurs again with the formation of b-keto acid (b-ketoacyl-CoA). This process is catalyzed by acyl dehydrogenases, the coenzyme of which is NAD+. And at the last stage, b-ketoacyl-CoA, interacting with free CoA, is cleaved into acetyl-CoA and acyl-CoA. The latter is shortened by two carbons compared to the original.

Daily fat requirement

The amount of fat in the diet is determined by various circumstances, which include labor intensity, climatic conditions, and a person’s age. A person engaged in intense physical labor needs more high-calorie food, and therefore more fat. The climatic conditions of the north, which require a large expenditure of thermal energy, also cause an increase in the need for fats. The more energy the body uses, the more fat is needed to replenish it.

The average physiological need for fat in a healthy person is about 30% of the total calorie intake. With heavy physical labor and a correspondingly high caloric intake of the diet, which ensures such a level of energy expenditure, the proportion of fat in the diet may be slightly higher - 35% of the total energy value.

The normal level of fat intake is approximately 1 -1.5 g/kg, i.e. 70-105 g per day for a person weighing 70 kg. The calculation takes into account all the fat contained in the diet (both as part of fatty products and the hidden fat of all other products). Fatty foods make up half of the fat content of the diet. The second half is accounted for by the so-called hidden fats, i.e. fats that are part of all products. Hidden fats are introduced into certain bakery and confectionery products to improve their taste.

Taking into account the body's need for polyunsaturated fatty acids, 30% of the fat consumed should be vegetable oils and 70% animal fats. In old age, it is rational to reduce the proportion of fat to 25% of the total energy value of the diet, which also decreases. The ratio of animal and vegetable fats in old age should be changed to 1:1. The same ratio is acceptable when the cholesterol level in the blood serum increases.

Dietary sources of fats

Table Sources of unsaturated and monounsaturated fatty acids.

Table Sources of polyunsaturated fatty acids.

Digestion of fats

Enzymes that break down fats are lipases. The effect of lipases on fats becomes possible after emulsification of fats, because Lipids are insoluble in water and they are exposed to lipolytic enzymes only at the interface and, therefore, the rate of digestion depends on the area of ​​this surface. When fats are emulsified, their total surface area increases, which improves the contact of fat with lipase and accelerates its hydrolysis. The main emulsifiers in the body are bile salts.

The synthesis of bile acids occurs on the membranes of the ER of hepatocytes under the action of hydroxylases (cytochromes, which include cytochrome P 450), catalyzing the inclusion of hydroxyl groups at position 7 α, 12 α, followed by shortening of the side radical at position 17 with its oxidation to a carboxyl group, hence the name - bile acids.

Rice. Synthesis and conjugation of bile acids.

Cholic and chenodeoxycholic acids produced in the liver are called primary bile acids. They are esterified with glycine or taurine, yielding paired (or conjugated) bile acids, and in this form are secreted into bile. Bile acids enter the conjugation process in active form in the form of HS-KoA derivatives. Conjugation of bile acids makes them more amphiphilic and thus increases detergent properties.

Bile acids synthesized in the liver are secreted into the gallbladder and accumulate in bile. When eating fatty foods, the endocrine cells of the epithelium of the small intestine produce the hormone cholecystokinin, which stimulates the contraction of the gallbladder, and bile is poured into the small intestine, emulsifying fats and ensuring their digestion and absorption.

When primary bile acids reach the lower small intestine, they are exposed to bacterial enzymes that first cleave glycine and taurine and then remove the 7α-hydroxyl group. This is how secondary bile acids are formed: deoxycholic and lithocholic.

Rice. A. Conjugation of bile acids in the liver. B. Formation of secondary bile acids in the intestine.

About 95% of bile acids are absorbed in the ileum and return through the portal vein to the liver, where they are again conjugated with taurine and glycine and excreted into bile. As a result, bile contains both primary and secondary bile acids. This entire path is called enterohepatic circulation of bile acids. Each molecule of bile acids undergoes 5-8 cycles per day, and about 5% of bile acids are excreted in feces.

Rice. Enterohepatic circulation of bile acids.

Bile acids form Na and K salts, which are the main emulsifiers of fats (they surround a drop of fat and contribute to its fragmentation into many small droplets), making them available for the action of lipases contained in pancreatic juice.

Features of the action

Lingual lipase

Found in infants. Catalyzes the breakdown of emulsified triglycerides of breast milk in the stomach. In adults it is of little significance.

Gastric juice

Lingual lipase

2. Gastric lipase

As part of liquid food (breast milk) received from the oral cavity. Catalyzes the breakdown of emulsified triglycerides in breast milk. In adults it is of little significance.

Catalyzes the breakdown of emulsified triglycerides

Pancreatic juice

1.Pancreatic lipase

2.Colipase

3. Monoglyceride lipase

4. Phospholipase A, lecithinase

5. Cholesterol esterase

In the cavity of the small intestine, it catalyzes the breakdown of triglycerides emulsified by bile. As a result of hydrolysis, 1.2 and 2.3-diglycerides are formed first, and then 2-monoglycerides. One molecule of triglyceride produces two molecules of fatty acids. Can be adsorbed in the glycocalyx of the brush border of enterocytes and participate in membrane digestion.

