In the most comprehensive study to date to examine the effects of soy on menopause, researchers have found that two daily servings of soy can reduce the frequency and severity of hot flashes by up to 26 percent, compared to a placebo.
Until now, the evidence for soy has been inconclusive with some studies showing benefit and others finding no effect.
The interest in soy for hot flashes began when researchers noticed only about 10 percent of Asian women experience menopausal symptoms. It's been speculated that Asian women are less likely to have hot flashes due to their high soy consumption.
The current report, published online in Menopause: The Journal of the North American Menopause Association, reviewed 19 randomized controlled trials that enrolled more than 1200 women taking soy isoflavone extracts (not soy foods) or placebo.
When all studies were combined, there was a clear and consistent positive effect for isoflavones. Compared to placebo, consuming at least 54 milligrams daily for six weeks to a year reduced hot flash frequency by 20 percent and severity by 26 percent.
Women who took isoflavones for at least 12 weeks experienced a threefold greater reduction in hot flashes than women who consumed isoflavones for a shorter duration.
Isoflavone supplements with higher levels of genistein - one of the two main types of isoflavones in soybeans - were the most effective at easing hot flashes.
Genistein is the primary isoflavone found in soybeans and soy foods, suggesting that adding soy to your diet, or using supplements made from whole soybeans, may work better than synthetic isoflavone supplements.
Each gram of soy protein in soybeans and traditional soy foods provides approximately 3.5 milligrams (mg) of isoflavones. Two cups (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.
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