High blood pressure, or hypertension, affects approximately 50 million Americans and 1 billion individuals worldwide, according to estimates from the American Heart Association. It can result in potentially fatal coronary heart disease. In fact, women with high blood pressure have four times the risk of heart disease as women with normal blood pressure.

For the study, led by Francine K. Welty, MD, PhD, of Beth Israel Deaconess Medical Center in Boston, 60 healthy post-menopausal women were instructed to eat two diets for eight weeks each in random order.

The first diet consisted of 30 percent of calories from fat (with 7 percent or less from saturated fat), 15 percent from protein and 55 percent from carbohydrates. It included 1,200 milligrams of calcium per day; two meals of fatty fish (such as salmon or tuna) per week; and less than 200 milligrams of cholesterol per day.

The other diet had the same calorie, fat and protein content, but the women were instructed to replace 25 grams of protein with one-half cup of unsalted soy nuts. Blood pressure and blood samples for cholesterol testing were taken at the beginning and end of each eight-week period.

At the beginning of the study, 12 women had high blood pressure (140/90 or higher) and 48 had normal blood pressure.

Soy nut supplementation significantly reduced systolic (top number) and diastolic (bottom number) blood pressure by in all 12 hypertensive women, the researchers reported. It lowered systolic BP by 9.9 percent and diastolic BP by 6.8 percent in that group. It also reduced systolic and diastolic blood pressure in 40 of the 48 women who had normal levels -- by 5.2 percent and 2.9 percent respectively.

In women with high blood pressure, the soy diet also decreased levels of low-density lipoprotein ("bad") cholesterol by an average of 11 percent. Cholesterol levels remained the same in women with normal blood pressure.

"A 12-millimeter of mercury decrease in systolic blood pressure for 10 years has been estimated to prevent one death for every 11 patients with stage one hypertension treated; therefore, the average reduction of 15 milligrams of mercury in systolic blood pressure in hypertensive women in the present study could have significant implications for reducing cardiovascular risk and death on a population basis," the authors write.