In interaction with lipase, it catalyzes the breakdown of triglycerides. As a result of hydrolysis, fatty acids, glycerol and monoglycerides are formed.

It is adsorbed in the glycocalyx of the brush border of enterocytes and participates in membrane digestion. Catalyzes the hydrolysis of 2-monoglyceride. As a result of hydrolysis, glycerol and fatty acid are formed.

Catalyzes the breakdown of lecithin. As a result of hydrolysis, diglyceride and choline phosphate are formed.

Catalyzes the breakdown of colesterol esters. As a result of hydrolysis, cholesterol and fatty acid are formed.

Not detected

Lipolytic enzymes exhibit maximum activity at pH = 7.8-8.2.

In an adult, fats in the oral cavity do not undergo chemical changes due to the absence of lipolytic enzymes.

The section in which the bulk of lipids is digested is the small intestine, where there is a slightly alkaline environment that is optimal for lipase activity. Neutralization of hydrochloric acid ingested from food is carried out by bicarbonates contained in pancreatic and intestinal juices:

HCl + NaHCO 3 →NaCl + H 2 CO 3

Carbon dioxide is then released, which foams the food and promotes the emulsification process.

H + + HCO 3 - → H 2 CO 3 → H 2 O + CO 2.

Pancreatic lipase is excreted into the duodenum in the form of an inactive proenzyme - prolipase. Activation of prolipase into active lipase occurs under the influence of bile acids and another enzyme of pancreatic juice - colipase.

Colipase enters the intestinal cavity in an inactive form, and is converted into an active form by partial proteolysis under the influence of trypsin. Colipase binds to the surface of emulsified fat with its hydrophobic domain. Another part of the colipase molecule promotes the formation of such a configuration of the pancreatic lipase molecule, in which the active center of the enzyme is as close as possible to the fat molecules, so the rate of the hydrolysis reaction increases sharply.

Rice. Action of pancreatic lipase.

Pancreatic lipase is a hydrolase that cleaves fatty acids from the α-position of the molecule at a high rate, therefore the main products of TAG hydrolysis are 2-MAG and fatty acids.

A peculiarity of pancreatic lipase is that it acts stepwise: first it cleaves off one IVH in the α-position, and DAG is formed from TAG, then it cleaves off the second IVH in the α-position, and 2-MAG is formed from DAG.

Rice. Cleavage of TAG by pancreatic lipase.

Features of TAG digestion in infants

In infants and young children, milk is the main food. Milk contains fats, which are composed mainly of short- and medium-chain fatty acids (4-12 carbon atoms). The fats in milk are already in emulsified form, so they are immediately available for hydrolysis by enzymes. Milk fats in the stomach of children are affected by lipase, which is synthesized in the glands of the tongue (tongue lipase).

In addition, the stomach of infants and young children produces gastric lipase, which is active at a neutral pH value, characteristic of the gastric juice of children. This lipase hydrolyzes fats by cleaving off mainly fatty acids at the third carbon atom of glycerol. Further, the hydrolysis of milk fats continues in the intestine under the action of pancreatic lipase. Short-chain fatty acids, being water-soluble, are partially absorbed in the stomach. The remaining fatty acids are absorbed in the small intestine.

Rice. Digestion of fats in the gastrointestinal tract.

Digestion of phospholipids

Several enzymes synthesized in the pancreas are involved in the digestion of phospholipids: phospholipase A1, A2, C and D.

Rice. Action of phospholipases.

In the intestine, phospholipids are primarily broken down by phospholipase A2, which catalyzes the hydrolysis of the ester bond at position 2, producing lysophospholipid and fatty acid.

Rice. Formation of glycerophosphocholine under the action of phospholipases.

Phospholipase A2 is secreted as an inactive prophospholipase, which is activated in the small intestine by partial proteolysis by trypsin. The coenzyme of phospholipase A2 is Ca 2+ .

Subsequently, the lysophospholipid is exposed to phospholipase A1, which catalyzes the hydrolysis of the ester bond at position 1, with the formation of glycerophosphatidyl bound to a nitrogen-containing residue (serine, ethanolamine, choline), which

1) or is broken down by the action of phospholipases C and D to glycerol, H 3 PO 4 and nitrogenous bases (choline, ethanolamine, etc.)

2) or remains a glyceropholpholipid (phospholipases C and D do not work) and is included in the micelles.

Digestion of cholesterol esters

In food, cholesterol is found mainly in the form of esters. Hydrolysis of cholesterol esters occurs under the action of cholesterol esterase, an enzyme that is also synthesized in the pancreas and secreted into the intestine.

Cholesterol esterase is produced in an inactive state and is activated by trypsin and Ca 2+. Hydrolysis products (cholesterol and fatty acids) are absorbed in mixed micelles.

Rice. Hydrolysis of cholesterol esters by cholesterol esterase.

