News & Perspectives

Soy: Can You Get Too Much of a Good Thing?

Compared with westerners, Asians have a relatively low risk of developing cancers of the breast and prostate. And the death rate from these cancers in Japan is about one quarter that in the United States.

Yet Asians who immigrate to the U.S. and start eating a western diet quickly see their cancer risk rise. Because soy is a staple of the Asian diet--Asians typically consume 20 to 50 times more soy than the average American--some scientists believe that eating more soy might help protect against these hormone-dependent tumors.

Other benefits have been ascribed to soy as well, including protection against heart disease, osteoporosis, and menopausal symptoms. At present, the evidence seems strongest for soy’s role in preventing cardiovascular disease. And in the fall of 1999, the federal government gave soy’s heart-healthy properties its official sanction, allowing some soy products to claim they can help lower cholesterol and reduce the risk of heart disease.

But now, just as Americans are eating more soy, there are disturbing hints that it may pose problems for certain people--namely, women with breast cancer, and perhaps older women in general.

So what’s the bottom line? Should you eat more soy, or wait for more information before loading up on tofu? And does it make sense to take soy components in supplement form? Well, here are some things we now know about soy, and some questions researchers are still trying to answer.


Soybeans contain protein and, depending on where they’re grown, varying levels of other substances that may have cancer-fighting properties.

Much attention has focused on soy’s isoflavones, or phytoestrogens--plant-derived compounds that produce estrogen-like effects in the body. Soy has a particularly high content of two isoflavones, genistein and daidzein, which exert hormonal effects similar to those of estrogen, but weaker.

Soy’s effects are complex. When there’s too much estrogen in the body, soy phytoestrogens may block its harmful effects, suppressing the growth of certain types of cells, including prostate and breast cancer cells. When there’s not enough estrogen, phytoestrogens may become estrogen substitutes and make up for the difference. Phytoestrogens may have benefits unrelated to estrogen as well.

Prostate Cancer

Intriguing evidence suggests that soy may help protect men against prostate cancer. Population studies have documented a low incidence of the disease in countries such as Japan, where soy intake is high. In addition, in mice, soy protein inhibits prostate cancer cell growth, with isoflavones a possible key anticancer component.

Such promising findings have prompted the U.S. Defense Department to fund a national pilot trial to test whether soy can reduce prostate cancer in men at risk. Meanwhile, a number of experts believe that increasing soy intake (while cutting fat) is a reasonable option for men seeking to decrease their prostate cancer risk.

Breast Cancer

Concerning soy’s possible role in breast cancer prevention, the picture remains a little murky at present.

On the one hand, several studies have shown that populations with a high soy intake, such as those in Japan and China, have a relatively low incidence of breast cancer. And other research suggests that soy may have cancer-fighting effects, inhibiting the growth of breast cancer cells.

On the other hand, other studies suggest that soy sometimes causes breast tissue to proliferate, a possible harbinger of cancer.

Soy’s effects on the breast may also differ at different times in a woman’s life. Dr. W. Mark Cline, associate professor at Wake Forest University, has observed, “It’s quite likely that exposure to soy early in life will turn out to be more important than exposure later in life.” This may help explain why Asians, who begin eating soy at a very young age, have lower rates of certain cancers.

Women's health specialist Dr. Tori Hudson does not regard the conflicting soy findings as a cause for concern. “I don’t think soy is a health risk for anybody,” she says. “The key is that soy is a weak estrogen mimicker, with only 1/400 to 1/1000 the potency of natural estrogen. I still advise soy for women of all ages, including breast cancer patients to treat menopausal symptoms.”

Dr. Hudson believes adding soy to the diet offers women a way of reducing the amount of estrogen they need to relieve symptoms of menopause. In some cases, she says, “A woman can take a half dose of estrogen replacement and fill the gap with soy.”

Heart Disease

A comprehensive review of 38 trials concluded that eating about 47 grams of soy protein daily can lower cholesterol by about 9%.

Recent research also indicates that soy can reduce levels of oxidized cholesterol, which is linked to the formation of dangerous plaques. And soy phytoestrogens also appear to help lower heart disease risk in men and women by increasing the elasticity of the arteries, which may decline with age.

Hot Flashes

Women living in Asian countries experience hot flashes less often than their counterparts in the West, suggesting that a high soy intake might protect against this menopausal symptom.

Studies examining soy’s effectiveness in easing hot flashes, however, have yielded mixed results. Some earlier research had suggested that high soy intake decreased the frequency and severity of hot flashes. However, a recent Mayo Clinic study found soy isoflavone pills to be no more effective than a placebo for this problem (Journal of Clinical Oncology, 3/00).

Eating soy foods may be a better bet for those with hot flashes.


The weakened bones of osteoporosis sufferers have been linked to declining estrogen levels after menopause, and researchers are trying to determine whether soy phytoestrogens might help prevent this disorder.

Several studies indicate that soy isoflavones can help conserve bone in animals. Data on the effect of natural isoflavones on osteoporosis are limited but suggest the compounds can help reduce bone loss caused by estrogen deficiency.

In addition, ipriflavone, a synthetic isoflavone, appears to help prevent bone loss in postmenopausal women.

Food vs. Supplement

Many experts feel the best way to increase your soy intake is to eat soy foods, such as tofu, soy milk, fresh soybeans, and roasted soybeans (soy nuts). They believe that it’s impossible to “overdose” on isoflavones by eating these foods.

According to Dr. Cline, strong evidence indicates that “there’s more of a benefit from the whole food” than from isolated phytoestrogens. Recent reports suggest that other soy components, not just isoflavones, have an anticancer effect.

Suggested dose

To help lower your cholesterol, the FDA recommends eating at least 25 grams of soy protein daily--the amount found in 1 cup of fresh soybeans, 1/3 cup of soy nuts, 3-1/2 cups of soy milk, or two soy burgers or soy dogs. (These amounts contain 70 to 140 mg of isoflavones.) Try some of the new soy foods on the market; many taste great.

Women’s health specialist Phuli Cohan, M.D., advocates higher amounts--about 50 grams daily, typically in the form of tofu (a quarter block) or soy protein powder, which is essentially dried whole soybeans (1/4 cup; check your brand).

Dr. Hudson also recommends soy protein powders or shakes and, in some cases, soy isoflavone supplements because, she says, “Most people in the U.S. aren’t thrilled about ingesting soy foods every day.”

Date Posted: 07/27/2001

Date Published: 07/26/2001
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