The potential dangers of soy don't appear to arise until a very large amount is consumed, and that level is extremely unlikely -- unless a person eats soy supplements or powdered soy isolate. "We do need to be cautious," Hendrich says. "All of the negative studies speak to people taking supplements. A tiny pill can give you isoflavone levels of two hundred, three hundred, four hundred milligrams, which is potentially dangerous." Soy-protein powders, which are often incorporated into smoothies, usually provide between 50 and 100 milligrams of isoflavones in one serving, a level most researchers consider acceptable. But beware of the "enhanced" powders that go beyond 100 milligrams.
"Even in populations that eat a lot of soy, most people eat an upper limit of 100 milligrams of isoflavones per day," says Barry Goldin, who studies plant estrogens at Tufts University School of Medicine. "There is no convincing evidence of adverse effects below that dosage." And the chances are very slim that you eat anything close to the quantity of soy that people in those populations do -- the equivalent of four half-cup servings of tofu per day, for instance, or 10 cups of soy milk. "You'd be hard-pressed to take in too much soy in the diet," Hendrich says. "Soy foods would have to replace practically everything else you ate." With supplements, on the other hand, you could easily exceed these amounts.
In any event, as Hughes points out, the question of whether or how much soy to consume is a very individual one. He cites the case of a woman in her 40s who, having prematurely entered menopause after chemotherapy for breast cancer, decided to eat 80 milligrams of isoflavones a day -- a daily intake range that Asians have lived with for centuries -- to mitigate her hot flashes. She achieved some relief, with only a slight risk of stimulating cancer growth in the remaining breast tissue, a compromise that she and Hughes considered reasonable. And, he adds, he would urge someone with a family history of heart disease to eat some soy almost daily, in addition to other healthy food choices. He would not, however, recommend high-dose supplements -- something no population has tried for a long period of time.
"We do know that populations that eat more soy have a lower incidence of certain chronic diseases, such as endometriosis, cancer, and heart disease," Goldin says. "Of course, other things about their lifestyle may be different, too -- more exercise, a lower-fat diet overall, lower body-mass indexes." The bottom line, he says, is that soy is a good source of protein without saturated fats.
"If you're replacing a hamburger with a soy burger, you're probably doing yourself a favor," Goldin says. "You're replacing something that might be bad for you -- that is more likely to cause obesity, heart disease, and certain cancers -- with something that might be good for you." That's reason enough to eat soy foods, and it leaves plenty of room for serving an elegant tofu entree at Saturday night's dinner party.
Return to Weighing the Health Benefits of Soy.