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Soy Isoflavones

What Is It?

Westerners have been slow to put tempeh, soy milk, tofu, and other soybean products on the table. But as evidence mounts that compounds in soy–isoflavones–have impressive health benefits, so too does consumption of this Asian dietary staple.

Soy isoflavones are powerful plant substances chemically similar to the female hormone estrogen. Their presence in soy foods may help to explain why people in countries where soy is a big part of the diet suffer from relatively few menopausal symptoms such as hot flashes. It may also explain why rates of certain hormone-related cancers are much lower in Asia than they are in many parts of the West, including the United States.

While soy isoflavones are now available in supplement form, it remains to be determined whether these pills can provide the same health benefits as soy isoflavones consumed as part of the diet. In fact, most research has been done on people who eat soy products rather than take soy supplements. It’s possible that isoflavones are just one of several therapeutic compounds in soy. Still, while it’s ideal to get soy isoflavones from one or two daily servings of soy foods, supplements are now an option for soy-leery eaters.

Health Benefits

Two particularly important isoflavones in soy–genistein and daidzein–appear to protect against hormone-related disorders such as breast cancer and endometriosis. They do this by competing for the same place on cells (receptor sites) that the body’s own estrogen does. Some of the risks of excess estrogen, including breast and uterine cancer, can apparently be lowered in this way.

Similarly, when the body’s natural levels of estrogen drop, as they do with menopause, soy isoflavones can compensate by binding to some of the cell receptor sites that estrogen once did. Menopausal symptoms may improve as a result.

Research findings suggest that soy isoflavones may also inhibit cancer-causing enzymes, provide antioxidant protection, and enhance the immune system.

Specifically, soy isoflavones may help to:

Control symptoms of menopause and perimenopause. When regularly ingested, soy may reduce the frequency and severity of hot flashes and other menopausal difficulties. In one study, women who added 45 grams of soy flour to their daily diet experienced a 40% reduction in hot flashes. The isoflavones in soy are believed to be responsible for these effects.

Guard against osteoporosis. Soy isoflavones may aid women (and men) in maintaining bone mineral 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. If the spine is weak, stooped posture and myriad complications may develop.

Counteract the effects of endometriosis. The phytoestrogens in soy products may help to offset the action of the body’s natural estrogen, which is often responsible for instigating the monthly pain, heavy bleeding, and other symptoms of endometriosis.

Protect against prostate problems. Eating soy products may protect against enlargement of the male prostate gland. The size of the prostate gland tends to increase with age, causing various types of urinary difficulties, including frequent nighttime awakenings.

Prevent various cancers. Preliminary studies show that regular consumption of soy foods or supplements may protect against hormone-related cancers of the breast, prostate, and endometrium. According to one study, women who ate the most soy products and other foods rich in phytoestrogens reduced their risk of endometrial cancer by 54%.
Integrating soy products may be especially important for women who have never been pregnant. One analysis found that women in this category who consumed little, if any, soy–less than a quarter ounce in a given day, on average–were at four times the risk for developing endometrial cancer. In animal studies, adding soy protein to the diet significantly reduced tumor formation and the likelihood that cancer, once developed, would spread. So even though more research is needed on soy’s cancer-fighting properties, researchers are hopeful. They speculate that the isoflavone genistein may block a protein called tyrosine kinase, which promotes the growth and proliferation of tumor cells.

Reduce heart disease risk. Heart-healthy actions have been attributed to isoflavone-rich soy. In 1999 the Food and Drug Administration declared that soy foods can be billed as products that reduce the risk of heart disease by lowering harmful cholesterol. Specifically, soy products have been shown to lower LDL (“bad”) cholesterol and significantly increase HDL (“good”) cholesterol.
In one study, people who drank a “milk shake” containing 25 g of soy protein for nine weeks experienced, on average, a 5% reduction in LDL cholesterol. And people with the highest LDL levels experienced an 11% drop. (For each 10% to 15% drop in the LDL level, the risk of a heart attack decreases 20% to 25%.) Because soy’s effects are less marked in people with near-normal cholesterol levels, individuals in this category will need to take larger amounts of soy to produce the same reductions. In addition, soy products appear to inhibit the oxidation of LDL cholesterol that can eventually clog arteries.

Note: Soy isoflavones have also been found to be useful for a number of other disorders. For information on these additional ailments, see our Dosage Recommendations Chart for Soy Isoflavones.



Dosage Information

Special tips:

The best way to get soy isoflavones is through soy foods. Such foods provide abundant protein without the saturated fat found in many other protein-rich sources, such as red meat.

