Noting that many of the health benefits of soy have been attributed to isoflavones, Canadian researchers from St Michael's Hospital and University of Toronto and the University of Guelph conducted a study to determine the effects of high- and low-isoflavone soy-protein foods on risk factors for coronary artery disease (CAD). Isoflavones are naturally occurring glucosides found in many plants, particularly soybeans.
Forty-one men and postmenopausal women were recruited for the study. All of them had elevated levels of lipids (fats and fatlike substances) in the blood. The study included three one-month diets, all of which were very low in saturated fat (less than 5% of calories from saturated fat) and cholesterol (less than 50 milligrams per day): a diet high in both soy protein (50 grams/day) and isoflavones (73 milligrams/day); a diet high in soy protein (52 grams) but low in isoflavones (10 milligrams); a control diet, based on low fat dairy foods. Researchers found that both soy diets resulted in meaningfully lower total cholesterol levels; ratios of total cholesterol to high density lipoprotein (HDL, "good") cholesterol; ratios of low density lipoprotein (LDL, "bad") cholesterol to HDL cholesterol; and ratios of apolipoprotein B to apolipoprotein A-I. Results were similar, irrespective of isoflavone content.
Apolipoprotein B is a blood faction that is found in LDL and very LDL cholesterol. Apolipoprotein A-I is the primary protein component of HDL and is instrumental in the excretion of cholesterol from the body. The soy diets resulted in significantly lower estimated CAD risk, as well. In men, systolic blood pressure was also significantly lower after each of the soy diets than after the control diet. "Substitution of soyfoods for animal products, regardless of isoflavone concentration, reduces the CAD risk," researchers wrote in American Journal of Clinical Nutrition (Vol. 76, No. 2, 365-372, August 2002), "because of both modest reductions in blood lipids and reductions in oxidized LDL, homocysteine and blood pressure." An elevated concentration of the amino acid homocysteine is an independent risk factor for diseases of the heart.