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The assimilation of dietary lipids and the synthesis and degradation of lipids; this article is restricted to mammals.
The principal dietary fat is triglyceride. This substance is not digested in the stomach and passes into the duodenum, where it causes the release of enterogastrone, a hormone which inhibits stomach motility. The amount of fat in the diet, therefore, regulates the rate at which enterogastrone is released into the intestinal tract. Fat, together with other partially digested foodstuffs, causes the release of hormones, secretin, pancreozymin, and cholecystokinin from the wall of the duodenum into the bloodstream.
Secretin causes the secretion of an alkaline pancreatic juice rich in bicarbonate ions, while pancreozymin causes secretion of pancreatic enzymes. One of these enzymes, important in the digestion of fat, is lipase. Cholecystokinin, which is a protein substance chemically inseparable from pancreozymin, stimulates the gallbladder to release bile into the duodenum. Bile is secreted by the liver and concentrated in the gallbladder and contains two bile salts, both derived from cholesterol: taurocholic and glycocholic acids. These act as detergents by emulsifying the triglycerides in the intestinal tract, thus making the fats more susceptible to attack by pancreatic lipase. In this reaction, which works best in the alkaline medium provided by the pancreatic juice, each triglyceride is split into three fatty acid chains, forming monoglycerides. The fatty acids pass across the membranes of the intestinal mucosal (lining) cells. Enzymes in the membranes split monoglyceride to glycerol and fatty acid, but triglycerides are reformed within the mucosal cells from glycerol and those fatty acids with a chain length greater than eight carbons: Short- and medium-chain fatty acids are absorbed directly into the bloodstream once they pass through the intestinal mucosa. See Cholesterol, Digestive system, Gallbladder, Liver, Pancreas, Triglyceride
Obesity is a condition in which excessive fat accumulates in the adipose tissue. One factor responsible for this condition is excessive caloric intake. In starvation, uncontrolled diabetes, and many generalized illnesses the opposite occurs and the adipose tissue becomes markedly depleted of lipid. See Adipose tissue, Diabetes, Lipid, Metabolic disorders, Obesity