Genistein is an isoflavone, a plant-source substance with estrogen-like qualities (a “phytoestrogen”). Soy is the most abundant common source of isoflavones, and genistein is the most abundant isoflavone in soy. Red clover is also a good source of genistein.
Like other phytoestrogens, genistein can have two opposite effects depending on the circumstances in which it is taken. In some circumstances, it acts as an antagonist to estrogen, decreasing the hormone’s effects in the body. In other situations, it acts as an estrogen substitute, producing estrogenic effects where the hormone is failing to do so. It can have these opposite effects because it binds to special sites on cells called “estrogen receptors,” and mildly stimulates them. This stimulation is not as strong as that produced by estrogen itself; however, genistein effectively occupies these sites and keeps estrogen from having any effect on them. The net result is that when women with high levels of estrogen take genistein, estrogenic activity in the body decreases. Conversely, in women with relatively little natural estrogen, genistein can partly make up for what is lacking.
This second effect is the rationale for using genistein as a treatment for problems associated with menopause. Estrogen reduces many symptoms of menopause, from hot flashes to menopause. Estrogen, however, is somewhat dangerous, increasing the risk of heart disease and cancer. It has been hypothesized that genistein and other isoflavones could “split the difference,” providing some of estrogen’s benefits without the risks.
There have been numerous studies of genistein for this purpose. Most have evaluated possible beneficial effects regarding osteoporosis . In 2007, a study was published that looked at genistein’s effects on hot flashes. This was a double-blind study that included 247 women who suffered from problematic hot flashes caused by menopause. Participants were given either placebo or genistein at a dose of 54 mg per day. The results indicated that use of genistein significantly reduced the rate and severity of hot flashes as compared to placebo. No adverse effects were seen in this study. Importantly, genistein did not have any harmful effects on the uterus. Estrogen, taken by itself, causes precancerous changes in the uterus, so this is a distinct advantage.
As noted above, estrogen is also thought to increase breast cancer and heart disease risk. Genistein most likely does not present the same issues, but this has not been conclusively proven.