High-fat diets reduce lymphocyte proliferation compared to low-fat diets, but the precise effects depend on the amount and type of fat. There are two major classes of polyunsaturated fatty acids (PUFAs) the n-6 and the n-3 families. Linoleic acid is the precursor of the n-6 family, and is found in plant oils, including corn and soybean oil. In animals, linoleic acid is converted to arachidonic acid, which can account for 25% of the total fatty acids in the plasma membranes of immune cells. The amount of arachidonic acid in the plasma membrane of immune cells is important because it is the precursor of several prostaglandins and leukotrienes that have potent inflammatory effects. The precursor of the n-3 PUFAs is a-linolenic acid, which in animal tissues is converted to eicosapentaenoic and docosahexaenoic acids. As op posed to n-6 PUFAs, which are inflammatory, n-3 PUFAs are anti-inflammatory.

Diets rich in n-3 PUFAs decrease inflammation in at least two ways. First, diets rich in n-3 PUFAs increase membrane levels of eicosapentaenoic and docosahexa-enoic acids at the expense of arachidonic acid. Thus, when immune cells are stimulated, there is less arachi-donic acid available to generate prostaglandins and leukotrienes, which are inflammatory in nature. Second, eicosapentaenoic acid is a substrate for the same enzymes that metabolize arachidonic acid. However, the products of eicosapentaenoic acid metabolism are less inflammatory than those derived from arachidonic acid.

Although it may be useful to consume high levels of n-3 PUFAs to decrease inflammation associated with autoimmune and neoplastic disease, or to reduce the risk of heart disease, these conditions are not especially relevant to food-animal production, and the immunosup-pression may render animals more susceptible to infectious disease. Thus, inclusion of fish or other n-3 PUFA-rich oils in animal diets should be approached with caution to avoid increased incidence of infections.

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