Diet and cholesterolemia. V. Effects of sulfur-containing amino acids and protein.

Interrelationships among the effects of dietary protein, sulfur-containing amino acids, and choline on the serum cholesterol concentration of the rat have been studied. Hypercholesterolemia was induced by feeding rats a diet containing cholesterol, cholic acid, and saturated fat. When the diet contained choline, additional casein alleviated the hypercholesterolemia. A similar effect was obtained with a supplement of methionine equal to the amount in the casein. Cystine and cysteine, but not taurine, were as effective as methionine. These and other observations suggest that the serum cholesterol-lowering effect of protein supplements is due largely to the sulfur-containing amino acids they provide. The omission of choline from the diet also alleviated the hypercholesterolemia; but when the choline-free diet was supplemented with either choline or methionine, serum cholesterol concentration increased. A supplement of choline alone caused a much greater rise than a supplement of methionine or combined supplementation with choline and methionine. Thus methionine appears to have two opposing effects on serum cholesterol concentration in rats fed a hypercholesterolemic diet lacking choline: ( a ) a cholesterol-elevating effect attributable to its ability to spare choline by providing a supply of preformed methyl groups, and ( b ) a cholesterol-lowering effect common to sulfur-containing amino acids and not, dependent upon t,he provision of methyl groups.