Experts have yet to pinpoint the amount of soy isoflavones needed to produce health benefits. In Asian countries, where the prevalence of disorders such as breast cancer and invasive prostate cancer is relatively low, people consume soy foods in amounts that provide an estimated 20 to 200 mg of soy isoflavones a day.

If you decide to take supplements, choose products that supply a mixture of isoflavones including genistein and daidzein.

High-quality soy-based protein powders are widely available and easy to use; simply add the recommended number of scoops to a cold or hot drink or a food such as oatmeal or soup.
For the majority of conditions mentioned: Try to ingest a total of 100 mg twice a day from supplements, foods, or a combination of the two.

Note: Soy isoflavones have also been found to be useful for a number of other disorders. For information on these additional ailments, see our Dosage Recommendations Chart for Soy Isoflavones.

Guidelines for Use

Take a pill containing soy isoflavones with a large glass of warm water before eating breakfast and dinner.

A high-fiber diet may interfere with the absorption of soy isoflavones. Therefore, if your diet is high in fiber, boost your soy consumption.

General Interaction

There are no known drug or nutrient interactions associated with soy isoflavones.

Possible Side Effects

Even large amounts of soy isoflavones obtained through foods pose no apparent health risks. However, much less is known about the potential side effects of taking soy isoflavones in concentrated capsule form, and research into the increased risk for breast cancer and other reactions is ongoing.

A woman with a strong family history of breast cancer in particular should be aware that in laboratory studies, soy stimulates the growth of breast cells. Whether this directly translates into an increased risk for breast cancer in humans is poorly understood, however. Consult your doctor for guidance.


A woman’s decision to take soy isoflavone supplements for any condition must be an individualized one made in consultation with a doctor familiar with her medical background. This is certainly the wisest approach until research can reveal more details about how soy isoflavones affect the body–in both positive and negative ways.

Because of their estrogenlike effects, pregnant or nursing women should not take soy isoflavone supplements. (Soy-rich foods pose no such problems, however.)

Avoid all soy supplements and soy-based foods if you have an allergy to soybeans.


Alcoholism 1-2 scoops twice a day mixed with juice
Cancer Follow package instructions (usually 2 scoops twice a day, mixed with juice).
Menopause 50-100 mg once or twice a day
Perimenopause 50-100 mg once or twice a day
Prostate Problems 2 500 mg capsules (standardized to contain 5% soy isoflavones) each morning

Doctor Recommendations
David Edelberg, M.D.

Soy is definitely good for you. It lowers cholesterol and helps prevent heart disease. It can help protect bones from osteoporosis. In societies where soy figures prominently in the daily diet, relatively few women suffer from such menopausal symptoms as hot flashes and night sweats. In these same countries, the rates of certain hormone-related cancers are remarkably low. Of course, whether the apparent healing compounds in soy foods–the isoflavones–will benefit you in the same ways once they’ve been isolated and packaged into pills isn’t so clear. We’ve still got a lot to learn.


The active ingredients in soy, called isoflavones, are compounds known as phytoestrogens, plant-based substances that can have a weak estrogenlike effect. As a woman’s natural levels of estrogen start to drop during perimenopause, these isoflavones can help to make up for the decline. Granted, soy is much weaker than your body’s own estrogen, and weaker still than commercial, animal-based estrogens such as Premarin. But if you’re like many women, soy may prove just strong enough to ratchet down the number of hot flashes and other perimenopausal symptoms you’re now enduring.


You essentially have three options when it comes to soy: Soy foods are by far the wisest choice: tofu, tempeh, and products made with soy flour. The list goes on and on. Soy protein powders made from protein concentrates of the soybean are yet another option for the many women who resist eating soy foods. Try blending the powder with juice and fruit for an appealing protein-packed punch. Capsules or tablets containing extracted soy isoflavones are widely available. Look for products that supply a mixture of isoflavones including genistein and daidzein. The pills are best taken with a large glass of warm water twice a day, before eating breakfast and dinner. Shopping tip: Soy sauce and soybean oil aren’t good choices: They’re made from soybeans but after processing are ultimately devoid of isoflavones or soy protein.


What about the latest news, with its warnings from concerned scientists, about a connection between soy supplements and cancer? Right now, it’s theory only. Many soy supplement studies are under way. And no clinical study has ever shown that a single patient developed breast cancer from eating inordinate amounts of soy to prevent hot flashes or osteoporosis. If you’re troubled by all this, the answer is simple. Don’t overdo it. Eat plenty of soy, but don’t bother with the isoflavone capsules. They may not even be necessary if your soy intake is high. Scientists still have a lot more to learn about the potential side effects of taking soy isoflavones in concentrated form.


If you have a strong family history of breast cancer, talk over any concerns you might have about soy and cancer with your doctor. Remember, the studies so far have been limited to the test tube.

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