Soy, which is rich in protein, is a staple of a vegetarian diet due to its purported health benefits. Isoflavones, which are found in soybeans, are a class of phytoestrogens, compounds derived from plants that are similar in structure to estrogen. At present, isoflavones are at the heart of many studies to determine their actual health benefits. Patients taking medication should consult their doctors before starting a soy-based diet or taking soy supplements.
Metabolism
Soy isoflavones exist as glycosides, meaning they are bound to sugar molecules. Digestion of the sugar molecule leaves one of three isoflavone aglycones: genistein, daidzein and glycitein. Oregon State University states that the biological effects of soy isoflavones depend on the type of bacteria present in the intestine. Daidzein may be broken down to equol, which possesses the strongest estrogenic effects, but studies have shown that only 33 percent of Westerners metabolize daidzein to equol.
Sources
According to the U.S. Soyfoods Directory, the isoflavones in the soybean are more concentrated in the hypocotyl, which is just below the leaves on the seedling, with moderate amounts found in the cotyledon, or the first layer of leaves to grow. There is some difference in the amount of isoflavone found in soybeans, depending on the type grown, growing conditions and location of the field. In general, roasted soy nuts, soymilk, tofu, tempeh and soy flour all contain 30 to 50 mg of isoflavones.
Therapeutic Doses
According to the University of Pittsburgh Medical Center, foods containing 6.5 g of soy protein are permitted to carry a heart-healthy label by the FDA. Studies have shown that 25 g of soy protein per day can lower cholesterol levels, which can be found in about 2.5 cups of soymilk or 1/2-lb. of tofu. One concern is that so many foods now contain soy protein as an additive that some people in the U.S. will consume more soy protein than anywhere else in the world.
Proposed Benefits
The National Center for Complementary and Alternative Medicine states that research shows daily ingestion of isoflavones may slightly lower the levels of LDL, the "bad" cholesterol. Because of their estrogen-like structure, isoflavones can occupy some estrogen receptors in the body, acting to reduce the symptoms of menopause after the body's levels of natural estrogen are reduced. It is possible, but is has not been proven, that soy isoflavones may reduce the amount of plaque that forms on artery walls. They may also reduce the risks of some cancers, such as breast and prostate, and help improve bone health.
Risks
The safety of long-term use of soy isoflaveones has not yet been established. It is possible to have an allergy to soy. Products containing soy protein or soy isoflavones should be avoided by those with allergies to soy. According to Memorial Sloan-Kettering Cancer Center, soy isoflavones may actually increase the risk of endometrial hyperplasia, bladder cancer and the chances for metastasis to occur. One soy isoflavone, genistein, can interfere with the anti-cancer drug tamoxifen.
