The effect, however, differed between African-Americans and whites: While white women lost more fat around their middles, black women showed greater overall reductions in body fat, researchers found.
Researchers have been interested in soy's potential for treating problems that affect women during and after menopause, from hot flashes to heart attack risk factors like high cholesterol. Because estrogen drops sharply when menstruation stops, the idea is that soy's estrogen-like properties could help with these symptoms. But studies have yielded conflicting results.
In the new study, Dr. Daniel R. Christie of the University of Alabama School of Medicine in Birmingham and colleagues used soy-stuffed shakes, which included 20 grams of soy protein and 160 milligrams of soy isoflavones, or placebo shakes containing a milk protein called casein. They assigned 16 African-American women and 17 white women to drink the shakes daily for three months.
X-rays found no difference in total body fat percentage between the soy and placebo groups after three months. But more precise computerized tomography (CT) scans did find a 7.5 percent less abdominal fat in women given soy, compared to 9 percent more of such fat in the placebo group.
Black women taking soy lost an average of 1.8 kilograms, or 4 pounds, while whites given soy put on 0.8 kilograms. But while white women lost visceral fat -the fat surrounding the organs in the abdominal cavity - black women gained it. Deep abdominal fat is believed to be particularly important in the risks of health conditions like diabetes, high blood pressure and heart disease.
Whites also showed greater abdominal fat reductions, while blacks mainly lost total body fat.
While the reason for the racial difference in fat loss is not clear, the study in the American Journal of Obstetrics & Gynecology said, white women have more visceral fat to begin with and so have more to lose in response to the treatment.
But another expert on soy and body composition who wasn't involved in the new study said that its size - just 33 patients - was too small to draw any conclusions about differences between ethnic groups. It was also very short, Dr. Oksana A. Matvienko of the University of Northern Iowa in Cedar Falls told Reuters Health.
In her own research, published in February, Matvienko and her team looked at 229 postmenopausal women who took a placebo or soy isoflavone tablets (at 80- or 120-milligram doses) for a year. They found no difference in body composition between the active and placebo groups after 12 months.
Matvienko said that CT scans are more sensitive than the X-ray technique, which could explain why Christie and his colleagues saw abdominal fat changes that she and her colleagues did not. It's also not clear, she added, that these changes would be large enough to help improve the women's health.
One possible explanation for the new study's positive findings, Matvienko said, is that women took soy protein along with isoflavones; women in her study only took isoflavones.
Recognizing how short the study was, the authors write that it is not clear if the effects of the soy supplement on abdominal fat reduction would persist if the supplement was continued.
Matvienko said more research is needed, but for now, I wouldn't recommend taking soy supplements just for the sake of controlling body composition or body weight.