Fats are insoluble in water : This inability to dissolve in water makes it difficult for fat digestion. Fats are hydrophobic in nature so, it tends to clump together and form large droplets as it moves through the digestive system. By the time fat reaches the small intestine, it has not been digested at all. So, dietary fat in the small intestine looks like a fairly large globule of fat.

These globules remain undigested until the bile, that is produced in the liver and stored in the gallbladder, mixes with the large fat droplets. Bile contains bile salts, which act as an emulsifier of lipids. The term ’emulsify’ means to break large fat droplets into smaller droplets. The bile salts break up the fat molecules and coat the fat to form much finer droplets. These finer droplets have more surface area, and this aids digestion because the fat-digesting enzyme pancreatic lipase can only act on the surface of the fat droplet. The enzymes of the small intestine are responsible for almost all of the fat digestion.

Epithelial Cells of the Small Intestine

Absorption takes place through the mucosal lining of the small intestine, and when these products pass through the mucosa, they enter the epithelial cells. Once inside the cells, the free fatty acids and monoglycerides enter the endoplasmic reticulum, which is a system inside the cell whose functions include synthesis and transport of lipids. Here, the digested products are resynthesized into triglycerides.

Triglycerides are the major form of fat stored by the body. So, an important fact to point out is that fats are reassembled in the epithelial cells of the small intestine. This makes fat absorption different than absorption of proteins and carbohydrates. With protein and carbohydrate absorption, we see their basic units, which are amino acids and monosaccharides, are able to pass through the intestinal epithelial cells without being altered.

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There are certain theories regarding the digestion and absorption of fats They are-

Lipolytic Theory ( given by Verzar)

It says that, during digestion, fats are completely hydrolyzed to glycerol and fatty acids. They are absorbed into the cells of the intestinal mucosa, where they are re-synthesized into fat inside the endoplasmic reticulum of the cells. About 70% of this fat finds a way into the lymph and 30% into the bloodstream. The lymph containing fat appears milky and is called as chyle.

Glycerol is water soluble, so it is easily absorbed through the intestinal mucosa, but fats are insoluble so there is a different way of its absorption.  During the digestion of fats in the small intestine, the fats are mixed with bile (produced by the liver and stored in the gallbladder). It is believed that after digestion of fats, fatty acids unite with bile salts (bile salts are derived from cholesterol and are a major end product of cholesterol metabolism)to form a complex, which is soluble and passes through the lining of villi. As soon as the absorption takes place, this complex breaks down and fatty acids are released from fats and the bile salt is carried to the liver to re-secrete bile.

Before triglycerides actually leave the epithelial cells, they’re coated by proteins, which results in the formation of chylomicrons, they are small globules composed of protein and lipid. The coating of protein gives the triglyceride a water-soluble coat, and this allows the chylomicron to travel outside of the cell. The newly formed chylomicrons leave the epithelial cell and enter the lymphatic capillaries, which are called lacteals. The lacteals are found in the fingerlike projections of the intestinal wall, called the villi. The lacteals represent another unique way fats are absorbed because lipids pass through the lymphatic system before they make their way back to your bloodstream.


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Partition Theory (Given by Frazer)

It says that fats are not completely hydrolyzed during digestion. Those molecules that are hydrolyzed are only partially broken down into mono and diglycerides and some fatty acids. Some of these glycerides and fatty acids form a complex with the bile salts which are very powerful emulsifying agents. As a result, the undigested fats together with mono and diglycerides are emulsified and absorbed directly into the lymph. The fatty acids are absorbed into the blood as a fatty acid bile salts complex and carried to the liver.

Absorption of sterols

The only sterol found in the foods that are absorbed by the human intestine is cholesterol. This is found in animal fats. It is absorbed into the lymph in combination with the bile salts and also as cholesterol esters. The D Vitamins which are sterol derivatives are also absorbed. Likewise, certain sex hormones and adrenal hormones used as medicines are absorbed.

Assimilation of Fats

The fat is stored in the fat deposits of the body, such as subcutaneous layers, mesenteries, etc. The fat stored is a readily available source of fuel for the cells. Fat has important insulating properties in connection with the conservation of heat and maintenance of body temperature.

Fat also plays a protective role as filling or around packing material and between organs. In the liver, phospholipids are formed which are returned to the blood to be used by all the cells. In the liver cells, the fats are converted into amino acids and carbohydrates. Vitamins, salts, and water are also useful for various metabolic processes.Fat is a good storage compound – it releases twice as much energy as carbohydrates when respired, and act as insulation in the skin. Some nerve cells form a myelin sheath from fat, to prevent electrical impulses from leaking out.

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