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How Safe is Soy?

I keep reading contradictory things about soy and women's health - that it relieves hot flashes, that it doesn't, and that it can cause breast cancer or not. Can you update me on the pros and cons of soy foods for women?

Answer (Published 8/6/2012)

The latest news about soy and women's health comes from a review of 17 previous studies looking at whether or not soy supplements can help relieve menopausal hot flashes. The researchers from Loma Linda University in California concluded that taking extracts of soy isoflavones, biologically active phytoestrogens (plant estrogens), led to a 21 percent reduction in hot flashes compared to a placebo. While the conclusions of the 17 studies varied, the researchers note that overall they showed a "pattern" of soy isoflavones providing more relief than placebos. (The median amount of soy isoflavones used in the studies was 54 mg taken daily for anywhere from six weeks to 12 months, and researchers reported that supplements providing more than 18.8 mg of the soy isoflavone genistein were more than twice as potent at reducing hot flash frequency than supplements containing less genistein.)

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Given the review of this evidence, the researchers suggest that soy isoflavones may be worth a try for some women. However, the study leader advises that women look elsewhere for relief if the soy isoflavones don't help within four weeks.

Here's a rundown of the impact of soy products on other aspects of women's health:

  • Breast cancer: Theoretically, the phytoestrogens in soy can attach to estrogen receptors and block the effect of estrogen, the hormone most types of breast cancer cells need in order to multiply. If so, consuming soy foods could reduce the risk of breast cancer, but study results have been conflicting: some show that soy consumption lowers the risk of breast cancer while others have found no such evidence; some studies have even suggested that concentrated supplements of soy proteins may stimulate the growth of breast cancer cells. Results of a big study from Shanghai published in 2009 found that women participants whose intake of soy protein was highest when they were teenagers or young adults had a risk of pre-menopausal breast cancer that was 60 percent lower than participants whose soy protein intake during those young years was lowest. As far as other types of cancer are concerned, no good evidence demonstrates that substances in soy foods are protective against endometrial, ovarian, colorectal, prostate and other cancers.
  • Heart disease: According to the American Heart Association, a daily diet that includes 50 grams of soy (the equivalent of 1.5 pounds of tofu or eight 8-ounce glasses of soymilk) can lower LDL ("bad") cholesterol by three percent. Earlier research was more optimistic: an analysis of the results of 38 controlled clinical trials suggested that substituting 50 grams of soy protein daily for animal protein reduced total cholesterol by 9.3 percent, LDL by 12.9 percent and triglycerides by 10.5 percent, adding up to a 20 percent reduction in the risk of heart attack, stroke or other types of cardiovascular disease. 
  • Memory: Here, too, study results conflict. A few suggest that eating soy could prevent age-related memory loss or decline of cognitive function while other research suggests that the opposite is true. A study published in 2000 in the Journal of the American College of Nutrition found that older women of Japanese ancestry who lived in Hawaii and ate their traditional soy-based diet were more likely to develop cognitive problems than similar women who switched to a Western diet. More research is needed here, but if this finding holds up, we need to learn what it is about a soy-based diet that could be at fault. 

Given the uncertainty of the study results, I continue to advise avoiding concentrated soy supplements entirely. However, I still recommend one to two servings a day of whole soy foods. A serving is a cup of soymilk or one half-cup of tofu, tempeh, edamame or soy nuts. At those amounts you'll get the benefits of soy without the theoretical risks of taking in too many isoflavones.

Andrew Weil, M.D.

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