Soy pills for the hot flashes and bone loss menopausal women may endure seemed like a great idea – a cheap way of getting the benefit of estrogen without the risks.
But alas, a new study concludes they don't work.
Woman who took a daily soy pill had no less bone loss after two years than others who took a sugar pill. (Women in both groups didn't know which pill they got.)
And women taking soy actually reported more hot flashes. That could be due to a strong placebo effect among women taking placebos, or perhaps the soy protein actually has an anti-estrogen effect. (Estrogens can work both ways, so perhaps phytoestrogens – plant estrogens found in soy – do too.)
The discovery will be a big disappointment to millions of women who started taking soy after the Women's Health Initiative showed in 2002 that estrogen supplements increase the menopausal women's risk of blood clots, stroke and cognitive problems. And estrogen plus progestin – a formulation necessary to protect against cancer in women who still have a uterus – also increases the risk of heart attacks. Those results led most menopausal women to reject so-called hormone replacement therapy (although millions still use it).
The study, in this week's Archives of Internal Medicine, is larger and longer than previous looks at soy pills. It enrolled 248 women, and although many women in both groups dropped out, the 182 who didn't were apparently enough to show that soy was no better than placebo.
"It is difficult to imagine that the results would have differed substantially with additional participants," say women's health experts Katherine Newton and Deborah Grady, authors of an accompanying commentary.
The study was done largely by Florida researchers with support from the National Institutes of Health.
Health experts have bought into the soy pill idea because Asian women, who typically eat lots of soy protein, have lower levels of osteoporosis, breast cancer and cardiovascular disease.
The notion makes sense, since soy contains phytoestrogens that weakly mimic the human hormone. The new study provided a soy dose equal to twice the typical Asian diet, and the hope was that would be enough to minimize hot flashes, which estrogen reduces by 60 to 95 percent. Researchers also thought it could retard the rapid bone loss of early menopause, which estrogen also does.
But this is hardly the first time that dietary supplements have disappointed. Beta-carotene, vitamin E, selenium and vitamin D supplements have all been acclaimed for their health benefits, but studies have failed to back up many of the claims or have produced conflicting results. (There's little question that getting these micronutrients from food, rather than in pill form, is a good thing.)
The new study isn't necessarily the end of the soy protein story. Newton and Grady point out that 25 to 50 percent of women make a substance called equol when they eat a soy protein called daidzein. Some think equol may be more biologically active, so there may be a rationale for studying it in women who are equol-producers.