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Nutrition Notes
Week of August 24, 2009
Contact: Mya R. Nelson, (202) 328-7744

Men Don’t Need to Fear Soy Foods

Karen Collins, MS, RD, CDN
American Institute for Cancer Research

It’s no wonder that men who have read the recent stories circulating about soy are confused. Some stories claim that soy foods, such as tofu and soy milk, can lead to breast growth or reduce testosterone levels. Other stories tout the benefits of soy, suggesting soy may help protect against prostate cancer. Women have wondered for years whether phytoestrogens affect their health, from menopausal hot flashes to breast cancer. Now men are left to wonder about soy foods possible unwanted effects.

Soy contains compounds called isoflavones that are known as phytoestrogens, which are weak plant versions of the hormone estrogen. Fears that the amount of phytoestrogens in moderate amounts of soy foods could stimulate men’s breast tissue or reduce their testosterone are not based on sound research. A study of children fed soy protein formula for more than six months showed no hormonal effects. A study of men in their 50s and 60s found that the men who ate soy foods twice a day for three months showed no change in testosterone levels, but had a 14 percent drop in levels of PSA (an indicator of prostate growth used in screening for prostate cancer).

Concerns about lowering sperm count and affecting fertility also do not seem to hold up in human studies of realistic dietary consumption.

Whether or not soy foods offer protection from prostate cancer is still an open question. One recent analysis that combined results from many studies found that men who consumed the most soy foods or the isoflavones they contain were 26 percent less likely to develop prostate cancer. Yet studies overall suggest that men may differ in their response to soy.

Asian populations tend to show a much greater drop in prostate cancer risk with soy foods than Western populations. Japanese men with highest consumption of soy or blood levels of isoflavones have shown 40 to 60 percent lower risk of prostate cancer than those with lowest consumption or blood levels. Yet European studies on the effect of soy are inconsistent.

The difference between Asian and Western populations could stem from several sources, such as the bacteria in our gut. All people carry millions of useful bacteria in our digestive tract that help metabolize food, yet different populations (and people) carry varied types of bacteria. Some bacteria break down food isoflavones into a compound (equol) that is linked with lower prostate cancer risk. Overall, Asian populations show a higher proportion of equol-producers, which could be one reason why soy is more often linked with prostate protection in these groups.

The type of soy food also makes the research challenging. Fermentation may change how easily our bodies absorb isoflavones, which means that fermented soy foods (miso, tempeh, natto) may have a different effect on prostate cancer risk than non-fermented soy foods (soy milk, tofu and edamame). Some studies suggest that the effect of soy foods may depend on its interaction with other food compounds; the lycopene from tomatoes is one such interaction under study.

While we wait for more answers about the possible protection from soy, men can feel safe eating one or two servings of soy foods daily. Soy foods are nutritious, low in saturated fat, and men have consumed them for generations without harm. However, until we know more about isoflavone safety, research does not show benefit or safety of consuming large amounts of isoflavone supplements in hopes of achieving protection.


Our Mission: The American Institute for Cancer Research champions the latest and most authoritative scientific research from around the world on cancer prevention and survival through diet, weight and physical activity, so that we can help people make informed lifestyle choices to reduce their cancer risk.

We have contributed over $105 million for innovative research conducted at universities, hospitals and research centers across the country. Find evidence-based tools and information for lowering cancer risk, including AICR’s Recommendations for Cancer Prevention, at www.aicr.org.

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