Fats entering the body from the outside are very important for maintaining its proper structure and normal functioning. Digestion and absorption occurs in the duodenum. It is there that water-soluble lipolytic enzymes are found with the lipids themselves. But for the interaction of the fat and water phases, intermediaries are needed - emulsifiers. And emulsification of fats is a process of mixing two phases and the dispersion of large fat droplets into microscopic ones.
Fat digestion
A person consumes about 80-100 grams of fat per day, as part of cholesterol esters, triacylglycerols and phospholipids. Of these, approximately 70% are animal fats and 30% are vegetable. Sources of fat are very numerous - these are oils, fruits and vegetables, nuts.
The absorption of fats, like other substances in our body, is a complex and multi-stage process. For its normal functioning, the participation of the entire gastrointestinal tract, liver with gall bladder, pancreas, hormones and enzymes, as well as other substances contained in food, is necessary. And in order for the emulsification of fats in the intestine to occur, food must turn into a chyme and go through a number of stages.
Oral cavity
The food received is processed by saliva, chewed and found with the first enzymes. Formed chyme, food lump. The environment in the oral cavity is alkaline. In this section of the gastrointestinal tract, the breakdown of simple carbohydrates (monosaccharides) begins actively.
Although there are glands excreting lipase on the back of the tongue, lipid digestion in adults is practically not observed. This is due to its small amount. Therefore, the enzyme simply mixes with the chyme and passes through the esophagus into the stomach.
Lipolysis in children
In infants, the amount of tongue lipase is much greater. She begins to stand out when the baby is sucking her breast. Up to a year, it is this lipase that plays a major 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 begins actively with the oral cavity and continues in the stomach, which greatly speeds up the absorption of milk.
Stomach
In the stomach of an adult, a very acidic environment is normally pH 1-1.5. And for maximum activity of the tongue lipase, a pH level of 5.5-7.5 is required. Therefore, the enzyme that is part of the chyme is inactive and its role in the digestion of lipids is extremely small. In the stomach, food is exposed to acid, here begins the active digestion of proteins and carbohydrates. But since fat emulsification occurs under the action of bile, it is lipids that pass almost unchanged, not emulsified, into the lower sections of the gastrointestinal tract. However, the small amount of fat that is absorbed at this stage turns into free fatty acids. And they, in turn, enter the intestines and facilitate emulsification by preparing chyme lipids for the action of pancreatic lipase.
Duodenum
First, when the chyme enters the duodenum, two hormones are released - secretin and cholecystokinin. Under the action of the first of the pancreas, bicarbonate ions begin to flow actively. They neutralize gastric hydrochloric acid, which came with chyme. In this case, the carbon dioxide emitted helps to soften and mix the food lump. Next, the processed alkaline food lump is ready for the breakdown of fats.
Pancreatic juice with lipases and intestinal juice are constantly present in the intestinal lumen. After eating fatty foods under the influence of cholecystokinin, the gall bladder contracts and bile is thrown into the lumen of the duodenum. In its composition are in large quantities bile acids, cholesterol and phospholipids.
Therefore, we can confidently say that emulsification of fats occurs under the influence of a whole complex of active substances. But due to the fact that pancreatic lipases are hydrophilic and fats are hydrophobic, without the process of emulsification, reactions between them can occur only at the phase boundary, and this is a very small surface. Emulsification of fats breaks up large droplets of fat into smallest ones, and the phase interface, respectively, multiplies. Active digestion with lipolytic enzymes becomes possible. Consider the process in more detail.
Emulsification of fat by bile
Bile is produced by hepatocytes of the liver and enters the gall bladder. In it, it accumulates until the moment fatty foods enter the stomach. After that, it contracts and bile flows into the duodenum.
The main necessary component for emulsification of fats are bile acids. They form in the liver cells, hepatocytes, and from there they enter the gall bladder. There they are present in the composition of paired salts - glycocholeate and taurocholeate.
There are several varieties of bile acids, but all of them consist of two components (paired bile acids): bile acid itself and glycine - 70% (glycocholic, glycodeoxycholic and glycochenodeoxycholic acids) or taurine - 30% (taurocholic, taurodeoxycholic, taurochenodeoxydeoxy).
Micelle formation
Accordingly, with peristalsis in the duodenum, fat emulsification occurs, where the components necessary for it are mixed: the molecules of triacylglycerols and cholesterol esters are hydrophilic and attracted to each other. Phospholipids are attracted to them by their hydrophilic part - a core is formed. With their hydrophobic part, phospholipids diverge to the sides, thus forming the outer surface. It looks like an emulsified tiny drop of fat, that is, a micelle. The particle diameter is about 0.5 microns. Hydrophobic bile acids adhere to the outer surface of the micelles; they seem to form a film on the surface of the bubbles, which prevents their adhesion.
Additionally, bile salts 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 the conditions for further lipid digestion using pancreatic enzymes are created.
This or similar emulsification of fats occurs in the body of all mammals and is especially relevant for those who consume animal fats, because their absorption due to the complex structure of the molecules is much slower.
Small intestine
After the formation of a finely dispersed solution with primary micelles, their treatment with pancreatic enzymes begins. At pH 8.0, the reaction of hydrolysis of ether bonds occurs in the molecules of triacylglycerides. As a result of the reaction, two fatty acid molecules and one beta-monoglyceride molecule are formed from each molecule of the triacylglyceride. A very small number of glycerol molecules are also formed. After that, already mixed micelles penetrate the intestinal epithelium. In epithelial cells, the reaction of resynthesis of these substances occurs. Part of the micelles in an unchanged form enters the human lymphatic system. The rest in the form of chylomicrons and high-density lipoproteins are sent to the general bloodstream. 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 in the debugged system for the absorption of fats, disturbances occur. They can be at different levels of this system. These are structural disorders of the liver, gall bladder, their ducts and intestines, as well as functional ones - insufficient production of enzymes, for example.
Possible causes of impaired emulsification of fats:
- reduction of bile formation in the liver (various liver diseases - acute, chronic and autoimmune hepatitis, cirrhosis, toxic lesions);
- violation of 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 outside (possibly in the presence of tumors or enlargement of adjacent organs).
At the same time, the amount of bile acids in bile decreases or a smaller amount of substance enters the intestinal lumen. As a result, fewer micelles are formed, and the hydrolysis reaction is weaker. Part of the undigested fat passes through the intestine and leaves unchanged, steatorrhea occurs. Also, sometimes fats begin to leave the body not in the traditional way, but, for example, with urine (lipuria) or mainly through the skin (seborrhea). These conditions are symptoms of disease.
Malabsorption
The most serious malabsorption is considered malabsorption syndrome. It is hereditary etiology and acquired. The main pathophysiological characteristic is a violation of the absorption of substances in the small intestine. Symptoms: painful swollen abdomen, diarrhea, steatorrhea, oily shiny stools. In severe forms, there is a significant weight loss, vitamin deficiency, a violation of the structure of the skin and mucous membranes - dryness, inflammation, brittle bent nails, hair loss. The nervous system also suffers - vegetative-vascular dystonia, weakness, sleep disorders can occur. In severe cases, fluid begins to accumulate in the abdominal cavity, there may be peripheral edema, paresthesia, and visual impairment.
Conclusion
Fat metabolism is a very complex process. In order for the digestive enzymes of lipase to be able to process a large amount of lipids, emulsification of fats is necessary. It is it that allows a person to absorb about 100 grams of fat per day. With insufficient emulsification, painful conditions arise, including life-threatening ones.