Lipids, such as oils, fats, cholesterol, triglycerides and fatty acids, are diverse chemical compounds that share the characteristic of being insoluble in water. Lipids form an integral part of your diet as they improve the taste and the satiety value of food. Upon consumption, digestion breaks down dietary lipids into simpler units for absorption. Metabolism of lipids is the process by which the body synthesizes and degrades the lipids for use by the body.
Essential Lipids
Although lipids are generally considered redundant, two fatty acids are essential and must be included in the diet. Linoleic acid, an omega-6 fatty acid, and linolenic acid, an omega-3 fatty acid, are necessary for brain development, blood clotting and to control inflammation. Lipids are required for absorption of fat-soluble vitamins and also insulate and protect the organs from injury. Extra lipids are stored as a source of energy in the adipose tissue.
Digestion of Lipids
Most of the lipids in food are present as triglycerides. Digestion splits them down into free fatty acids for absorption. Even though lipid-digesting enzymes are present in the mouth as lingual lipase, and in the stomach as lipase, very little digestion takes place here. Once the lipids enter the small intestine, bile from the gallbladder emulsifies the lipids and suspends them as tiny globules. Pancreatic lipase, which comes from the pancreas, acts on the emulsified triglycerides and releases free fatty acids.
Transport of Dietary Lipids
The intestinal cells absorb the free fatty acids. Since lipids cannot mix with water-based blood for transport, intestinal cells form chylomicrons, lipoproteins that contain triglycerides. Blood transports the chylomicrons to different parts of the body. When cells need lipids, an enzyme, lipoprotein lipase present in the inner cell walls of the blood vessels, breaks down the chylomicrons. The cells absorb the free fatty acids, which are either used for energy by the muscle cells or stored in the adipose tissues.
Lipid Metabolism
Stored fat become a source of energy when needed. Lipolysis breaks down the stored triglycerides to fatty acids and glycerol. The process of beta-oxidation further metabolizes free fatty acids to form acetyl coenzyme A, which releases energy in the Krebs cycle. On a very low carbohydrate diet, lipids become the main source of energy by the process of ketogenesis in the liver. Another process called lipogenesis synthesizes non-essential fatty acids from the free fatty acids to carry out various functions in the body.
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