Humans need fat in the diet in order to survive, grow, and prosper. Over 60 years ago, Burr and Burr' demonstrated that fat was an essential component of the diet. Rats reared on a fat-free diet failed to grow and reproduce and also developed renal disease, fatty liver, dermatitis, and necrosis of the tail.

Later studies identified the deficient components of the diet as polyunsaturated fatty acids with two or more double bonds.* Until recently, linoleic acid, an n-6 fatty acid* commonly found in vegetable oils and many other foods, was deemed the primary essential fatty acid, together with its derivative n-6 fatty acids, of which arachidonic acid (20:4) was the most important.

However, it now appears that two classes of fatty acids are both essential for health. The second series of highly polyunsaturated fatty acids includes the n-3 fatty acids, a-linolenic acid (18:3) and its longer-chained, more polyunsaturated derivative, docosahexaenoic acid (22:6). n-3 fatty acids are also contained in a wide variety of natural foodstuffs, including some but not all liquid vegetable oils, particularly soybean and rapeseed oils, and green leafy vegetables.

The higher derivative, docosahexaenoic acid, is found only in the plants of the sea, phytoplankton, or, as one ascends the food chain, in shellfish, fish, and sea mammals.

Docosahexaenoic acid is especially prominent in brain, retina, and spermatozoa but is also found in phospholipid membranes throughout the body, including the myocardium.

Department of Medicine, Oregon Health Sciences University, Portland 97201.
PMID: 1608561

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