Feb. 4, 2003 -- You may have heard that diets rich in soy that contain phytoestrogen help protect against breast cancer. Now, new research from The Netherlands indicates that diets containing phytoestrogens typically found in Western diets do not show a protective effect.
Researchers studied the association between the consumption of plant-based compounds that have estrogen-like activity, known as phytoestrogens, and breast cancer risk in a study involving more than 15,000 middle-aged and elderly Dutch women.
The main sources of phytoestrogens in the women's diets -- as is the case for most women in the U.S. -- were not soy and flaxseed, but grains, fruit, nuts, and seeds. And like most people eating primarily Western diets, the overall consumption of phytoestrogens was low.
Breast cancer risk among women eating diets containing the highest amounts of these phytoestrogens was similar to those eating lower amounts. The findings are published in the latest issue of the American Journal of Clinical Nutrition.
Most previous studies examining the role of dietary phytoestrogens in breast cancer have focused on soy. Soy researcher Marc Cline, PhD, tells WebMD that the evidence to date remains more than a little contradictory.
Population-based studies seem to overwhelmingly support a protective role for soy phytoestrogens. American women have a breast cancer risk that is as much as six times higher than women who live in Asian countries, where diets rich in soy phytoestrogens are commonly eaten. When Asian women immigrate to the U.S. and adopt more Western diets, they develop a higher risk of breast cancers.
"These are the studies that got people excited about soy," Cline tells WebMD. "But there are a lot of other factors, in addition to diet, that can explain this difference, Asian women typically consume fewer calories overall, they begin to menstruate later in life, they exercise more, eat more vegetables and are thinner than women living in the West."
Studies directly measuring the impact of soy consumption on breast cancer risk show a "weak protective effect", with women who eat soy throughout life and starting at an early age appearing to derive the most benefit, Cline says.
Dietary Soy Intake May Also Promote Tumors
But animal studies show that for women with a history of breast cancer or who are at high risk for the disease, dietary soy may actually promote tumor growth.
Epidemiologist Regina G. Ziegler, PhD, says it is understandable that women are confused about soy and other plant-derived foods that have compounds that act similar to estrogen. In an editorial published with the Dutch study, the National Cancer Institute researcher concluded that the research, to date, does not support the need for women in the U.S. to increase their dietary phytoestrogen intake to the level consumed by women in Asia.
"There is a lot of inconsistency in the literature," she tells WebMD. "My personal belief is that telling women with a history of breast cancer or who are at high risk to take soy supplements or eat large amounts of soy is imprudent. On the other hand, I don't think the message should be that they shouldn't eat soy at all. We just don't know enough to say."