Fats that enter the body from the outside are very important for maintaining its proper structure and normal functioning. Digestion and absorption takes place in the duodenum. It is there that water-soluble lipolytic enzymes meet with the lipids themselves. But for the interaction of the fatty and aqueous phases, intermediaries are needed - emulsifiers. And fat emulsification is the process of mixing two phases and dispersing large fat droplets into microscopic ones.
Fat Digestion
A person consumes about 80-100 grams of fat per day, consisting of cholesterol esters, triacylglycerols and phospholipids. Of these, approximately 70% are animal fats and 30% are vegetable. Sources of fats are very numerous - these are oils, fruits and vegetables, nuts.
Assimilation of fats, like other substances in our body, is a complex and multi-stage process. For its normal functioning it is necessaryparticipation of the entire gastrointestinal tract, liver with gallbladder, pancreas, hormones and enzymes, as well as other substances contained in food. And in order for fat to be emulsified in the intestines, food must turn into chyme and go through a series of stages.
Oral cavity
Incoming food is processed by saliva, chewed and meets with the first enzymes. Chyme is formed, a food lump. The environment in the oral cavity is alkaline. In this section of the gastrointestinal tract, the breakdown of simple carbohydrates (monosaccharides) actively begins.
And although there are glands on the back of the tongue that excrete lipase, lipid digestion in adults is practically not observed here. This is due to its small amount. Therefore, the enzyme simply mixes with chyme and passes through the esophagus to the stomach.
Lipolysis in children
In infants, the amount of tongue lipase is much higher. It begins to stand out when the baby sucks the breast. Up to a year, it is this lipase that plays the main role in the digestion of breast milk fats. At this age, the peak of its activity occurs at pH 4-4, 5, respectively, in the stomach for this ideal conditions. Milk itself is an emulsified solution, i.e. water and small drops of fat are mixed in it. Therefore, the emulsification of fats in children actively begins with the oral cavity and continues in the stomach, which significantly accelerates the absorption of milk.
Stomach
In the stomach of an adult, a very acidic environment is normally pH 1-1, 5. And for maximumthe activity of tongue lipase requires a pH level of 5.5-7.5. Therefore, the enzyme that is part of the chyme is inactive and its role in lipid digestion is extremely insignificant. In the stomach, food is exposed to the action of acid, and active digestion of proteins and carbohydrates begins here. But since the emulsification of fats occurs under the action of bile, it is the lipids that pass almost unchanged, non-emulsified into the lower sections of the gastrointestinal tract. However, the small amount of fat that is absorbed at this stage is converted into free fatty acids. And they, in turn, enter the intestine and facilitate emulsification, preparing chyme lipids for the action of pancreatic lipase.
The duodenum
First, when chyme enters the duodenum, two hormones are released - secretin and cholecystokinin. Under the action of the first from the pancreas, bicarbonate ions actively begin to flow. They neutralize the gastric hydrochloric acid that came with the chyme. At the same time, the released carbon dioxide contributes to the softening and mixing of the food bolus. Next, the processed alkaline food bolus is ready for the breakdown of fats.
In the intestinal lumen there is always pancreatic juice with lipases, intestinal juice. After eating fatty foods, the gallbladder contracts under the action of cholecystokinin and bile is ejected into the lumen of the duodenum. It contains a large amount of bile acids, cholesterol and phospholipids.
So it's safe to saythat the emulsification of fats occurs under the action of a whole complex of active substances. But due to the fact that pancreatic lipases are hydrophilic and fats are hydrophobic, without an emulsification process, reactions between them can only occur at the interface, and this is a very small surface. Emulsification of fats breaks large fat droplets into tiny ones, the phase interface, respectively, increases many times over. Active digestion by lipolytic enzymes becomes possible. Consider the process in more detail.
Emulsification of fats with bile
Bile is produced by hepatocytes of the liver and enters the gallbladder. In it, it accumulates until the receipt of fatty foods in the stomach. After that, it contracts and bile flows into the duodenum.
Bile acids are essential for emulsifying fats. They are formed in the liver cells, hepatocytes, and from there they enter the gallbladder. There they are present in the composition of paired s alts - glycocholeate and taurocholeate.
