Soy contains plant estrogens known as isolfavones. These plant estrogens may act like estrogen produced naturally in the body and could have a drug-like effect in the body. Studies looking at these chemicals have had mixed results. Some studies indicate benefit, while others indicate risk for breast cancer. This scientific conundrum has researchers urging caution. As with all steroids, estrogen can be a two-edged sword, conferring both benefits and risks. Plant estrogens are thousands of times weaker than natural estrogens, but they circulate in the blood at levels thousands of times higher than natural estrogens.
So far seven studies have compared soy protein shakes, bars, muffins, and flour to placebos. Two studies found that soy curbed hot flashes. In one study drinking a soy protein beverage twice a day reduced the severity of hot flashes but not the number. In an Italian study the average number of hot flashes in the soy drinkers declined from 11 a day to six. In the placebo group the number dropped from 11 to eight. In five other studies, soy products were no better than soy-free placebos at relieving menopausal symptoms. No good studies have looked at foods like tofu or soy cheese.
Taking soy supplements with much higher levels of isoflavones, or isoflavone supplements are not recommended. No one knows what effects such large amounts will have on the body. As with estrogen, large amounts can potentially be dangerous.
There are other reasons to include whole soy foods in your diet. Last October, The Food and Drug Administration gave food manufacturers permission to put labels on products high in soy protein indicating that these foods may help lower heart disease risk. No sooner had the FDA proposed the health claim regulation, than concerns arose about certain components in soy products, particularly isoflavones. This should come as no surprise. Every dietary health claim that has ever been published has met with controversy.
Scientists agree that foods rich in soy protein are heart healthy. A year long review of the available human studies in 1999 prompted the FDA to allow a health claim on labels stating that a diet consuming 25 grams of soy protein daily may reduce the risk of heart disease. Much of the research concentrated on soy in the form of whole foods, such as tofu, soymilk, or as soy protein added to foods. The recently raised concerns focus on specific components of soy, such as isoflavones (daidzein and genistein), not the whole food or intact soy protein. Soy by itself is not a magic food. It is an example of a healthy food that, when combined with a healthy diet, can have a positive effect on health.
Unlike the controversy surrounding soy isoflavones, the evidence on soy protein is much clearer. Which is why the FDA approved the health claim on labels. Soy protein can be a good substitute for animal products because, like animal products, they contain all the essential amino acids, but much less saturated fat. Diets with four daily soy servings (6.25 grams of soy protein is considered a serving) can reduce levels of LDL by as much as 10 percent (a one percent drop in total cholesterol can equal a 2 percent drop in heart disease risk).