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Digestion of fat is intricate because fats are water-insoluble. Mechanical breakdown, emulsification by bile salts, chemical breakdown primarily by pancreatic lipase to monoglycerides and free fatty acids, micelle formation for transport, absorption into enterocytes, re-synthesis into triglycerides, packaging in chylomicrons, and transport via lymph into the circulation.
Typology: Lecture notes
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Fats, also scientifically referred to as lipids, are an integral component of our diet. They occur in such foods as butter, oils, nuts, avocados, and fatty flesh.
They supply us with considerable energy, facilitate the absorption of certain vitamins (A, D, E, K), and serve to form cell membranes (1). The principal form of fat we consume is a triglyceride. Visualize the molecule of glycerol (a small backbone) with three fatty acid chains in addition to it – that is, a capital 'E' with a short vertical line and three horizontal lines of lengthy proportions (1, 2). The major issue with fats is that they're hydrophobic, i.e., they "shun" water and are not miscible in it. Our digestive tract is largely an aqueous (watery) medium. Hence, digestion and absorption of fats require some unique steps (2).
action continues mechanical digestion, breaking down fat into smaller droplets. The stomach also secretes gastric lipase. The enzyme also acts on triglycerides, particularly those with short or medium- chain fatty acids. Again, fat digestion is limited here, as in the mouth, and constitutes perhaps 10- 30% of total fat digestion (1, 2). The stomach's acid environment assists to a small extent. Example: Cheese in your belly mixes up, and gastric lipase clips a few more fat molecules.
Processing Factory! This is where the serious action occurs for fat digestion.
Most of it happens in the duodenum, the beginning of the small intestine (2).
When the fatty chyme (partially digested food) arrives in the small intestine, it stimulates the release of a hormone named cholecystokinin (CCK). CCK instructs the gallbladder to release bile (2, 4). Bile is produced in the liver and is stored in the gallbladder. Bile functions like soap or a detergent. It emulsifies large globules of fat into minute microscopic droplets. It doesn't digest the fat chemically, but it greatly increases the surface area of the fat droplets so that it is much easier for enzymes to act upon them (1, 4).
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Other small pancreatic enzymes such as cholesterol esterase (hydrolyzes cholesterol esters) and phospholipase A (hydrolyzes phospholipids) are also involved (1).
The products of fat digestion (monoglycerides, free long-chain fatty acids) and bile salts, cholesterol, and fat- soluble vitamins aggregate to become small water-soluble packets known as micelles (1, 2). Micelles contain a core that holds the fatty material and an outer surface that can dissolve in water. This enables them to bring the digested fats across the watery environment of the intestine to the intestinal absorptive cells' surface (enterocytes).
Body
The micelles migrate to the brush border (intestinal cell surface). The free fatty acids and monoglycerides here diffuse out of the micelles and enter the cell membrane into the enterocytes (1, 5). Short-chain fatty acids are absorbed into the bloodstream without the requirement for micelles. Bile salts are primarily left behind in the intestinal lumen and reabsorbed lower down in the ileum to be recycled by the liver (enterohepatic circulation) (2).
When within the enterocyte, something unusual occurs: The monoglycerides and long-chain fatty acids are re-esterified – they are rebuilt back into triglycerides (1, 5).
The lacteals are tiny lymph vessels in the center of each intestinal villus (1, 2). The lymph (fluid within the lymphatic system) that these chylomicrons are in becomes milky white and is referred to as chyle. The chyle makes its way into the lymphatic system and finally into the blood supply close to the heart (through the thoracic duct) (1, 2). So most dietary fat never even sees the liver at first.
Once in the blood, chylomicrons distribute their triglycerides to tissues throughout:
There is an enzyme known as lipoprotein lipase, located within the walls of the
blood vessels in the tissues, which hydrolyzes the triglycerides in chylomicrons so that the fatty acids may enter the cells (1).
Digestion of fat is intricate because fats are water-insoluble. Mechanical breakdown, emulsification by bile salts, chemical breakdown primarily by pancreatic lipase to monoglycerides and free fatty acids, micelle formation for transport, absorption into enterocytes, re-synthesis into triglycerides, packaging in chylomicrons, and transport via lymph into the circulation (1, 2, 5). References 1: Gropper SS, Smith JL. Advanced nutrition and human metabolism. 7th ed. Boston (MA): Cengage Learning; 2018.
