Soybeans provide an excellent source of protein and polyunsaturated fats, including omega-3 fats. In addition, soybeans contain isoflavones (or phytoestrogens), a type of phytochemical. Each gram of soy protein is associated with approximately 3.5 mg of isoflavones and is found in foods like tofu, tempeh, soy milk, and miso. Asian populations have been consuming soy foods as a regular part of the diet for centuries. At 30 grams per day, Japan's population consumes more soy protein than any other nation. Americans consume less than one gram per day, on average.
image from Soy Connection
In 1999, the FDA approved a health claim for soy linking its intake with the prevention of heart disease. Since that time, thousands of scientific papers have been written linking soy with the reduced incidence of cardiovascular disease, osteoporosis, breast & prostate cancers, type 2 diabetes, and menopausal symptoms. However, much more research is still needed before anything can be said conclusively.
One of the major questions surrounding soy is whether its consumption is contraindicated in women with estrogen-sensitive breast cancer. According to soy expert Mark Messina, PhD, no human studies have linked soy or isoflavones adversely with markers of breast cancer risk. According to Messina, tumor growth is positively related to the degree to which the soy product has been processed. Therefore, it is best to choose soy products that have been minimally processed (eg, whole soy flour).
Another common question regarding soy intake is whether it has feminizing effects on men. According to a recent article examining data from over 150 studies, published in the journal Fertility and Sterility, soy consumption was not found to have any effect on circulating levels of testosterone or estrogen, sperm count, semen quality, breast size, or erectile function in men. It has also been noted that many concerns about high doses of soy isoflavones have come from the use of pills or powders that provide much more than that supplied in the diet.
Bottom Line: Stay away from soy supplements and go for the real, unprocessed stuff!