If hormone replacement therapy has gotten a drubbing of late, so too has a much-celebrated food that many women eat to help ease the symptoms of menopause and to protect against heart disease and osteoporosis: the soybean.
Some scientists are increasingly wary about Americans going hog-wild for soy, soy protein and the estrogen-like chemicals (isoflavones) that soy protein contains.
A soon-to-be-published study linking tofu to a faster decline in mental abilities is particularly unwelcome news for any woman who sees eating soy as risk-free, either as well as or in addition to conventional hormone replacement therapy. How seriously should she view this new twist to the soy story? Should she toss out her tofu? Swear off soy burgers?
Most soy scientists simply preach moderation.
"The majority of evidence indicates that soy is a safe food that's eaten by two-thirds of the world's population," says Dr. David Heber, director of UCLA's Center for Human Nutrition, adding that it does appear to have health benefits. But Heber and others warn against eating soy to the exclusion of other foods: A varied diet is important to good health. And they caution against consuming very large amounts of soy protein or popping isoflavone pills available in stores.
"There's a tendency in our culture to think if a little is good, then a lot's better," says Mary Anthony, a soy researcher at Wake Forest University School of Medicine in Winston-Salem, N.C. "But I personally am very concerned about isoflavone pills and soy protein supplemented with extra isoflavones."
Isoflavones, after all, seem to act like hormones or drugs in our body--even if for regulatory purposes they are classified as nutritional supplements.
Some of the druglike effects of soy protein or the isoflavones they contain could be good for us. Several dozen clinical studies have reported, for instance, that eating an average of 25 grams of soy protein daily for several weeks to months lowers blood levels of "bad" (or LDL) cholesterol and may help elevate "good" (or HDL) cholesterol levels--in other words, may help protect against heart disease.
Based on such studies, the U.S. Food and Drug Administration now allows foods with at least 6.25 grams of soy protein per serving to be labeled with claims stating that they could help reduce the risk for heart disease.
Soy protein (probably because of its isoflavones) might also help keep bones strong, as well as ward off prostate or breast cancer.
Yet some scientists warn that soy may have a dark side.
For instance, the new study of more than 3,000 Japanese American men living in Hawaii "raises the specter that there may be--even if there are benefits--a cost to be paid," says Dr. Lon White of the Pacific Health Research Institute on Honolulu, author of the study.
In that study, the diets of the men who ate tofu at least twice a week were assessed in midlife and their mental abilities assessed several decades later. In addition to the more rapid decline in mental abilities, compared with non-tofu-eaters, the men experienced a reduction in brain size.
Other potential downsides to soy, some scientists say, include the possibility that soy chemicals might reduce the body's ability to absorb minerals, might cause fertility problems, alter sexual development, cause abnormalities of the thyroid and contribute to cancers of the breast or pancreas. In particular, these scientists have warned against feeding babies soy-based formula.
The Food and Drug Administration, in reaching its decision to allow soy products to carry health labels, concluded there was insufficient evidence to warrant such concern (though it didn't weigh in on the issue of soy formula).
"Most of the concerns are red herrings," says Stephen Barnes, professor of pharmacology and toxicology at the University of Alabama in Birmingham.
Two studies addressing the breast cancer risk, for instance, did detect increases in the amount of fluid that women eating soy secreted from their nipples--a potential danger sign. But the more detailed study didn't find any change in a variety of other measurements that are more direct signs of heightened risk, such as increased growth of breast cells.
Although soy does contain chemicals called trypsin inhibitors that might adversely affect the pancreas, there is little evidence that this happens in humans. Nor is there proof that the chemical called phytate in soy interferes with absorption of minerals like iron or that isoflavones interfere with hormones of the thyroid gland and cause a disorder known as goiter.
What's more, these possibilities--since many plants contain similar chemicals--are not confined to the soybean.
"There are a lot of foods that could potentially cause goiter--including cabbage," Heber says.
Dr. Claude Hughes, endocrinologist and chairman of the Center for Women's Health at Cedars-Sinai Medical Center, says he does worry about exposure of babies to isoflavones through formula. He also worries about too much soy for women who have had breast cancer. As for the possible dementia link, "we sure can't blow it off."
The best bet, for now, he suggests, is for women to make decisions about eating soy that best matches their individual risk.
"Personally, I eat tofu or edamame several times a week--people in my family get heart attacks and strokes, whereas Alzheimer's disease has never been an issue in my mom's or my dad's family," he says. "I would offer that if there's a family history of heart disease then the concern should be about doing everything you realistically can to minimize progression of heart disease. But if lots of people in your family get Alzheimer's, then you may decide, 'Well, I don't want to do this.' "