Genistein, a naturally occurring chemical present in soy, has attracted scientific interest for its possible benefits in cancer and heart disease prevention. Genistein is a type of chemical called a phytoestrogen—an estrogen-like substance present in some plants. There are two main types of phytoestrogens: isoflavones and lignans. Soy is the most abundant source of isoflavones, with genistein the most abundant isoflavone in soy. Red clover is also a good source of genistein.

Like other phytoestrogens, genistein can work in two ways: either by increasing or decreasing the effects of estrogen. This happens because genistein binds to special sites on cells called estrogen receptors. Genistein stimulates these receptors, but not as strongly as real estrogen; at the same time, it blocks estrogen itself from attaching. The net result is that when there is a lot of estrogen in the body, such as before menopause, genistein may partly block its effects. Since estrogen appears to increase the risk of various forms of cancer, regular use of genistein by premenopausal women might help reduce this risk. On the other hand, if there is little human estrogen present, such as after menopause, genistein can partly make up for it. This is one rationale for using genistein to treat menopausal symptoms and to prevent osteoporosis.

Genistein might also be helpful for reducing heart disease risk.

Genistein is found in high quantities in soy and in negligible quantities in a few other foods. Most soy foods contain about 1 mg to 2 mg of genistein per gram of protein.3

For more information on the proper dosage of isoflavones in general, see the full Isoflavone article.