Taking extra soy supplements did not help stave off artery hardening in a new study of postmenopausal women.
But, researchers said, soy showed a possible benefit in young women just a few years after menopause -- warranting more research in that age group.
The findings add to conflicting data about the role of soy in lowering blood pressure and cholesterol and reducing the risk of heart disease.
"I was not surprised" about the lack of a clear benefit, said William Wong, who studies nutrition and chronic disease at Baylor College of Medicine in Houston and wasn't involved in the new study. "So far the research (on soy) has been contradictory."
Researchers have theorized that since rates of cardiovascular disease are lower in Asia -- where soy is an important component of the diet -- there could be something protective about the nutrient-rich plant.
For the current study, Dr. Howard Hodis from the Keck School of Medicine of the University of Southern California in Los Angeles and colleagues randomly assigned 350 healthy postmenopausal women to get either 25 grams of soy protein (including about 90 milligrams of isoflavones) or soy-free milk protein each day. Both types of protein were delivered in drink powder and food bars.
Women -- and the doctors treating them -- didn't know what kind of protein they were getting.
Every six months for up to three years, researchers brought the women in for an ultrasound to measure the build-up in their carotid arteries -- those responsible for supplying blood to the brain.
More than 700,000 Americans suffer a stroke each year, and most of those occur when blood clots form on plaque in those arteries and cut off blood flow to part of the brain.
In general, there was not a clear difference in artery hardening among women taking soy or non-soy protein supplements. In the soy group, researchers measured artery thickening of nearly five micrometers per year, compared to about six micrometers in the "placebo" group, on average -- but that differences may have been due to chance.
It seemed more likely, the researchers write in the journal Stroke, that women less than five years out of menopause got some artery-protecting benefit from soy protein, based on their build-up measurements. Again, they couldn't confirm that wasn't also a chance finding, but Hodis said it would make sense.
Soy contains estrogen-like chemicals called isoflavones, he said, and women's bodies seem to stop responding to estrogen as they get farther away from menopause.
Women getting high doses of soy protein reported more breast pain and vaginal bleeding or spotting during the study.
"If a woman goes on this stuff, especially at this dosage... she still needs to see her gynecologist," Hodis told Reuters Health. "Those things need to be monitored," he said, in case they hint at future disease.
Soy isoflavone tablets can be bought over the counter and cost about 25 to 50 cents per day.
The amount given to women in this study, Hodis said, is much more than Americans usually get -- but is similar to what would be found in a typical Asian diet.
For women who would rather get extra soy from food, Wong recommended soy beans, bean curd and soy milk, which "nowadays is pretty tasty," he said in an interview.
The possibility of a benefit in young women "is encouraging, but we need to do more study to confirm it," Wong said. For now, he said he doesn't see much harm if women want to add more soy-based food to their diets. "A little bit of soy isn't going to hurt you."