More women taking soy isoflavone tablets reported constipation, too, although the differences were not significant from a statistical point of view.
The researchers did find that spinal bone loss was smaller in a subgroup of women taking the soy isoflavone tablets: women whose vitamin D levels were less than 20 nanograms per milliliter.
Interest in soy supplements increased after the Women's Health Initiative study, halted in 2002, found increased risk of strokes and heart attacks in women who took combination hormone therapy using estrogen and progesterone. Soy has been promoted as an alternative to estrogen, providing comparable benefits of bone preservation and symptom relief without the risks.
Levis said that if a patient were to ask her now about taking soy for menopausal symptoms or bone health, "I would not suggest they start for hot flash [relief] or bone loss prevention."
Dr. Deborah Grady, professor of medicine and epidemiology at the University of California San Francisco, co-wrote a commentary to accompany the study. In an interview, she said that, "I personally would say we have spent enough time and effort on this. And to do more trials on soy doesn't seem to be an efficient way to spend money if we want to come up with better treatments for menopausal women."
Many studies have looked at soy, she said. Every time one has had negative results, it seems, people say perhaps the dose was wrong or perhaps the study was not long enough, she noted.
But the results of the new study are definitive, Grady said. "This one is important because it was funded by the NIH, was long-term and they gave a huge dose of soy."
The bottom line: "Paying money to buy soy supplements doesn't make sense," she said.
Costs vary, but soy isoflavone supplements are widely sold on the internet. A month's supply can be bought for about $7.
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