May 2000 -- Once thought of as an exotic Asian food or a meat substitute for vegetarians only, soy has become increasingly familiar to American consumers. And why not? Soy is gaining a reputation as something of a miracle food with the ability to relieve symptoms associated with menopause as well as lower the risks for osteoporosis, heart disease, breast and prostate cancer. It is no wonder that more Americans are incorporating soy into their diets. But is this wise?

There is no doubt that soy is extraordinary. It contains compounds that mimic the female hormone estrogen, estradiol: daidzein and genistein. Although they are weaker than estrogen, these substances can bind with the estrogen receptor and act like estrogen or, if they displace estrogen, as an anti-estrogen.

Despite the fact that soy is a traditional part of many Asian diets and is growing in popularity in the West, how it effects health is still unclear. Recent studies have tried to shed light on the relationship of soy to breast cancer. Case-control (retrospective) studies in human and experiments in animals have both shown mixed results. In animals, soy consumption reduced tumors in some reports but not in others.

In China, case-control studies were conducted in the cities of Shanghai and Tianjin to test the effects of soy on breast cancer. Women who had the highest rates of soy in their diets showed no increase or decrease in the rates at which they developed breast cancer as compared to those who ate the lower amounts of soy. These studies are important because the level of soy consumption was high.

Interestingly, a preliminary report from a recent study in Shanghai did suggest that high soy intake during adolescence may lower the risk of developing breast cancer later in life. If finding this holds true, then timing the consumption of soy may be an important factor in our ability to enjoy the protective benefits.

There are, however, some important reasons to be skeptical about the benefits of high soy consumption for reducing breast cancer. For one thing, women in Asia have lower rates of breast cancer across the board compared to women in the West, and many of these Asian populations don't consume soy. Also, studies in the United States among members of the Seventh Day Adventist church who followed vegetarian diets and consumed larger amounts of soy showed the same rates of breast cancer as in the general U.S. population. Most worrisome was the result of a trial of soy protein supplementation given to women receiving breast biopsies. After one month of a high soy diet, these women actually showed an increase in the number of breast cells undergoing multiplication compared with women on their usual diet.

Evidence for soy as a palliative for the symptoms of menopause are no more encouraging. It is true that soy-consuming Japanese women reported fewer night sweats and hot flashes than did Canadian women, and, Australian women who took soy for 12 weeks reported a 40% reduction in symptoms. But the Australian control group who ate no soy did almost as well, suggesting that time itself, rather than soy, could account for the improvement. After all, menopausal symptoms do improve with time. Adding to the difficulty in judging effectiveness is the fact that symptoms are subjective. One woman might judge the severity of her symptoms quite differently from another.

What of the other health claims made for soy? Does it lower cholesterol? What effect does it have on rates of prostate cancer, osteoporosis, and cognitive function? When it comes to prostate cancer and osteoporosis, the jury is still out. The FDA has allowed manufactures to advertise soy's ability to lower cholesterol but it would take eating over a pound of tofu each day to realize the benefit. A recent study on cognitive functioning again raised concerns about the dual nature of soy's estrogen-like compounds. In that among Hawaiian adults of Japanese descent, higher levels of cognitive impairment were seen in those who had the highest rates of soy consumption. The results were not conclusive, but emphasize that we cannot assume that high soy consumption is completely safe.

Much remains to learn about soy. But, in the meantime what should consumers do? If you have traditionally consumed soy as a regular part of your diet, there is not enough evidence to suggest change. If, however, you did not grow up eating the traditional Asian diet and if you have not been a regular soy consumer in the past, it might be best not to add large amounts of soy products to your diet. It is probably reasonable to include soy and tofu in the diet 4 or 5 times a week. Ongoing research should continue to reveal the complex relationship between soy and health.