While the existing scientific data strongly support the value of increasing soy protein as described in the health claim, questions have been raised about individual components of soy, especially when consumed as concentrated supplements by some segments of the population.
"FDA continues to monitor the debates about the relative safety of these individual soy components and the scientific research that will eventually resolve them," says the Center for Food Safety and Applied Nutrition's Lewis, "If new results suggest an increased risk, the agency will modify or refine its policies in light of the new information."
A number of studies already are under way or in the planning stages now. In one study, Barry Delclos, Ph.D., a researcher at FDA's National Center for Toxicological Research (NCTR), is overseeing a long-term, multigeneration study in rats of the soy component genistein. Early data using rats suggest that genistein alone may prompt undesirable effects such as the growth of breast tissue in males. The study will analyze the relationship between dosage and any adverse outcomes.
The National Institutes of Health is sponsoring a long-term follow-up study on the safety of soy infant formula. The study is a "longitudinal retrospective epidemiological" assessment in which young adults who consumed soy formula as infants will be compared with young adults who consumed milk-based formulas as infants. They will be evaluated for any adverse effects from infancy into their childbearing years.
NCTR's Sheehan says research is needed in this area because an earlier study, published in 1997 in the medical journal The Lancet, showed that infants consuming soy formula had five to 10 times higher levels of isoflavones in their blood serum than women receiving soy supplements who show menstrual cycle disturbances. He says these levels may cause toxicological effects. "Infants receive higher doses of soy and isoflavones than anybody because it is their only food and they are consuming it all the time." The American Academy of Pediatrics, however, has published guidelines showing that in some cases, soy protein-based formulas "are appropriate for use in infants" when cow's milk cannot be tolerated.
Sheehan also expresses concern about the effects soy may have on the function of the thyroid gland. Animal study results, some of which date back to 1959, link soy isoflavones to possible thyroid disorders, such as goiter. A 1997 study in Biochemical Pharmacology identified genistein and daidzein as inhibitors of thyroid peroxidase, which data suggest may prompt goiter and autoimmune disorders of the thyroid. Critics of these studies suggest that iodine deficiency may be a factor that needs to be considered when evaluating study results.
Though the research community has varying degrees of concern about a possible "dark side" to soy consumption, one thread runs consistently through its messages: the need for more research for new uses of soy components. The health claim, however, focuses on uses of soy protein that are generally accepted among health professionals as useful for heart-healthy diets.
Sales of soy foods probably will continue to rise steadily for the foreseeable future, says Sansoni of the Grocery Manufacturers of America. "We're seeing a 'buzz' with soy products that intrigues people and they want to try them," he says. "But I don't believe soy is a fad. It's a continuing trend that's here for the long haul."
With the rising interest, the health claim for soy protein appears to have succeeded. It has provided specific guidelines to help the public improve the heart-healthiness of its diet and has stimulated the industry to produce new food products high in soy protein. These trends, in the end, should be good for those trying to lower their risk of heart disease.
John Henkel is a member of FDA's public affairs staff.
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