The Influence of Dietary Protein on Plasma Lipids in Human Subjects

Abstract

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 (15), however, noted that the serum cholesterol levels of elderly patients frequently were elevated by increased protein intakes. Olson (16) fed diets supplying 100 g of protein per day followed by isocaloric, iso-fat diets supplying 25 g of protein per day to 9 subjects. The lower serum cholesterol levels observed during the low protein intake were attributed to a low dietary level of choline and sulfur-containing amino acids. In a more recent study Walker, Morse and Overley (17) fed two groups of young female subjects diets supplying 45 to 50 g of protein per day from animal or vegetable sources. The serum cholesterol level decreased in both groups but more so in the group receiving vegetable protein.In view of the existing controversy concerning the influence of dietary protein on serum lipids in man, the present study was initiated. The results reported were obtained in a carefully controlled study employing young male volunteers as subjects. EXPERIMENTAL METHODSThe subjects for this study were 7 healthy, young, male volunteers, whose average age was 21.6 years (range, 19 to 23). Their average initial weight was 77.4 kg (range, 70.2 to 97.5). The subjects were housed in a metabolic ward and were permitted off the ward for supervised activity. Body temperature, blood pressure, and body weight were recorded daily for each man.The study ...