Settling The Soy Controversy
Soy products are remarkably versatile. Manufacturers have found ways to turn them into soymilk, veggie burgers, hot dogs, ice cream, yogurt--you name it. One day, they'll probably turn soy into snow tires. Because soy products are so widely consumed, some people have raised the question as to whether they are safe. The biggest question mark was whether they affect the risk of breast cancer and, for women who have been diagnosed with breast cancer, whether they would help or harm their chances for survival.
Fibroids
Soy products may reduce the risk of fibroids, knots of muscle tissue that form within the thin muscle layer that lies beneath the uterine lining. A study of Japanese women found that the more soy women ate, the less likely they were to need a hysterectomy, suggesting that fibroids were less frequent (6). In a study of women in Washington State, soy did not seem to help or hurt, perhaps because American women eat very little soy, compared with their Japanese counterparts (7). What did have a big effect in this study were lignans, a type of phytoestrogens found in flaxseed and whole grains. The women consuming the highest amounts of these foods has less than half the risk of fibroids, compared with the women who generally skipped these foods. So, again, phytoestrogens seem beneficial, countering the effects of a woman's natural estrogens, although in this case the benefit comes from foods other than soy.
Soy and Male Hormones
How about men? Although compounds in soy products have been likened to very weak female hormones, they have no adverse effects on men and may actually help them prevent cancer. A meta-analysis to be published in Fertility and Sterility, based on more than 50 treatment groups, showed that neither soy foods nor isoflavone supplements from soy affect testosterone levels in men (8). An analysis of 14 studies, published in the American Journal of Clinical Nutrition showed that increased intake of soy resulted in a 26 percent reduction in prostate cancer risk (9). Researchers found a 30 percent risk reduction with nonfermented soy products such as soy milk and tofu.
Protein: A Little Goes a Long Way
Many soy products are high in protein. Manufacturers have exploited this fact, packing isolated soy protein into shakes and turning it into meat substitutes. But some have raised the concern that pushing protein intake too high--from any source--might not be wise. The concern is that an overly high protein intake may boost the amount of insulin-like growth factor (IGF-I) in the bloodstream (14), a phenomenon known to occur with cow's milk (15). High IGF-I levels are linked to higher cancer risk. Some reassurance comes from the fact that soy intake is linked to lower, not higher, cancer risk, and simple soy products, such as tempeh, edamame, or soynuts, are unlikely to affect IGF-I levels, in any case.
In summary, evidence to date is reassuring. Soy products may reduce the risk of breast cancer and breast cancer recurrence. They do not appear to have adverse effects on male hormone function or on the thyroid gland, but may reduce the absorption of thyroid medications.
Having said that, soy products are certainly not essential. Many people who start a healthful vegan diet, as I and many other doctors recommend, seem to feel they must have soy products. But the fact is, a vegan diet can follow a Mediterranean tradition, focusing on vegetables, fruits, beans and pasta. Or it might follow a Latin American tradition of beans, rice, and corn tortillas. Soy products come from an Asian tradition with many healthful delights and the most enviable health statistics on record. So soy is handy, but it is certainly not essential. If you choose to include soy products in your routine, you'll have science on your side.