Isoflavones
Isoflavones are polyphenols with the same double ring structure as other flavonoids. Their structure is also very similar to estrogen, thus giving them the classification of phytoestrogens. Their activity in the body is very similar to that of estrogen, although on a much weaker scale, which gives them the ability to increase the effects of a low estrogen level, or balance the effects of a high estrogen level. Soybeans are the richest source of isoflavones, where they are found bound to a sugar molecule in a form known as a glycoside. When soy products are either fermented or digested, the sugar molecule is released and the isoflavone is now an aglycone. This makes the isoflavone available to be absorbed and metabolized by the body. The isoflavones in this bioavailable form without sugar molecules attached are genistein, daidzein, and glycitein.
Isoflavones work by binding to estrogen receptors which are located throughout the body, not only in the reproductive system but also in the heart, bones, liver, brain and other tissues. When they bind to estrogen receptors, the isoflavones may either mimic the activity of estrogen, or they may block the effects of estrogen. Blocking estrogen in some tissues may reduce the risk of cancers caused by this hormone, such as breast, uterine and prostate cancers. Mimicking the effects of estrogen can help the body to increase bone density and lessen the unpleasant symptoms that go along with menopause. Isoflavones also work in the body in ways unrelated to their estrogen-like properties. They inhibit the activity of some enzymes, and in the lab they are found to have antioxidant properties.
Isoflavones are the subject of numerous studies Some of these are to determine if they can help lower serum cholesterol, effects on arterial function, whether they can prevent or treat hormone-related cancers, decrease the risk of osteoporosis, and prevent cognitive decline. Results for most of these areas seem to be all over the map at the moment, with studies showing isoflavones have beneficial effects, and studies showing no effect. One factor may be that people metabolize isoflavones differently. In some people the isoflavone daidzein may be metabolized in the intestine to equol, which has even greater estrogenic activity than daidzein. In others it may be metabolized into compounds that are less estrogenic. Studies have shown that only about 33% of individuals from Western populations metabolize daidzein to equol.
Many legumes, grains and vegetables contain small amounts of isoflavones, but the best source by far is from soybeans, and the many products made from them. Isoflavones in fermented soy products are more easily absorbed, since they already are separated from the sugar molecule. Soy protein separated with an alcohol wash loses most of the isoflavones, so choose products processed with an aqueous wash instead. Soy protein, miso, tempeh, tofu, soy milk, soybeans, soy yogurt, edamame and soy meat and cheese subsitutes all contain isoflavones. Supplements and extracts of isoflavones are also available over the counter in the US, but make sure you get them from a reputable company, as tests have shown these products have poor quality control.
Breast cancer survivors should use caution and check with their doctor before using a lot of soy products, as they may interfere with drugs that inhibit tumour growth. The effects of a high isoflavone intake on pregnancy have not been established, so also a good idea to avoid these supplements if you are pregnant. A high soy intake has also been found to interfere with some drug therapies such as the anticoagulant warfarin, and thyroid hormone replacement drugs.
The information on this page is not meant to be used in treatment of medical conditions. Please seek the advice of a physician about any medical condition or symptom. Those with medical conditions should consult a medical professional about the appropriateness of taking dietary supplements or diet therapy, and how these methods will interact with their medications.
LINKS
Linus Pauling Institute has information about isoflavones.
Sloan-Kettering Cancer Center information on soy.