Isoflavones are water-soluble chemicals found in many plants. In this article, we will discuss a group of isoflavones that are phytoestrogens, meaning that they cause effects in the body somewhat similar to those of estrogen. The most investigated phytoestrogen isoflavones, genistein and daidzein, are found both in soy products and the herb red clover. Soy additionally contains glycitein, an isoflavone that is more estrogenic than genistein and daidzein, but is usually present in relatively low amounts. Red clover also contains two other isoflavones: biochanin (which can be turned into genistein) and formonenetin (which can be turned into daidzein).

Certain cells in the body have estrogen receptors, special sites that allow estrogen to attach. When estrogen attaches to a cell’s estrogen receptor, estrogenic effects occur in the cell. Isoflavones latch onto estrogen receptors too, but produce weaker estrogenic effects. This leads to an interesting two-part action. When there is not enough estrogen in the body, isoflavones can stimulate cells with estrogen receptors and partly make up for the deficit. However, when there is plenty of estrogen, isoflavones may tend to block real estrogen from attaching to estrogen receptors, thereby reducing the net estrogenic effect. This may reduce some of the risks of excess estrogen (for example, breast and uterine cancer) while still providing some of estrogen's benefits (such as, preventing osteoporosis).

Isoflavones also appear directly to reduce estrogen levels in the body, perhaps by fooling the body into thinking that it has plenty of estrogen.

Isoflavones are widely thought to be the active ingredients in soy products. However, growing evidence suggests that there are other active ingredients as well, such as proteins, fiber, and phospholipids.

Although isoflavones are not essential nutrients, they may help reduce the incidence of several diseases. Thus, isoflavones may be useful for optimum health, even if they are not necessary for life like a classic vitamin.

Roasted soybeans have the highest isoflavone content: about 167 mg for a 3.5-ounce serving. Tempeh (a cake of fermented soybeans) is next, with 60 mg, followed by soy flour with 44 mg. Processed soy products such as soy protein and soy milk contain about 20 mg per serving. The same isoflavones found in soy are also contained in certain red clover products.

When purified isoflavones from red clover or soy are used, the dose generally ranges from about 40 mg to 80 mg daily. This is considerably higher than the average isoflavone intake in Japan, which is about 28 mg daily.109(Post-menopausal Japanese women may consume closer to 50 mg daily.17)

There are three major isoflavones found in soy: genistein, daidzein, and glycitein. Each of these isoflavones can occur in two types or states. The first type, predominant in raw soy products, is called an “isoflavone glycoside.” In an isoflavone glycoside, the isoflavone is attached to a sugar-like substance known as a “glycone.” The second type, predominant in fermented soy products, is called an “isoflavone aglycone.” These consist of isoflavones without a glycone attached, and are also called “free isoflavones.” Since isoflavone aglycones are the most pure form of isoflavones, it has been hypothesized (but not proven) that they are more effective than other forms.193