Over the last several years, millions of Americans who had turned a deaf ear to the virtues of soy have had a change of heart. Sales of the lowly bean, which has been a staple of the Asian diet for millennia, have been skyrocketing because preliminary research suggested that soy has many life-enhancing benefits, from preventing bone density loss to easing some symptoms of menopause.
In October, the federal government put its imprimatur on soy when it allowed food companies to make the claim that soy protein reduces cholesterol and the risk of heart disease. The other claims are still unproved, even though soy in all its forms, from tofu and veggie burgers to shakes and supplements, is being heavily promoted by its sellers as a panacea. The news media, too, have been almost unanimous in praising its safety and efficacy.
Against the backdrop of widespread praise, however, there is growing suspicion that soy _ despite its undisputed benefits _ may pose some health hazards. The scientific world is divided over many of the claims for efficacy and over some safety issues, but there are two points on which there is agreement:
+ Soy is useful in reducing cholesterol;
+ There may be an increased risk of cancer associated with consuming the components of soy called isoflavones in supplement form, particularly for post-menopausal women; and for these women, there may also be hazards in adding soy foods to their diets.
Isoflavones, particularly genistein and daidzein, are phytoestrogens (plant chemicals that have estrogenlike hormonal effects on the body) that occur naturally in soybeans and foods made from them. Compared with chemical estrogens, the kind taken by women to reduce the symptoms of menopause, phytoestrogens are weak, but they act the same way: they can both inhibit and stimulate the growth of certain types of cells.
Not one of the 18 scientists interviewed for this story was willing to say that taking isoflavones was risk-free. Some particularly cautioned against it. Dr. Margo Woods, an associate professor of medicine at Tufts University School of Medicine, who specializes in nutrition and breast cancer, said: "As a food, soy does a lot of great things, but once you start looking at different components like phytoestrogens, you are talking about pharmacological things. It's wiser to talk about soy and soy foods. A whole food behaves very differently in the body than when you take one compound. We are looking into the components, but we haven't been studying in the area long enough. I would not recommend to anyone that they take isoflavones."
Even before the Food and Drug Administration approved the cholesterol-lowering health claim for soy, sales were booming. In 1998, 770,000 metric tons of soybeans were sold in this country to be turned into food products; in 1999 the figure rose to 1.007-million metric tons. Total sales of soy foods in supermarket chains during the 12 months ending in October were almost $420-million, up 45 percent from the previous year, according to Spins, a natural products market research company in San Francisco. In natural food stores, sales in the six months ending in October were up 37 percent from the previous six-month period. Many large companies like Kellogg's, General Mills, Campbell Soup and Con Agra are developing new soy products in response to the demand.
The biggest jump has been in soy supplements, the isoflavone pills, whose sales were up 246 percent in the 12 months ending in October. But the carefully worded health claim the Food and Drug Administration permits for cholesterol reduction is for soy protein, not for isoflavones. To have that health claim on its label, a food must be low in fat, saturated fat and cholesterol and contain 6.5 grams of soy protein per serving. For the cholesterol-lowering effect, 25 grams of soy protein must be added daily to a diet low in saturated fat and cholesterol.
The scientists are worried that the public is interpreting the approval of soy protein as a recommendation to take soy supplements, which generally have higher levels of isoflavones than occur naturally in food.
The highest levels of naturally occurring isoflavones are found in soy beverages, cooked soybeans and tempeh, and the range is wide. Some processed products, like sports bars, have added isoflavones. Supplements can contain more than 85 milligrams of isoflavones in a single pill, and some manufacturers advise taking two pills a day. There are soy protein concentrate powders with as much as 160 milligrams of isoflavones in a single serving.
"People don't distinguish isoflavones from soy protein," said Dr. Daniel Sheehan, a research biologist at the FDA's National Center for Toxicological Research in Jefferson, Ark. He also directs a program that studies endocrine disrupters, which are chemicals like isoflavones with hormonal activity that disrupt the endocrine system. "The approval of soy protein for cardiovascular disease is going to lead to tremendous increase in the use of isoflavones, and this rachets up concern levels," he added. For that reason, Sheehan opposed the FDA's health claim label.
Dr. Gregory Burke, chairman of the public health sciences department at Wake Forest University School of Medicine in Winston-Salem, N.C., worries that Americans will overdose on these supplements. "To Americans, more is better," he said. "Adding soy to a healthy diet makes a lot of sense. If you have to take soy through food, you are not going to overdose on isoflavones, but if you take it through pills you could take doses up to 10 times what folks consume in Japan" _ where soy is a staple of the diet. "I don't know what the risk-benefit is on those high doses," Burke said.
No one is really willing to put a number on the maximum safe level of isoflavones, because human studies have not been done, said Dr. Daniel Doerge, a research chemist in biochemical toxicology at the National Center for Toxicological Research. "Nobody knows what high levels are," he added.
Dr. William Helferich, an associate professor of nutrition at the University of Illinois, who specializes in diet and breast cancer, pointed out that regulators, food companies and some scientists are willing to make the assumption that 50 milligrams of isoflavones a day are safe. One gram of soybeans contains about one milligram of isoflavones. "That amount may be perfectly safe and beneficial for the vast majority of the population," he said. "However, the potential for some individuals to be harmed, such as post-menopausal women with estrogen-dependent breast cancer, still remains unclear. Because it is a natural food component, there is no requirement to put any indication or contraindication on the products. I don't know how anyone can commit to any number, because data are not there for all populations." The typical daily consumption in Japan, for example, is 25 to 50 milligrams of isoflavones.
Whether soy or its isoflavones reduce the risk of breast cancer or increase it has not been sorted out. Data on estrogens are mixed so far, Helferich said. "Estrogens are a mixed bag," he said. "Given early they will prevent, given late they will likely promote. Taking natural estrogens late in life to prevent symptoms of menopause may be a real problem."
Epidemiological studies suggest that soy may reduce the risk of breast cancer, but there are animal and even human studies that suggest that soy may increase the risk. Dr. Stephen Barnes, a professor of pharmacology and toxicity at the University of Alabama, said that in countries like Japan, where there is a lot of soy in the diet, women have lower rates of breast cancer than American women. But American-born daughters of Japanese immigrants have higher levels of breast cancer than their Japanese-born mothers. This statistic is often cited as proof that soy reduces the risk of breast cancer. Barnes said that consumption of soy early in life may make the difference. "But," he added, "I don't know what happens if you are 50 years old and go on soy."
Yet another study, reported in the American Journal of Clinical Nutrition in December 1998, showed that there was increased breast cell proliferation in women given soy. The greater the cell proliferation, the greater the chance for cancer cells to develop.
A number of studies that may answer these questions are being underwritten by the National Cancer Institute, but Dr. Peter Greenwald, the director of the institute's division of cancer prevention, said: "The results are mixed and far from definitive. I don't think we are in a position to give advice for or against."