1.2. Digestion and absorption of fats

The daily requirement for fats is 50-100 g. They provide up to 50% of an organism's energy requirements.

Digestion of fats occurs in thin intestines. In the duodenum there is a neutralisation of hydrochloric acid of gastric juice got to the intestine with food by the bicarbonates contained in pancreatic and intestinal juice. Bubbles of carbon dioxide excreted thus promote agitating of food mass. There is fat emulsification, basically by the salts ofbile acids which get to a duodenum with bile. Bile contains basically cholic, deoxycholic and chenodeoxycholic acids. Usually they are conjugated with glycine or taurine.

Pancreas secretes inactive prolipase. Its transformation into active lipase occurs with the participation of bile acids and protein of pancreatic juice colipase. Colipase joins lipase and makes its resistant to trypsin influence, and also promotes connection of lipase with micelles. Lipases рН optimum is 8-9, in the presence of bile it shifts to 6.

Lipase is absorbed on the surface of micelles and hydrolyzes an ester group in triacylglycerine. Main degradation products are 2-monoglycerides and fatty acids.

Fatty acids with a short carbon chain and glycerin are well soluble in water. They are absorbed in intestine and move to blood of portal vein, and then into the liver. The absorption of fatty acids with the long carbon chain and of monoglycerides happens with the assistance of bile from micelles. Fatty acids and monoglycerides are absorbed, and salts of bile acids remain in an intestine lumen. Their basic part is absorbed by blood in ileum, enters the liver and then is excreted with bile. This process is called enterohepatic circulation. During the day it occurs 6-8 times.

In cells of an intestinal wall epithelium there is a lipids resynthesis. It is carried out in two ways.

The first is β-monoglyceride pathway. Fatty acids form their active form - acyl-CoA. Then there is an acylation of β-monoglyceride. Reactions are catalyzed by the enzymetic complex triglyceridesynthetase.

The second is α-glycerophosphate pathway. Glycerine is phosphorylated. The donor of the phosphoric acid residue is ATP. Then α-glycerophosphate interacts with acyl-CoA.

If mainly fatty acids enter into cells, synthesis of lipids goes along α-glycerophosphate pathway, if fatty acids with β - monoglycerides - then β-monoglyceride pathway.

Lipids are practically insoluble in water and in organism fluids. Therefore special mechanisms are necessary for their transport.

Transport of lipids is provided by special particles - lipoproteins. They have spherical form. Their superficial part is formed by phospholipids and proteins (apolipoproteins). Phospholipids are oriented with the hydrophilic ends outside. Hydrophobic ends are “solved” in the lipid phase inside particles. Intrinsic lipid phase contains basically triacilglycerins and cholesterol ethers (fig. 1).

Lipoproteins (LP) differ in composition and contents of lipids and proteins. They density increases with increasing of protein content. LP can be separated by centrifugation. They also differ by the electrophoretic mobility. There are divided into:

- chylomicrons (CM) (diameter - 0,1-5 microns);

- very-low density lipoproteins (VLDL);

- low-density lipoproteins (LDL):

- high-density lipoproteins(HDL).

Fig. 1. Structure of lipoprotein

Lipoproteins are formed in cells of mucosa of intestine (chylomicrons and VLDL), in blood plasma (LDL and HDL), in hepatocytes (VLDL and HDL). Chylomicrons and VLDL serve basically for transport of fats along the bloodstream, and LDL and HDL - for cholesterol transport. The fats synthesised in cells of intestine, are included mainly in CM. CM enter the lymphatic system and then into the general blood stream.

In about 1-2 hours after intake of fatty meal we observe alimentary hyperlipemia. This is a physiological phenomenon when the concentration of triglycerides in blood rises, and CM appears in it. Its peak is observed in 4-6 hours, and after 10-12 hours the number of triglycerides comes to norm. CM disappear from blood.

Triglycerides of CM are hydrolyzed on the surface of endothelium capillars of adipose tissue under the action of the enzyme lipoprotein lipase. Fatty acids and glycerine are formed. Glycerine is transported into liver where it can be used for synthesis of fats. The most of fatty acids penetrates into tissues. In adipose tissue they are deposited in the form of fats, in a cardiac muscle and working skeletal muscles they are used as an energy source. Residual chylomicrons are captured by liver cells.