I'm aware of Internet paranoia on the subject of soy and the contention that only fermented soy is safe to consume. That is simply not true. Some of the best forms of soy - edamame, tofu and soy nuts - are unfermented and are much more likely to help you than hurt you.
Claims that unfermented soy foods (such as tofu and soy milk) contain toxins that block the action of enzymes needed to digest protein, and that these toxins cause pancreatic enlargement, cancer and stunted growth in animals are misleading. While soy does contain substances (trypsin inhibitors) that may adversely affect the pancreas in animals, there's no solid evidence that they cause similar problems in humans. Furthermore, trypsin inhibitors are found in all of the vegetables of the cabbage family as well as in beans other than soy.
Other concerns about soy safety focus on the following issues:
- Breast cancer: Here, the idea is that high levels of isoflavones, active ingredients in soy that behave like estrogen in the body, may increase the risk of breast cancer. While high levels of isolated isoflavones may do so, it appears that the total mix of weak plant estrogens in soy protects the body's estrogen receptors. This protection may reduce the effects of excess estrogen exposure from such external sources as meats and dairy products from hormone-treated cows as well as artificial chemicals and industrial pollutants that act as foreign estrogens. Japanese women whose diets contain a lot of soy foods have only one-fifth the rate of breast cancer that occurs among Western women.
- Thyroid Problems: Excess consumption of soy can affect thyroid function, but only if you have a thyroid disorder to begin with or if you're not getting enough iodine in your diet (a rare deficiency in the United States). If you take medication for hypothyroidism (low thyroid), and are concerned about the effect of eating two daily servings of soy, have your thyroid levels checked regularly.
- Mineral absorption: The idea that substances in soy called phytates block absorption of essential minerals is also in circulation, but there is no scientific data suggesting that soy consumption leads to mineral deficiency in humans.
All told, based on the evidence to date, I see no reason to worry about eating soy foods, whether fermented or not. I still recommend consuming one to two servings of soy per day, an amount equivalent to one cup of soy milk, or one half cup of tofu, soy protein (tempeh) or soy nuts.
Andrew Weil, M.D.