Effects of including soy protein in a blood cholesterol-lowering diet on markers of cardiac risk in men and in postmenopausal women with and without hormone replacement therapy

Background: Increased consumption of soy foods has been associated with reduction in low-density lipoprotein cholesterol (LDL-C) in both clinical and observational studies. However, it is not clear whether adding soy to a low-fat diet has additional lipid-lowering benefits, and few previous studies have examined these effects in women using hormone replacement therapy (HRT⁺). Methods: We tested whether adding soy protein to a low-fat, high-fiber, Step I diet improved cardiac risk markers in 18 postmenopausal women and 14 men with hypercholesterolemia and examined whether concurrent use of HRT altered diet responsiveness. Diets were matched for macronutrient content, and all food was provided. After 3 weeks on the Step I diet, subjects were randomized to diets (6 weeks) containing 25 g/day protein isolate from soy or cow's milk, with crossover. The soy treatment contained 90 mg/day isoflavones. Lipids, vascular cell adhesion molecule-1 (VCAM-1), p-selectin, and urinary isoflavonoids were measured at the end of each diet. Results: In men and HRT^- women only, there were significant reductions in LDL-C (-17.3%), high-density lipoprotein cholesterol (HDL-C) (-15.3%), and triglycerides (-11.5%) during the Step I diet, and soy had no additional effects. At study entry, HRT⁺ women had lower LDL-C and higher triglycerides than men. Their LDL-C was unchanged, and triglycerides were significantly reduced (-15.1%) by the Step I diets. Isoflavonoid excretion was unrelated to diet response or HIRT status. Conclusions: In men and HRT^- women, the diets significantly lowered LDL-C, independent of soy intake. In HRT⁺ women, the diets improved triglycerides without lowering HDL-C.