Dietary protein and atherosclerosis

Interest in the effect of protein on lipid metabolism and atherosclerosis dates back to the first decade of this century. In the 1940s Meeker and Kesten showed that soy protein was more atherogenic for rabbits than casein. Carroll and his colleagues demonstrated that, in general, proteins of animal origin were more cholesterolemic for rabbits than were plant proteins, although there was a wide range of individual effects within both classifications. Carroll also showed that partial hydrolyzates of casein or soy protein were less cholesterolemic than the intact proteins. We have hypothesized that the lysine/arginine ratio (L/A) of the protein influences its effects on lipid metabolism. The L/A of casein is about 2 and that of soy about 1. Addition of enough arginine to casein to lower its L/A to 1 reduces its atherogenicity; adding enough lysine to soy isolate to raise its L/A to 2 enhances atherogenicity. The atherogenicities of fish protein (L/A, 1.44), casein (L/A, 1.94) and whole milk protein (L/A, 2.44) are correlated directly with their L/A (p<0.05). Other amino acids (methionine, glycine, arginine) have been shown to affect cholesterolemia. Casein lowers the excretion of steroids in rabbits and lengthens the turnover time of cholesterol. This may be the mechanism underlying the animal/vegetable protein effects.