Comparison of actions of soy protein and casein on metabolism of plasma lipoproteins and cholesterol in humans.

Several reports claim that vegetable proteins cause a lowering of plasma cholesterol when substituted for animal proteins. The present study was carried out to determine whether soy proteins alter plasma lipoproteins or cholesterol balance when compared to casein protein in humans investigated on a metabolic ward. Fourteen men were studied in two periods, each of 1 month's duration. In period I the diet contained 30% of calories as fat (in the form of lard), 55% as carbohydrate and 15% as casein protein. Period II was identical except that soy protein was used instead of casein. Ten patients had normal levels of plasma triglycerides with varying concentrations of plasma cholesterol. The substitution of soy protein for casein protein produced no consistent change in plasma concentrations of cholesterol, triglycerides, low-density lipoprotein, or high-density lipoprotein. Estimated synthetic rates of cholesterol and bile acids also were unchanged. In contrast, soy protein caused a significant lowering of plasma triglycerides in three of four patients with hypertriglyceridemia. Thus, the only significant alteration induced by soy proteins in this study was a reduction of plasma triglycerides when levels were elevated; soy proteins had no discernable effects on the metabolism of cholesterol.