Cholesterol

An analysis of many trials has proven that soy reduces both total and LDL (“bad”) cholesterol. Trials showing significant reductions in cholesterol have generally used more than 30 grams per day of soy protein; if soy protein replaces animal protein in the diet, as little as 20 grams per day may reduce both total and LDL cholesterol. Isoflavones found in soybeans appear to be a key cholesterol-lowering ingredient. While the cholesterol-lowering effect of soy protein or soy isoflavones is inconsistent in people with normal cholesterol levels, the effect of soy protein in people with high cholesterol is consistently beneficial.

Menopause

Soybeans contain compounds called phytoestrogens, which are related in structure to estrogen; research has yet to determine the extent to which these or other compounds in soybeans are responsible for soy’s effect in both premenopausal and menopausal women. Soy is known to affect the menstrual cycle in premenopausal women, and societies with high consumption of soy products have been linked to a low incidence of hot flashes during menopause.

Doctors often recommend that women experiencing menopausal symptoms eat tofu, soy milk, tempeh, roasted soy nuts, and other soy-based sources of phytoestrogens. Soy sauce and many processed foods made from soybean concentrates have low levels of phytoestrogens.

Breast cancer

The commonly held belief that consuming soybeans or isoflavones such as genistein will protect against breast cancer is far from proven.

While Asian countries in which people consume high amounts of soy generally have a low incidence of breast cancer, the dietary habits in these countries are so different from diets in high-risk countries that attributing protection from breast cancer specifically to soy foods is premature. Similarly, women who frequently consume tofu have been reported to be at low risk of breast cancer. However researchers acknowledge that consumption of tofu might only be a marker for other dietary or lifestyle factors that are responsible for protection against breast cancer.

Some studies suggest that consuming soybeans in childhood—but not adulthood—may ultimately be proven to have a protective effect. Still other studies suggest that consuming soy might, under some circumstances, increase the risk of breast cancer.

Scientists who remain hopeful about the potential for soy to protect against breast cancer under some circumstances recommend consumption of foods made from soy (such as tofu)—as opposed to taking isoflavone supplements. Several substances in soybeans other than isoflavones have shown anticancer activity in preliminary research.

Prostate cancer

Genistein is an isoflavone found in soybeans and many soy foods, such as tofu, soy milk, and some soy protein powders. Some research has shown that genistein inhibits growth of prostate cancer cells, helps kill these cells, and has other known anticancer actions.

Some researchers believe that genistein may eventually be a potential treatment for prostate cancer; others are more conservative, saying only that enough evidence exists to recommend that future genistein research be devoted to prostate cancer prevention. Doctors remain hopeful that soy-based foods containing genistein and related isoflavones may eventually be proven to help protect against prostate cancer.

Fibrocystic breast disease

Fibrocystic disease has been linked to excess estrogen. When people with fibrocystic disease are put on a low-fat diet, their estrogen levels decrease; after three to six months, the pain and lumpiness also decrease. The link between fat and symptoms appears to be most strongly related to saturated fat. Foods high in saturated fat include meat and dairy products; fish, nonfat dairy, and tofu are possible replacements.

Osteoporosis

Soy foods may be beneficial in preventing osteoporosis. Isoflavones from soy have protected against bone loss in animal studies. In one trial involving postmenopausal women, supplementation with 40 grams of soy protein powder (containing 90 mg of isoflavones) per day protected against bone mineral loss in the spine. And many trials show that a synthetic isoflavone, ipriflavone, reduces the incidence of osteoporotic bone fractures. Although the use of soy in the prevention of osteoporosis looks hopeful, no long-term human studies have examined the effects of soy or soy-derived isoflavones on bone density or fracture risk.