WEDNESDAY, April 5, 2006 (HealthDay News) -- Consuming soy may be associated with a small reduction in breast cancer risk, but the reduction is not big enough or clear enough to suggest women should be taking soy supplements.
"When you put it all together, we came up with evidence of a small protective effect but a lot of reasons to be a little wary of how accurate that effect may be," said study author Bruce Trock, an associate professor of urology at the Johns Hopkins School of Medicine.
"The bottom line is we don't know if it's helping, hurting or doing anything at all," added Dr. Jay Brooks, chairman of hematology/oncology at the Ochsner Clinic Foundation in Baton Rouge, La. Brooks was not involved with the study, which appears in the April 5 issue of the Journal of the National Cancer Institute.
Asian women have lower breast cancer rates (39 per 100,000) than Western women (133 per 100,000) and, when Asian women migrate to the United States, their breast cancer rates tend to go up. This suggests that an environmental factor, perhaps related to diet, is at play.
Attention has zeroed in on soy products (consumed more in Asia) as they contain high quantities of isoflavones, molecules that affect pathways that could change breast cancer risk. Indeed, more and more women are taking high-dose soy or isoflavone supplements because of their perceived benefits, which include lowering LDL ("bad") cholesterol.
Hard evidence on the subject has been lacking, however.
This paper was a meta-analysis of 18 epidemiologic studies looking at soy exposure and breast cancer risk, which were published from 1978 through 2004.
When the data was pooled, researchers found a 14 percent relative reduction in the risk of breast cancer among women who had a high soy intake. The association was somewhat higher in premenopausal women.
But the studies included in the meta-analysis suffered from a number of weaknesses, experts said.
For one thing, the studies got their estimates of intake from questionnaires filled out by the women themselves. "These questionnaires don't capture everything," Trock said. Soy is particularly problematic because products differ so greatly and because soy is also added to various other foods (such as instant coffee and baked products). "Reported intake is not a good estimate of real intake," Trock said.
Trock and his colleagues also did not see a dose response. "If something in our diet is influencing our risk of cancer, there would be a dose response, meaning more is better, at least up to a point, and we didn't see that," he said. In fact, Western women seemed to have at least as much protection as Asian women, despite lower levels of intake.
It's also possible that soy eaten earlier in life may be most important. The papers included in this meta-analysis only measured soy intake later in life.
Studies which rely on more precise measures may help clarify the picture, but they are expensive to do and take time, Trock said.
In the meantime, Trock said, "women probably should not be taking these high-dose supplements." One study showed that giving more refined soy to rats actually contributed to tumor growth.
On the other hand, many soy-based foods have known health benefits. "If women would like to hedge on the side that it may be protective, they should eat soy foods like soy milk, soy nuts and tofu," Trock said.
Overall, the findings are not much of a surprise. "Given the complicated interplay of events and factors that go into developing cancer, we shouldn't be surprised that one food doesn't give us a clear signal," Trock said. "We shouldn't expect it to be so simple."
The study is just one of many coming out that are trying to clarify the role of different supplements or vitamins in the genesis of cancer. Another study in the same issue of the journal found that low levels of vitamin D may be associated with an increased incidence of cancer in general and mortality in men.
This contradicts previous studies which indicated that exposure to sunlight and increased vitamin D intake might reduce the risk of certain cancers.
To learn more about soy and health, head to the U.S. Food and Drug Administration.