Studies in Adults With Elevated Cholesterol Levels
Although there have been some conflicting results in studies in adults with elevated serum cholesterol levels, most studies report total and LDL cholesterol reductions after the addition of soy protein to a diet that is low in saturated fat and cholesterol. In a recent study, postmenopausal women on a diet low in saturated fat and cholesterol (NCEP Step I diet) consumed 40 g/d of soy protein with either 56 or 90 mg of isoflavones daily or casein for 6 months. Both soy groups had significantly better blood lipid profiles (average change from baseline, 8.2% decrease in non-HDL cholesterol and a 4.4% increase in HDL cholesterol) than the casein group. However, no differences in lipids were seen between the 2 isoflavone levels.15 HDL significantly increased 7% from baseline with consumption of 32 g soy protein as soymilk in both women and men with hypercholesterolemia.16 Crouse et al17 reported reductions of 4% and 6% in total and LDL cholesterol, respectively, in hypercholesterolemic individuals consuming 25 g soy protein with isoflavones (see below) as part of a diet low in saturated fat and cholesterol.
A 9-week human study comparing the effects of soy protein (25 g/d) containing varying levels of isoflavones with those of casein found that consumption of the highest isoflavone level (62 mg/d) resulted in significantly lower total and LDL-cholesterol values than those of the casein group. Subjects with the highest LDL-cholesterol levels (top 50%) also experienced significant decreases in total and LDL cholesterol with 37 mg/d of isoflavones. However, those consuming soy protein with lower isoflavone levels (≤27 mg/d) did not have any significant cholesterol-lowering effect.17
Researchers have also addressed the question of the threshold of dietary soy protein needed to reduce cholesterol. A dose-response study in hypercholesterolemic men on an NCEP Step I diet used 20, 30, 40, or 50 g/d of soy protein compared with casein. After 6 weeks, all levels of soy consumption led to significantly greater reductions in non-HDL cholesterol (1.5% to 4.5%) than did the casein, with higher levels being more effective.18 An earlier study by Bakhit et al19 showed cholesterol lowering with as little as 25 g/d of ISP in hypercholesterolemic but not normocholesterolemic men. Thus, 20 to 50 g soy protein/d improved blood lipid levels (1.5% to 4.5%) in mildly hypercholesterolemic persons. The FDA recently published its final ruling on a food-labeling health claim for soy protein and cholesterol reduction stating that 25 g/d of soy protein, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease.7