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Dietary fats serve various needs. They are sources of concentrated energy. Fats provide the building units for biological membranes. Fats or lipids are sources of essential fatty acids and lipid-soluble vitamins. Animals cannot synthesize fatty acids containing double bonds in either the omega-3 (n-3) or the omega-6 (n-6) position. Both linoleic acid (derived from omega-6, 18:2) and linolenic acid (derived from omega-3, 18:3) are usually consumed with plant products. Linoleic acid is converted by animals to arachidonic acid (20:4, n-6) and linolenic acid to eicosapentaenoic acid (EPA, 20:5, n-3). EPA and arachidonic acid can eventually be converted by various tissue lipoxygenases and cyclooxygenases (COX) to a family compound identified as eicosanoids, e.g., prostaglandins, prostacyclins, thromboxanes and leukotrienes. Eicosanoids have profound physiological effects (hormonelike) at extremely low concentrations as well as pharmacological effects at higher concentrations.About 30 to 55% of the dry weight of the hepatic endoplasmic reticulum is lipid, comprising cholesterol esters, free fatty acids, triglycerides, cholesterol, and phospholipids.
Feeding diet deficient in linoleic acid depresses the activities of certain toxicant-metabolizing enzymes, and therefore lipid quality is an important factor. High-fat diets are known to promote the spontaneous incidence of cancer. This may be due, in part, to a low dietary intake of lipotropes. [Choline, methionine, glycine, folate, vitamin B12, pyridoxal, polyunsaturated fatty acids, and phosphate make up the dietary lipotropes, which are required for the synthesis of phospholipids and biological membranes. Dietary deficiencies in the lipotropes choline and methionine lead to a decrease in some cytochrome P450 isomers and to enhanced tumorigenic effects of chemical carcinogens.
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