Aug. 8, 2011 -- Women looking for an alternative to hormone replacement therapy for problems with menopause will get little help from soy supplements. A new study reported in Archives of Internal Medicine found there was no difference between soy and a placebo in relieving symptoms such as bone loss or vaginal dryness.
Researchers randomly assigned 182 women within the first five years of menopause to either daily doses of a soy supplement or a placebo. The soy supplement contained high levels of isoflavones. Those are plant compounds that act like the hormone estrogen in the body.
After two years, there was no difference in the rates of bone loss between the two groups. The percentage of women who had other common complaints, such as vaginal dryness, loss of interest in sex, and difficulty sleeping, was also the same in the two groups. The one exception was hot flashes.
Soy and Hot Flashes
At the study's start, about 50% of women reported having hot flashes. By the end of the study, more women in the soy group reported hot flashes compared to the placebo group, 48% vs. 32%.
"We know that soy isoflavones are a very weak estrogen," says researcher Silvina Levis, MD. "And so we don't expect it to provide much relief, and it didn't." Levis is an associate professor in the department of medicine and director of the Osteoporosis Center at the University of Miami's Miller School of Medicine.
"It could be that the soy isoflavones are partially blocking the estrogen receptor," Levis tells WebMD. That would prevent a woman's own natural estrogen from binding there. Lower estrogen levels are thought to increase hot flashes.
Soy Offers Little Benefit
Experts had mixed reactions to the study's findings. "This study, I think, is pretty much where the story ends for isoflavones and bone," says Mark Messina, PhD. Messina is an adjunct associate professor at Loma Linda University and an expert consultant for the United Soybean Board.
Messina points to another study published in 2010 in the Journal of Clinical Nutrition. That study tested similar doses of soy against a placebo in 224 early menopausal women. It also concluded that soy offered little protection against bone loss, which accelerates for most women in the first few years after menopause.
"These studies make it very difficult to promote this idea that isoflavone supplements, at least, are good for bone health," Messina says. But he thinks the study's finding that soy didn't help hot flashes and possibly made them worse was wrong.
Messina says he recently reviewed 19 studies of soy for hot flashes. The results of his review, which were presented at a medical conference, found a clear reduction in the frequency and severity of hot flashes. Compared to placebo, the use of soy resulted in about a 50% reduction.
Another analysis of 19 studies also found a benefit in soy supplements for hot flashes. The benefit, though, was very slight. That review, which was published in the journal Menopause in 2010, found that on average, soy supplements cut the number of daily hot flashes women had by less than half a single hot flash.
"If it works," says Deborah Grady, MD, a professor of medicine at the University of California at San Francisco, "it's a tiny effect and probably not worth taking great big amounts of soy." Grady wrote an expert commentary on the new study but was not involved in the research.