Short notes on the digestion of fats in the small intestine

Short notes on the digestion of fats in the small intestine

The human diet consists of number of fatty substances and oils. These are digested basically by the enzyme lipases (lipid = fat). The fatty substances are not influenced by the saliva nor is there any fat emulsifying agent in the buccal cavity. The digestion of the fat begins in the stomach where it is partially broken down and it is fully digested in the intestine.

The stomach also does not have any fat emulsifying agent. The gastric juice however consists of gastric lipase which can convert some fats into monoglycerides and fatty acids. This digestion of fats is highly limited in the stomach and most of the fat is digested only in the intestine. In the intestine three secretions of alkaline nature are combined together. These are the bile, the pancreatic juice and the intestinal juice.

The bile juice is secreted by the liver but it does not contain any fat digest­ing enzyme. The bile salts like sodium glycocholate and sodium taurocholate have the capacity to reduce the surface tension of large drop­lets of fat and bring about emulsification. As a result of emulsification, fine droplets of fat called micelles are produced in the intestine.

The ac­tion of bile salts is to break down large droplets of fat into smaller droplets of fats. This makes the function of fat digesting enzymes like lipases easy. The pancreatic juice consists of the pancreatic lipase which is the main fat digesting enzyme. Under the influence of lipase, fats are digested in sev­eral stages ultimately to yield glycerol and fatty acids (Fats are defined as esters of alcohol like glycerol and fatty acids.

Usually three molecules of fatty acids combine with glycerol to form a molecule of fat. Hence fats are generally called triglycerides) In the first stage, the trigyceride breaks up into a diglyceride with the elimination of one fatty acid; in the second stage a diglyceride breaks up to release an­other molecule of fatty acid and a monoglyceride; in the third stage the monoglyceride is hydrolysed to the trihydnc alcohol glycerol and a fatty acid molecule.

The intestinal juice also consists of a fat digesting enzyme intenstinal li­pase. This enzyme is produced by the epithelial cells of the intestine and can hydrolyse fats into glycerol and fatty acids just like the pancreatic lipase.

The end products of fat digestion are fatty acids glycerol and in some cases monoglycerides (fatty acid + glycerol). The glycerol is soluble in water and will be absorbed directly. But the fatty acids are water insoluble. Hence their absorption by the small intestine poses some problems. Two theories have been proposed to account for the absorption of fats. These are the lipolytic theory and the partition theory.

According to lipolytic theory fatty acids which are released from fats due to the action of lipases combine with the bile salts and form soluble complexes which are then absorbed by the intestinal walls. After absorption the complex is broken down to bile salts and fatty acids.

According to the partition theory, fats are not completely hydrolysed and these partially hydrolysed fats form emulsions called micelles under the influence of bile salts. These emul­sions are absorbed by the intestinal wall.