Women on HRT had fewer hot flashes, on average, than women who took soy - and both groups fared better than women who took a placebo.
"The bottom line for someone who is very disturbed by hot flashes: the best treatment is hormones, and the next tier would be soy," said Dr Gloria Bachmann, professor of obstetrics and gynecology at the Robert Wood Johnson Medical School in New Jersey.
The landmark study
However, a 2002 landmark study by the National Institutes of Health (NIH) found that HRT can increase the risk of breast cancer, heart disease and stroke. That means that if women do take hormones, Dr Bachmann said, it should be the lowest dose and for the shortest possible period of time.
Postmenopausal women typically suffer between 30 and 150 hot flashes a month, said Dr Rafael Bolanos-Diaz, professor of health economics at San Marcos University in Lima, Peru, who co-authored the study.
Women who took hormones in his study had an average of 24 fewer hot flashes per month, while those who took soy extracts had 12 per month fewer. The study did not look at whether soy in food has the same effect on hot flashes as supplements, Dr Bolanos-Diaz said.
Altogether, he and his colleagues analysed data from 19 studies comparing hormones or soy to placebo, in a total of 760 women who had treatment with 770 who did not.
No study on soy
Soy hasn't been studied as much as hormone replacement to treat hot flashes, and doctors don't know exactly how it works. It's thought to have similar effects to estrogen on the brain, bones, and blood vessels, Dr Bolanos-Diaz told.
Soy supplements also have side effects that include nausea, bloating, and constipation, according to the National Institutes of Health. A month's supply costs about $12, while a month of hormone tablets runs between $40 and $60.
"For some women, hormone therapy is the best option, for some soy is best, and for some, it's just watchful waiting until the hot flashes subside," Dr Bachmann said.
The study was published online in Menopause: The Journal of the North American Menopause Society.(Reuters Health/ March 2011)