Animal studies and small studies in humans have offered some evidence that these compounds, known as isoflavones, could help build muscle mass and reduce fat mass, Dr. Oksana A. Matvienko of the University of Northern Iowa in Cedar Falls and her colleagues write in the journal Menopause.
It's unclear why such compounds might have any effect on body composition, but it's possible that they might act as estrogen does, to affect hormones that play a role in fat and sugar metabolism.
To investigate whether isoflavones might help women avoid putting on fat after menopause, the researchers enrolled 229 postmenopausal women in their study, none of whom were severely obese. The women were randomly assigned to take a placebo - also known as a dummy pill -- a tablet containing 80 milligrams of soy isoflavones, or a 120-milligram isoflavone tablet every day for a year.
The isoflavones had no significant effect on women's bodies, the researchers found, nor did they affect levels of insulin, leptin, ghrelin, or adiponectin, hormones that help regulate appetite. The researchers did find a relationship between all of these "appetitive hormones"-aside from insulin-and body composition, but the only factor that predicted changes in fat mass was the amount of fat in a woman's diet.
And the longer it had been since a woman's last period, the higher her fat mass.
While the current findings found no effect of isoflavones, Matvienko and her colleagues say, it is possible that other compounds found in soy could influence body composition.
For now, though, the researchers say that they "cannot recommend" that healthy women take soy isoflavones to reduce body fat.