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Soy?s Secret Ingredient

What gives soy its health-promoting power

Soy-based foods and supplements are associated with several major health benefits, including protection against cardiovascular disease and cancer, the two major killer diseases. The active ingredients in soy thought to be responsible for that are phytoestrogens, or estrogens derived from plant sources. Phytoestrogens have three primary forms: 1) isoflavones, found chiefly in soy; 2) lignans, found in flaxseed; and 3) coumestans, found in sprouting plants such as alfalfa.

The two main phytoestrogens in soy are genistin and diadzein. Both are precursors, in that they are converted into biologically active forms by the action of intestinal bacteria. Thus, genistin converts to genistein, while diadzein converts to equol. A new study examining the functions of equol in rats discovered that it has an interesting and potentially useful property.1

In rats, the study found, equol completely inhibited the effects of dihydrotestosterone, a metabolite of testosterone. DHT is linked to such maladies as male-pattern baldness, acne and prostate growth and cancer. Physicians routinely treat cases of prostate enlargement by prescribing a drug called finesteride, which inhibits 5-alpha reductase, the enzyme that converts testosterone into DHT, thus lowering levels of DHT circulating in the blood. Another form of the same drug, marketed as Propecia, contains one-fifth the dose used to treat prostate problems and is prescribed as a treatment for male-pattern baldness.

Equol differs from those drugs because it doesn’t inhibit the synthesis of DHT but instead totally ties up the hormone, preventing it from acting. Equol prevents DHT from binding to androgen receptors in cells, and if DHT can’t interact with such receptors, it does nothing. In one part of the new study, rats’ testes were removed, thus eliminating DHT synthesis. When researchers injected DHT into the rats, their prostate glands enlarged. But when the rats got equol, nothing happened. Providing equol alone also prevented any apparent effects of DHT.

While the protective effects of soy have often been attributed to the weak estrogenic activity of isoflavones, the new study shows that the true effect derives from equol’s blocking of DHT activity. Equol has almost no estrogenic activity, though like estrogen it does have antioxidant effects. Equol also works with sex-hormone-binding globulin (SHBG), a protein made in the liver that binds with testosterone in the blood. Only the 2 percent of testosterone not bound to SHBG is active and able to interact with cellular androgen receptors. It’s possible that by binding with SHBG, equol may slightly increase free-testosterone levels. The binding of DHT by equol also appears to promote release in the pituitary gland of luteinizing hormone, which regulates testosterone synthesis.

By inhibiting DHT, equol may be of use in treating everything from male-pattern baldness to acne to prostate enlargement and other conditions. Equol doesn’t affect circulating testosterone, so it wouldn’t interfere with test’s anabolic features. If it proves cost effective, equol may find its way into bodybuilding supplements. In the meantime, soy products are the only reliable source of equol.


1 Lund, T.D., et al. (2004). Equol is a novel anti-androgen that inhibits prostate growth and hormone feedback. Biol Reprod. 70:1188-93.

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