Influence of Dietary Protein on Plasma Lipids

Most studies concerning the influence of dietary protein on serum or plasma lipids have been conducted with experimental animals. A hypocholesterolemic effect has been demonstrated for high dietary protein levels in growing swine, rats, and chicks (1-6). In the rat a hypercholesterolemic diet, containing cholesterol, or cholic acid, or both, is required to demonstrate a substantial depressant effect of high protein on cholesterol; however, this effect has been demonstrated in swine and chicks fed essentially cholesterol-free diets (1, 3, 4). Certain studies have indicated that the hypercholesterolemic effect of low protein is primarily the result of a sulfur amino acid deficiency; Cebus monkeys (7) and rats (8, 9), receiving diets adequate in choline but deficient in sulfur amino acids, develop a hypercholesterolemia that can be overcome by the addition of methionine to the diets. It has been further demonstrated that the serum cholesterol depression resulting from methionine supplementation of a deficient diet is independent of the growth response observed (8, 10) in growing animals. Methionine supplements appear to function differently in choline-deficient rats. Rats fed diets deficient in choline and sulfur amino acids have shown lowered serum cholesterol and elevated liver cholesterol levels; dietary supplementation of either sulfur-containing amino acids, or choline, or both to such animals results in a return of serum and liver cholesterol values to normal levels (11, 12). This hypocholesterolemia resulting from a choline deficiency has not been observed in the chick (6), probably owing to the lack of a lipotropic effect of choline in this species as compared with the rat (13). Studies on the influence of dietary protein and