Soy protein in the diet produces health benefits, regardless of isoflavone content

Many health advantages have been attributed to soy isoflavones, which are components of soy-protein foods, yet it is still unclear to what extent the isoflavones themselves are responsible for these benefits. In a new study published in the American Journal of Clinical Nutrition, Jenkins et al. compared the effects of a low-saturated fat control diet, and soy-protein diets with either a high or low isoflavone content in the context of their effects on risk for coronary artery disease. Compared with the control diet, both soy diets significantly improved the subjects’ cholesterol profiles and reduced systolic blood pressure in men. There were no major differences between the high- and low-isoflavone soy protein diets in their beneficial effect on blood lipids and blood pressure.

Twenty-three men and 18 postmenopausal women with elevated cholesterol levels participated in the study. The subjects rotated through three 1-month diets that were all very low in saturated fat. In the control diet, the main protein-containing foods such as meats and fish were replaced with low-fat dairy products and egg substitute. Low-fat soymilk and a variety of soy-based meat substitutes such as soy hot dogs and tofu burgers took the place of the usual protein sources in the two soy protein-containing diets. Body weight, blood lipids, and blood pressure were measured before and after each diet. After each soy diet, total cholesterol, the ratio of low-density lipoprotein (LDL) to high-density lipoprotein (HDL) cholesterol, homocysteine concentrations, and estimated overall cardiovascular risk were lower than they were after the control diet. Additionally, serum concentrations of LDL were lower after the high-isoflavone diet. The only significant difference between the sexes was a tendency toward reduced blood pressure in men after the high-soy isoflavone diet.

A wide range of small but beneficial effects were associated with the substitution of soy-protein for animal protein foods in the subjects’ diets that did not differ significantly between the high- and low-isoflavone diets, indicating that even low-isoflavone soy foods can produce favorable results. Two unique aspects of the study were that a variety of soy-protein foods were used to substitute for the usual protein sources and that the soy diets continued to improve the subjects’ blood lipid profiles even after dietary saturated fat consumption was maximally reduced.