Herbs & Supplements:
Genistein
Principal Proposed Uses
•
Osteoporosis Prevention, Cancer Prevention
Other Proposed Uses
•
Cholesterol Reduction, Improving Menopausal Symptoms, Amyotrophic Lateral Sclerosis (Lou Gehrig's Disease)
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 course 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 prevent osteoporosis.
Genistein might also be helpful for reducing heart disease risk.
Requirements/Sources
Genistein is found in high quantities in soy and in negligible quantities in a few other foods. Most soy foods contain about 1 to 2 mg of genistein per gram of protein.3
Therapeutic Dosages
The optimum dosage of genistein is unknown. In Asia, population groups who eat soy foods daily containing 20 to 80 mg of genistein have lower rates of breast and prostate cancer than do groups in the West with less genistein in their diets.4 However, we don't know whether genistein (or even soy isoflavones generally) are responsible for this effect.
Therapeutic Uses
Scientists are studying genistein as a possible treatment or preventive agent for osteoporosis, cancer, heart disease, and amyotrophic lateral sclerosis (ALS). However, most of the current evidence for genistein comes from test tube studies and animal studies, and therefore must be regarded as extremely preliminary. Human studies have tended to focus on soy foods and soy isoflavones in general, not specifically on genistein.
What Is the Scientific Evidence for Genistein?
Osteoporosis
Estrogen has a powerfully protective effect on bone. In women, osteoporosis (bone loss) most often occurs after menopause when the ovaries stop putting out estrogen. Several preliminary studies indicate that genistein can help protect bone as well. 5-7
Unlike estrogen, which primarily helps prevent the destruction of bone, two studies found that genistein might assist in creating new bone.8,9
Interestingly, in one of the rat studies, a small dose of genistein helped protect the rats' bones, while a larger dose of genistein seemed to have the opposite effect—causing increasing bone destruction.10 Studies in humans are needed to determine whether genistein is truly effective, and to find the optimum dose.
Other studies have evaluated the effects of soy products containing other constituents besides genistein. For more information, see the full soy article.
Cancer
Genistein may help reduce risk of various forms of cancer. In one study, newborn female rats treated with genistein had less breast cancer later in life than those treated with placebo.16 However, other studies suggest that genistein or other isoflavones could promote breast cancer under certain conditions. (See Safety Issues)
In the test tube, genistein has been found to suppress the growth of a wide range of cancer cells, including forms of cancer that are not affected by estrogen.11,12 For example, genistein has been found to inhibit skin cancer when it was applied to the skin of mice or fed to rats.14,15
Furthermore, in test tube studies, genistein has been found to enhance the effects of chemotherapy drugs.13
Heart Disease
Soy protein has been found to lower cholesterol in numerous trials. Some evidence suggests that the isoflavones in soy, including genistein, may be responsible for much of this effect. However, this conclusion isn't certain. For more information, see the full soy article.
Genistein may help prevent heart disease in other ways. Test tube studies suggest that genistein may help keep cholesterol in the blood from depositing on blood vessel walls, as occurs in atherosclerosis.20 In addition, very early test tube research suggests genistein may also inhibit the formation of blood clots, which are a major cause of heart attacks.21
Other Conditions
Like estrogen, genistein might be helpful for reducing symptoms of menopause. However most of the studies on menopausal symptoms, such as hot flashes, have involved soy protein or a combination of soy isoflavones rather than pure genistein. For more information, see the full article on soy isoflavones.
Preliminary animal studies suggest genistein may be helpful for treating amyotrophic lateral sclerosis (ALS).22
Safety Issues
Studies in animals have found soy isoflavones including genistein essentially nontoxic.23
However, because of its effects on estrogen, there are at least theoretical concerns that genistein may not be safe for women who have already had breast cancer. While most test tube and animal studies have found that genistein inhibits cancer growth, some animal studies indicate that various forms of genistein might stimulate breast cancer cells under certain conditions.31-33
Furthermore, evidence from two preliminary studies in humans found changes suggesting that soy or its isoflavone constituents might slightly increase breast cancer risk.24,39 While other studies in women have found reassuring results,40prudence nonetheless suggests that women who have had breast cancer, or are at high risk for it, should consult a physician before taking any isoflavone product.
Some evidence suggests that genistein might impair immunity. One study in mice found that injected genistein has negative effects on the thymus gland (an organ that is important for immunity) and also causes changes in the prevalence of various white blood cells consistent with impaired immunity.34 Although the genistein was injected rather than administered orally, the blood levels of genistein that these injections produced were not excessively high; they were comparable to (or even lower than) what occurs in children fed soy milk formula. In addition, there are several reports of impaired immune responses in infants fed soy formula.35-38 While it is too early to conclude that genistein impairs immunity, these findings are a cause for some concern.
In addition, genistein and other soy isoflavones seem to impair thyroid function, although this inhibition may only be significant in individuals who are deficient in iodine.28,41,42 Other soy products have been observed to reduce absorption of thyroid medication.29,30,43 However, to make matters more confusing, studies of healthy humans and animals given soy isoflavones or other soy products have generally found that soy either had no effect on thyroid hormone levels, or actually increased levels.44-48
The bottom line: In view of soy’s complex effects regarding the thyroid, individuals with impaired thyroid function should not take large amounts of genistein or other soy products except under the supervision of a physician.
Soy products may impair thyroid function or reduce absorption of thyroid medication, at least in children.28,29,30 For this reason, individuals with impaired thyroid function should use soy with caution.
There are also concerns that use of soy isoflavones by pregnant women could exert a hormonal effect that impacts unborn fetuses.25,26 Talk with your doctor before taking genistein or large amounts of soy while pregnant or nursing.
Finally, while fears have been expressed by some experts that soy isoflavones might interfere with the action of oral contraceptives, one study of 36 women suggests that such concerns are groundless.27
View References
Last reviewed September 2002 by Medical Review Board
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