Soy-rich diets have been linked to lower rates of heart disease, but soy supplements alone may not do anything for older women's blood pressure, according to a U.S. study.
The findings, reported in the American Journal of Clinical Nutrition, add to the mixed evidence on the health benefits of soy isoflavones - compounds that are thought to have weak estrogen-like effects in some body tissue.
Researchers have long known that Asian populations with soy-rich diets have lower rates of heart disease compared with people who eat "Western" diets, but it has not been clear whether soy isoflavone supplements have cardiovascular benefits, such as cutting blood pressure or cholesterol levels.
A number of studies have found that intravenous infusions of soy isoflavones may boost the body's production of nitric oxide and help blood vessels dilate.
"But we don't take soy by infusion," said William Wong, a nutrition researcher at Baylor College of Medicine in Houston, who led the study.
On top of that, he told Reuters Health, those studies looked at short-term effects on blood vessel function, and not whether there are "sustained" benefits for blood pressure.
So for their study, Wong and his colleagues randomly assigned 24 menopausal women to take either soy isoflavones or placebo tablets for six weeks. The supplement gave a daily dose of 80 milligrams of isoflavones.
All of the women started the study with moderately elevated blood pressure. After six weeks, Wong's team found that women on the soy supplement were faring no better than those on the placebo.
On average, systolic blood pressure was 136 mm/Hg in the placebo group, and 137 mm/Hg in the soy group. Systolic blood pressure is the first number in a blood-pressure reading, and levels of 140 mm/Hg or above are considered high.
As for diastolic blood pressure, the average in both groups landed at about 80 mm/Hg, or the upper threshold of "normal."
Wong's team also did special blood tests to see whether the soy supplement affected the women's production of nitric oxide, a chemical that dilates blood vessels. They found no effect.
"It was disappointing," Wong said.
While the study only lasted for six weeks, Wong said that the time period should have been long enough to detect blood pressure benefits if there were any.
"If we didn't see anything in six weeks, we doubt there would be effects (longer term)," he said.
He said that the health benefits linked to soy in Asian populations may be the result of a lifetime of eating those foods and taking supplements later in life may simply not have enough of an impact.
"I think that if people are looking for a magic bullet against high blood pressure, this is not it," he added.