The findings, published in Menopause: The Journal of the North American Menopause Association, reviewed 19 previous studies that examined more than 1,200 women.
Although the effectiveness of soy in alleviating hot flashes has been inconclusive, with some studies suggesting soy to be beneficial and others suggesting otherwise, much of the discrepancy is due to small sample sizes and inconsistent methodology, according to the authors.
"When you combine them all, we've found the overall effect is still positive," said Melissa Melby, a medical anthropology professor at the University of Delaware and co-author of the study.
Examining the impact of soy isoflavones, chemicals found in soy that exert a mild estrogen-like effect, Melby and her colleagues found:
- Ingesting at least 54 milligrams of soy isoflavones daily for six weeks to a year reduces menopause hot flash frequency by 20.6 percent and severity by 26 percent, compared to a placebo.
- The total reduction in frequency and severity might be even greater due to the placebo effect.
- In longer duration studies (where women consumed soy isoflavones for 12 weeks or more), the decrease in hot flash frequency was approximately threefold greater than in shorter-duration trials.
- Isoflavone supplements with higher levels (at least 19 milligrams) of genistein, one of the two main types of isoflavones, were more than twice as effective at reducing hot flash frequency than lower amounts.
Each gram of soy protein in soybeans and traditional soyfoods provides approximately 3.5 mg of isoflavones. Two glasses (16 oz) of soymilk or seven ounces of tofu provide approximately 50 mg of isoflavones.
The interest in soy and menopause stems from observational evidence in Japan, where researchers have found the low frequency of hot flashes in Japanese women might be attributed to the high soy consumption that often begins in utero and continues throughout their lifespan.
"Soy is probably more effective in these women," Melby said. "But if you're 50 and you've never touched soy, it's not too late. We've found that it still helps."