There are several varieties of bile acids, but they all consist of two components (paired bile acids): bile acid proper and glycine - 70% (glycocholic, glycodeoxycholic and glycochenodeoxycholic acids) or taurine - 30% (taurocholic, taurodeoxycholic, taurochenodeoxycholic acid).
Micelle formation
Accordingly, during peristalsis in the duodenum, emulsification of fats occurs, wherethe components necessary for it are mixed: the molecules of triacylglycerols and cholesterol esters are hydrophilic and are attracted to each other. Phospholipids are attracted to them with their hydrophilic part - a core is formed. With their hydrophobic part, phospholipids diverge to the sides, thus forming the outer surface. This is what an emulsified tiny drop of fat looks like, that is, a micelle. The particle diameter is about 0.5 µm. Hydrophobic bile acids stick to the outer surface of the micelles, they seem to form a film on the surface of the bubbles, which prevents them from sticking together.
Additionally, bile s alts at the interface reduce the surface tension, whereby the resulting fat emulsion becomes stable. That is, in fact, they act as a natural stabilizer in the body. After that, all conditions are created for further digestion of lipids with the help of pancreatic enzymes.
This or similar emulsification of fats occurs in the body of all mammals, and it is especially relevant for those who consume animal fats, because their absorption is much slower due to the complex structure of the molecules.
Small intestine
After the formation of a finely dispersed solution with primary micelles, their processing with pancreatic enzymes begins. At pH 8, 0, a hydrolysis reaction of ester bonds occurs in triacylglyceride molecules. As a result of the reaction, each molecule of triacylglyceride produces two molecules of fatty acids and one molecule of beta-monoglyceride. A very small amount of glycerol molecules is also formed. After that, already mixed micelles penetrate the intestinal epithelium. In epitheliocytes, the reaction of resynthesis of these substances occurs. Part of the micelles in unchanged form enters the human lymphatic system. The rest in the form of chylomicrons and high-density lipoproteins are sent to the general circulation. These are the main transport forms of fats in the human body. And bile acids from micelles through the cells of the intestinal villi enter the portal vein and from there return to the liver.
Fat emulsification disorders
Sometimes a well-functioning fat absorption system breaks down. They can be at different levels of this system. These are structural disorders of the liver, gallbladder, their ducts and intestines, as well as functional disorders - insufficient production of enzymes, for example.
Possible causes of impaired fat emulsification:
- decrease in the formation of bile in the liver (various liver diseases - acute, chronic and autoimmune hepatitis, cirrhosis, toxic lesions);
- impaired contractility of the gallbladder (dyskinesia and akinesia of the gallbladder);
- blockage of the bladder or its ducts from the inside (possibly with gallstone disease, tumors inside it);
- compression of the gallbladder or duct from the outside (possible in the presence of tumors or an increase in neighboring organs).
At the same time, the amount of bile acids in the bile decreases or a smaller amount of the substance enters the intestinal lumen. As a result, fewer micelles are formed, and the reactionhydrolysis is slower. Part of the undigested fat passes through the intestines and exits unchanged, steatorrhea occurs. Also, sometimes fats begin to leave the body in a non-traditional way, but, for example, with urine (lipuria) or mainly through the skin (seborrhea). These conditions are symptoms of diseases.
Malabsorption
Malabsorption syndrome is considered the most serious malabsorption disorder. It is of hereditary etiology and acquired. The main pathophysiological characteristic is malabsorption of substances in the small intestine. Symptoms: painful swollen abdomen, diarrhea, steatorrhea, fatty shiny stools. In severe forms, there is a significant weight loss, beriberi, a violation of the structure of the skin and mucous membranes - dryness, inflammation, brittle curved nails, hair loss. The nervous system also suffers - vegetovascular dystonia, weakness, and sleep disorders may occur. In severe cases, fluid begins to accumulate in the abdominal cavity, there may be peripheral edema, paresthesia, visual impairment.
Conclusion
Fat metabolism is a very complex process. In order for the digestive enzymes lipase to be able to process a large amount of lipids, emulsification of fats is necessary. It is she who allows a person to absorb about 100 grams of fat per day. Insufficient emulsification causes painful conditions, including life-threatening ones.
