Significance of the amino acid composition of dietary protein in the regulation of plasma cholesterol.

Three approaches were employed to identify the amino acid residue(s) that is responsible for the different effects of dietary proteins on the plasma cholesterol level in rats fed cholesterol-enriched diets. 1) Experiments on the effects of individual amino acids added to a 25% casein diet showed that sulfur-containing amino acids have the most potent effects on the plasma cholesterol level. Under the dietary conditions used, methionine significantly increased the level of plasma cholesterol while cystine decreased it. It was found that glycine can prevent the methionine-induced enhancement of plasma cholesterol. 2) There was a significant negative correlation between cystine content of dietary proteins and plasma cholesterol levels when animals were fed 7 kinds of animal and plant proteins. 3) Experiments with amino acid mixtures varying in methionine, cystine, and glycine content showed that diets high in methionine and low in cystine and glycine content tend to increase the plasma cholesterol level and diets of opposite amino acid content tend to decrease the plasma cholesterol level. From these results, it was suggested that sulfur-containing amino acids and glycine in dietary proteins are responsible, at least in part, for the alteration of plasma cholesterol level by dietary proteins.