Isoflavones are typical constituents of certain food plants from human nutrition and animal feeding, such as soy, red clover or alfalfa (lucerne). Their chemical structure resembles that of the flavones, typical contrituents of many fruits and legumes - the syllable "iso" tells the chemist that a part of the molecule is differently attached.

The best-known isoflavones are genistein, daidzein and glycitein from soy and red clover, and biochanin A and formononetin from red clover. The latter are transformed to the former in the human gastrointestinal tract.

A differentiation is made between "glycosides" and "aglycones": within the plant, secondary plant metabolites such as the isoflavones are usually present with a sugar moiety attached to it. This carbohydrate part is removed by fermentation or during digestion in the gastrointestinal tract. It is the aglycone part which is made responsible for the effects of isoflavones in the human organism. This aglycone part corresponds to approximately 60 percent of the weight of the glycosides: 100 mg of isoflavone glycosides correspond to approximately 60 mg of isoflavone aglycones.

In the soybean itself and in most soy products, genistein, daidzein and gycitein account for approximately 50-55%, 4-45%, and 5-10% of total isoflavones, respectively (Murphy et al. 1999). Isoflavones are also called "phyto-estrogens" (better: selective estrogen receptor modulators, SERMs), and in many publications the similarities of the structure of genistein and estrogen is stressed. However, this similarity is misleading, as the effect is not exactly the same as that of estrogen. Genistein does not prefentially bind to and activate the classical estrogen receptor alpha (which is responsible for many of the hormonal effects), but to the estrogen receptor beta, which has protective effects in the control of hormonal functions (Heldring et al. 2007).

Most health related effects of isoflavones can be explained by their differential effects at the estrogen receptors, and especially at the estrogen receptor beta.

References

Heldring N., Pike A., Andersson S., Matthews J., Cheng G., Hartman J., Tujague M., Strom A., Treuter E., Warner, M. and Gustafsson J.A. (2007). Estrogen receptors: how do they signal and what are their targets. Physiol Rev., 87(3), 905-931.

Murphy P. A., Song T., Buseman G., Barua K., Beecher G. R., Trainer D. and Holden J. (1999). Isoflavones in retail and institutional soy foods. Journal of Agricultural and Food Chemistry, 47(7), 2697-2704.