Soy Isoflavones

Soy has shown great promise in reducing the uncomfortable hot flashes of menopause, and new research indicates that it may also help protect against certain chronic diseases, including osteoporosis, heart disease, and some cancers.

What Is It

Found in soybean products such as tofu and soy milk, and sold in supplement form, isoflavones are powerful compounds known as phytoestrogens. These plant-based substances are chemically similar to the hormone estrogen, produced in the body, but are much weaker. Phytoestrogens, however, can bind to estrogen receptors in the cells and produce various important health benefits. Most research on soy isoflavones has been done with people who regularly ate soy products, so even though most supplements contain the major isoflavones in soybeans, genistein and daidzein, it’s not clear whether isoflavones are the only beneficial compounds in soy.

What Does It Do

As phytoestrogens, soy isoflavones have two important effects. First, when estrogen levels are high, phytoestrogens can block the more potent forms of estrogen produced by the body and may help prevent hormone-driven diseases, such as breast cancer. Second, when estrogen levels are low, as they are after menopause, phytoestrogens can substitute for the body’s own estrogen, possibly reducing hot flashes and preserving bones. Soy isoflavones may also have antioxidant and anticoagulant effects.

Common Uses

  • Reduce the frequency and severity of hot flashes and other menopausal symptoms.
  • May protect against coronary heart disease.
  • May forestall certain cancers.
  • May help prevent osteoporosis.


Research indicates that soybean products help protect against heart disease by lowering LDL (‘bad”) cholesterol, and significantly increasing HDL (‘good’) cholesterol. Soy seems most effective in people with high cholesterol levels. In those with near-normal cholesterol levels, its effects are less powerful, and larger amounts are needed to produce the same benefits. Soy products may also inhibit the oxidation of LDL cholesterol, the first step in the accumulation of artery-clogging plaque. In addition, laboratory studies show the genistein in soy helps prevent blood clots from forming.

In Asian countries where soy is a dietary staple, rates of certain cancers are much lower than they are in the United States. Preliminary studies indicate that regular consumption of soy foods or supplements may protect against cancers of the breast, prostate, and endometrium. And in animal studies, adding soy protein to the diet significantly reduces tumor formation and the likelihood that cancer, once developed, will spread. The phytoestrogens in the soy are most likely responsible for this effect. Researchers speculate that the isoflavone genistein may block a protein called tyrosine kinase, which promotes the growth and proliferation of tumor cells. This effect may be why soy is also associated with a lower risk of prostate cancer. Genistein has potent antioxidant properties as well, and for these reasons, it may one day prove useful against cancer, though more research is clearly needed.

Additional Benefits

Studies show that hot flashes and other symptoms of menopause are relatively rare in Asia, where women eat a lot of soy products. In addition, in one Western study, women who added 45 grams of soy flour to their daily diet experienced a 40% reduction in hot flashes.

Soy isoflavones may also help women maintain bone density. One study of postmenopausal women found that consuming 40 grams of soy protein a day resulted in a significant increase in bone mineral density in the spine, an area often weakened by osteoporosis.

How To Take It


Experts don’t know the number of soy isoflavones needed to produce a therapeutic effect. In Asian countries, isoflavone consumption ranges from 25 to 200 mg a day. Some researchers believe that an intake of 50 to 120 mg a day might be the minimum amount necessary. The supplements on the market vary in the types of isoflavones they contain and the total amount of isoflavones per pill. Choose a product that supplies a mixture of isoflavones (it should include both genistein and daidzein) and take enough pills to obtain 50 to 100 mg isoflavones a day.

Guidelines For Use:

Most experts recommend that you try to get your soy isoflavones from soy foods. In addition to their isoflavone content, these foods are good sources of protein, so they can replace red meat and other foods high in saturated fat.
The number of isoflavones in soybeans, and therefore any product made from them varies. In general, eating one to two servings of soy products a day is probably sufficient. (A serving equals: 3.5 ounces tofu or miso, 1 cup soy milk, or 1/2 cup soy flour, cooked soybeans, or textured vegetable protein.) If eating this much soy is not to your taste, you might want to get your isoflavones from a combination of foods and supplements. Another alternative is soy powder, which contains both soy protein and isoflavones; mix it into juice, milk, or shakes. Take soy supplements with a large glass of warm water right before eating breakfast and dinner.

Possible Side Effects

Soy products, even in large quantities, are not known to produce side effects. However, the small percentage of people who are allergic to soybeans should avoid all soy supplements and soy-based foods.

Facts and Tips

A diet high in fiber may interfere with the absorption of isoflavones. If you eat a high-fiber diet, be sure to increase your consumption of soy supplements or soy foods. Even though they’re made from soybeans, soy sauce and soybean oil contain no isoflavones.

Latest Findings

  • In a recent study, people with moderately high cholesterol levels drank a daily “milkshake” containing 25 grams of soy protein, either with or without isoflavones. After nine weeks, those who consumed the isoflavone-rich shake experienced, on average, a 5% reduction in LDL (“bad”) cholesterol levels. People with the highest LDL levels had an 11% drop. (For each 10% to 15% drop in LDL levels, the risk of a heart attack decreases 20% to 25%.)
  • Women who ate the most soy products and other foods rich in phytoestrogens reduced their risk of endometrial cancer by 54%, according to one study. Soy products may be especially important for women who have never been pregnant. Among this group, eating less than an average of 1/4 ounce of soy products a day was associated with a fourfold increase in the risk of endometrial cancer.

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