Plasma amino acids and the insulin/glucagon ratio as an explanation for the dietary protein modulation of atherosclerosis.

The amino acid composition of the diet influences the postprandial levels of plasma amino acids along with the hormones insulin and glucagon in humans fed single test meals identical in composition except for protein source. Soy protein (hypocholesterolemic), vs. casein (hypercholesterolemic), contains a higher amount of arginine and glycine and induces an increase in postprandial arginine and glycine. Soy protein induces a low postprandial insulin/glucagon ratio in both hypercholesterolemic and normocholesterolemic subjects. Casein induces a high postprandial insulin/glucagon ratio among hypercholesterolemic subjects. Amino acids such as arginine and glycine are associated with a decrease, while lysine and branched-chain amino acids are associated with increased serum cholesterol levels. Our data are consistent with the hypothesis that the control of cholesterol by insulin and glucagon is regulated by dietary and plasma amino acids. From this hypothesis the insulin/glucagon ratio is proposed as an early metabolic index of the effect of dietary proteins on serum cholesterol levels, a risk factor and a common mechanism through which dietary and lifestyle factors influence cardiovascular disease.