This conclusion may come as a surprise to many women who have come to depend on soy supplements to limit their risk of either developing breast cancer, or reducing breast cancer recurrence, says Robert Clarke, Ph.D., D.Sc., a professor at Georgetown's Lombardi Comprehensive Cancer Center.
"Soy supplements contain high levels of isoflavones, such as genistein, that have estrogenic properties, while in soy foods isoflavones are just a part of the package. Estrogens make the breast tumor cells grow," says co-author Leena Hilakivi-Clarke, Ph.D., a professor of oncology whose laboratory at Lombardi has long studied the role of food-based estrogens in cancer prevention. "We found that soy food intake was associated with a reduced breast cancer risk. However, we have no idea what soy supplements might do," she adds
After research more than a decade ago showed that soy could play a role in decreasing LDL (known as the "bad" cholesterol) levels in humans, U.S. sales of soy-based food, drinks and supplements increased. U.S. sales of soy-based food and drinks were $3.9 billion in 2003, but sales have slowed in recent years, according to the Soyfoods Association of North America, an association funded by the soy industry.
"Because soy foods and soy supplements are widely used, we conducted this first true meta-analysis to understand what role soy foods might have on breast cancer risk," said Clarke. "This objective examination of all of the studies tells us that currently, the data are not adequate to provide a clear answer to recommend soy foods to prevent breast cancer."
Although the study did not address the link between soy use and breast cancer recurrence, Clarke and Hilakivi-Clarke again cautioned against overuse of soy supplements, particularly by women at high risk for breast cancer or breast cancer survivors.
In the same way that estrogen-based hormone replacement therapy may be contraindicated for some women at high risk to develop breast cancer, soy isoflavone supplementation may not be a good idea for these women either, the researchers said. Co-author Bruce Trock, Ph.D., of the Johns Hopkins School of Medicine added, "High-dose soy or isoflavone supplements have biological effects that are different from normal soy foods, and no one has studied the effect of long-term consumption of such supplements."
"Soy foods like tofu and soy milk are readily available now if women want to add them to their diet," Trock said.
Because soy has recently become a widely used food additive in the United States, many Americans get some soy that they are not aware of, said Hilakivi-Clarke. "Our data suggests that women do not need to consume very much soy to possibly benefit from it. Higher soy intake does not provide additional risk reduction," she said.
To conduct the meta-analysis, Clarke, Hilakivi-Clarke, and Trock looked at 18 epidemiologic studies published between 1978 and 2004 that examined the association between soy intake and breast cancer risk. Some of these studies were based on the hypothesis that Asian women have reduced incidence of breast cancer, compared to American women, because they have traditionally eaten a diet rich in soy foods, such as tofu. In some studies, soy foods were just one of the food items studied in connection to breast cancer risk.
The results of the meta-analysis indicated a small association between soy intake and a reduced risk of breast cancer. Risk appeared to be reduced to a somewhat greater extent in premenopausal than postmenopausal women, however only 10 of 18 studies evaluated the effect separately by menopausal status. Studies that did not address menopausal status provided less evidence for the risk reducing effect.
While the data were too inconsistent to recommend soy as a breast cancer preventive, the authors say there is no evidence to suggest that consumption of soy foods in amounts consistent with an Asian diet is detrimental to breast health. Any overall health benefits offered by traditional soy foods in the diet, modest though they may be, likely outweigh any risks for the population as a whole, the authors said.
About Lombardi Comprehensive Cancer Center
The Lombardi Comprehensive Cancer Center, part of Georgetown University Medical Center and Georgetown University Hospital, seeks to improve the diagnosis, treatment, and prevention of cancer through innovative basic and clinical research, patient care, community education and outreach, and the training of cancer specialists of the future. Lombardi is one of only 39 comprehensive cancer centers in the nation, as designated by the National Cancer Institute, and the only one in the Washington, DC, area. For more information, go to http://lombardi.