A new study finds that the health benefits attributed to soy protein may not depend on the isoflavone content.
While many health advantages have been attributed to soy isoflavones, (components of soy-protein foods), and supplement manufacturers often emphasise levels of isoflavone content, it is still unclear to what extent the isoflavones themselves are responsible for health benefits associated with soy.
Research published in the recent issue of the American Journal of Clinical Nutrition investigated 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. However 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 one-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. This showed that even low-isoflavone soy foods can produce positive results.
Researchers pointed out that there were two unique aspects to the study - a variety of soy-protein foods were used to substitute for the usual protein sources and the soy diets continued to improve the subjects' blood lipid profiles even after dietary saturated fat consumption was maximally reduced.