April 6, 2006 -- The answer to the question, "Does eating a soy-rich diet lower breast cancer risk?" is an unqualified "maybe," according to the largest analysis of studies ever to examine the issue.
Johns Hopkins and Georgetown University researchers pooled the results of 18 studies and concluded that women who eat soy products may have a slightly lower risk of developing breast cancer than those who don't.
But they also found that inconsistencies and design limitations within the studies raised doubt about soy's cancer prevention benefits.
They warned that high-risk women who take soy supplements, instead of getting soy through food sources, could potentially be doing themselves more harm than good. The data are mixed, but some animal and cellular studies suggest that soy at very high doses can stimulate the growth of estrogen-sensitive breast tumors.
"At this point, women should not be taking high-dose soy supplements, especially those who are breast cancer survivors and women at increased risk for the disease," says researcher Bruce Trock, PhD. Brock is an associate professor at Baltimore's Johns Hopkins' Brady Urological Institute and Kimmel Cancer Center.
East vs. West
Women who live in Asian countries have much lower rates of breast cancer than those living in Western ones. Because this protection is not seen among Asian women who immigrate to the U.S., researchers believe that lifestyle differences -- specifically the difference between Eastern and Western diets -- explain the risk disparity.
Soy-based foods have long been a staple of Asian diets, but they were absent from most American diets until about the last decade.
Soy contains isoflavones, which may have biologic effects that could potentially reduce breast cancer risk. It also has estrogen-promoting properties, which may spur the growth of breast cancers fueled by the hormone.
In an effort to better understand the role of soy in breast cancer risk, Trock and colleagues Leena Hilakivi-Clarke, PhD, and Robert Clarke, PhD, of Georgetown University, combined the results from studies conducted between 1978 and 2004. All were epidemiologic, meaning that participants were asked questions designed to identify their dietary soy exposure and overall breast cancer risk.
Their findings are reported in the April 5 issue of the Journal of the National Cancer Institute.
Health Claims Aren't Proven
Trock tells WebMD that the 18 studies did not prove or disprove the protective benefits of eating soy foods. The risks vs. benefits of taking soy supplements were not addressed in the study because the women were asked about the foods they ate.
None of the studies indicated that eating soy foods increased breast cancer risk, but Trock says the jury is still very much out on soy supplements.
Nutritionist and epidemiologist Maria Elena Martinez, PhD, says many high-risk women and breast cancer survivors may be taking soy supplements instead of hormone therapy for menopausal symptoms because they believe them to be safer.
"The truth is, we don't know if these products are safer because they haven't been studied," she says. "And because they aren't regulated, you don't know what you are getting."
Complexity of Cancer
Ads for soy supplements make a wide range of health claims about the products. Studies examining the role of soy in reducing hot flashes and other menopausal symptoms have been mixed, however, and most have not shown a protective benefit for lowering cholesterol and preventing Alzheimer's disease and osteoporosis.
Trock says it is not surprising that the soy studies and most other studies examining the role of specific foods in cancer risk have failed to show clear associations.
"Cancer takes decades to develop and it is influenced by many things," he notes. "People want to know in simple terms what they can do to reduce their risk, and we can't always tell them. We have to get across the idea that cancer is a complicated process. If we are lucky we may find that a single food or a single nutrient is protective, but so far that hasn't happened."