Micelle formation

Water-soluble glycerol, H 3 PO 4, fatty acids with the number of carbon atoms less than 10, nitrogen-containing substances are absorbed diffusely into the portal vein.

The remaining hydrolysis products form a micelle, which consists of 2 parts: internal- core, which includes cholesterol, fatty acids with more than 10 carbon atoms, MAG, fat-soluble vitamins and outdoor– the outer shell, which contains bile salts. Bile salts have a hydrophobic group facing inward of the micelle, and a hydrophilic group facing outward, toward the water dipoles.

The stability of micelles is ensured mainly by bile salts. The micelles approach the brush border of the cells of the small intestinal mucosa, and the lipid components of the micelles diffuse through the membranes into the cells. Together with the products of lipid hydrolysis, fat-soluble vitamins A, D, E, K and bile salts are absorbed.

The absorption of medium-chain fatty acids, formed, for example, during the digestion of milk lipids, occurs without the participation of mixed micelles. These fatty acids from the cells of the mucous membrane of the small intestine enter the blood, bind to the protein albumin and are transported to the liver.

Rice. Structure of a micelle.

Bile salt micelles function as transport intermediaries to transport monoglycerides and free fatty acids to the brush border of the intestinal epithelium, otherwise the monoglycerides and free fatty acids will be insoluble. Here monoglycerides and free fatty acids are absorbed into the blood and bile salts are released back into the chyme to be used again for the transport process.

Resynthesis of fats in the mucous membrane of the small intestine

After absorption of the products of fat hydrolysis, fatty acids and 2-monoacylglycerols in the cells of the mucous membrane of the small intestine are included in the resynthesis process with the formation of triacylglycerols. Fatty acids enter into the esterification reaction only in the active form in the form of derivatives of coenzyme A, therefore the first stage of fat resynthesis is the fatty acid activation reaction:

HS CoA + RCOOH + ATP → R-CO ~ CoA + AMP + H 4 P 2 O 7.

The reaction is catalyzed by the enzyme acyl-CoA synthetase (thiokinase). Acyl~CoA then participates in the esterification reaction of 2-monoacylglycerol to first form diacylglycerol and then triacylglycerol. Reactions of fat resynthesis are catalyzed by acyltransferases.

Rice. Formation of TAG from 2-MAG.

As a rule, only fatty acids with a long hydrocarbon chain participate in fat resynthesis reactions. Fat resynthesis involves not only fatty acids absorbed from the intestine, but also fatty acids synthesized in the body, therefore, the composition of resynthesized fats differs from fats obtained from food. However, the ability to “adapt” during the process of resynthesis the composition of dietary fats to the composition of fats in the human body is limited, therefore, when fats with unusual fatty acids, for example, lamb fat, are supplied with food, fats containing acids characteristic of lamb fat (saturated branched fatty acids) appear in adipocytes ). In the cells of the intestinal mucosa, active synthesis of glycerophospholipids occurs, which are necessary for the formation of the structure of lipoproteins - transport forms of lipids in the blood.

Instructions

The digestion process usually begins in the mouth with the help of enzymes contained in saliva. However, this does not apply to fats. There are no enzymes in saliva that can break them down. Next, the food enters the stomach, but even here the fats are not amenable to local digestive enzymes. Only a small proportion is decomposed by the enzyme lipase, very insignificant. The main process of fat digestion occurs in the small intestine.

Fats cannot dissolve in water, but they need to be mixed with water first. Only in this case can they be exposed to enzymes dissolved in water. The process of mixing fats with water is called emulsification, and it occurs with the participation of bile salts. These acids are then secreted into the gallbladder. After fatty foods enter the body, cells in the small intestine begin to produce a hormone that causes contractions of the gallbladder.

The gallbladder releases bile into the duodenum. Bile acids are located on the surface of fat droplets, which leads to a decrease in surface tension. Drops of fat break down into small ones; contractions of the intestinal walls also help this process. As a result, the surface area between the fat and water phases increases. After emulsification, hydrolysis of fats occurs under the influence of pancreatic enzymes. Hydrolysis refers to the decomposition of a substance when it interacts with water.

Next, fat molecules are broken down by the pancreatic enzyme lipase. It is secreted into the cavity of the small intestine and acts on emulsified fat together with the protein colipase. This protein binds to euulsified fat, which significantly speeds up the process. As a result of cleavage by lipase, glycerol and fatty acids are formed.

Fatty acids combine with bile acids and penetrate the intestinal walls. There they combine with glycerol to form a fat triglyceride. Triglyceride, in combination with a small amount of protein, forms special substances, chylomicrons, which penetrate into the lymph. From lymph to blood, then to lungs. These substances contain absorbed fat. Thus, the products of fat breakdown enter the lungs.

The lungs contain cells that can trap fat. They protect the blood from excess fat. Fatty acids are also partially oxidized in the lungs, and the heat released warms the air entering the lungs. From the lungs, chylomicrons enter the blood, from where some move to the liver. A lot of fat accumulates in the liver when it is consumed in excess.