The Effects of Taking Soy Isoflavones on Men

For much of the past decade, the soybean has been touted as something of a "superfood" that could prevent cancer and heart disease, lower cholesterol levels, and relieve menopause symptoms. The American Heart Association officially recommended soy protein in 2000 as a part of a healthy low fat diet. Upon closer inspection, however, many experts are questioning those claims, raising some concerns about the possible adverse effects of soy consumption. Of particular concern are the estrogen-like phytochemicals called "isoflavones" that are found in soybeans and the potential side effects of consuming isoflavones in large quantities by men.

Male Fertility

• Consumption of soy isoflavones can lead to lower levels of testosterone and higher estrogen levels in men, according to Kaayla T. Daniel, a clinical nutritionist and author of "The Whole Soy Story: The Dark Side of America's Favorite Health Food." Daniels cites a study conducted by lead researcher Dr. Steven Zeisel at the University of North Carolina, Chapel Hill, in which male patients were given 300- to 600-mg doses of soy isoflavone. Some of the symptoms experienced by the subjects included breast enlargement, nipple discharge, and a decrease in testosterone levels. But, since the level of soy isoflavones administered to the test subjects was approximately 30 times what would be consumed in a normal diet, Dr. Zeisel said there was no need for men to avoid soy products. Not everyone agrees with that conclusion, however. In July 2005, the Israeli Health Ministry issued a warning to adults regarding soy consumption because of fertility concerns.

Prostate Cancer

• Some researchers speculate that soy may be beneficial in preventing or slowing prostate cancer. Omar Kucuk from the Barbara Ann Karmanos Cancer Institute at Wayne State University in Detroit points to the fact that men in the United States die from prostate cancer with a much greater frequency than Chinese and Japanese men, who regularly include soy in their diet. Kucek conducted a study in which he gave 200 mg of soy isoflavone pills a day to men with prostate cancer for six months, and finds that the patients' cancer metastasized at a slower rate or not at all. The study sample was rather small, however, and the results were not compared to a placebo. Other studies conducted on healthy men have not demonstrated any prostate cancer prevention benefits, according to Mark Messina, a soy expert at Loma Linda University in California. Messina also says that other studies conducted on prostate cancer patients have had mixed results. Kucek states that "all evidence we have is preliminary," and that he "can't make any recommendations yet about consuming soy foods or supplements to avoid prostate cancer."

Effects on the Brain

• "There should be some caution about recommending large amounts of isoflavones for men," states Pauline Maki of the University of Illinois at Chicago. Maki's concern comes from an observational study that shows middle-aged Hawaiian men who consume large amounts of tofu are more likely to suffer cognitive impairment and brain atrophy as they age. Jay Kaplan at the Department of Comparative Medicine at Wake Forest University in Winston-Salem, NC, conducted a study in which the researchers fed 44 adult male monkeys a diet rich in soy isoflavones for 15 months, giving half the monkeys the human equivalent of 60 milligrams, which is the same amount found in two servings of soy milk. The other half were given the equivalent of 129 milligrams. Kaplan found that the monkeys who consumed the most soy isoflavones became more aggressive and demonstrated antisocial behavior, while those who consumed the lesser amount suffered no harmful effects. Kaplan concludes that "consuming large amounts of isoflavones, particularly in supplement form, may have effects that we don't yet know about and that could as easily be adverse as beneficial."