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The findings are consistent with the results of previous studies that examined the cancer prevention benefits of the dietary supplements, said lead researcher Dr. Seema Khan, a professor of surgery at the Robert H. Lurie Comprehensive Cancer Center of Northwestern University.
The study included 98 women who were randomly assigned to receive a mixed soy isoflavones supplement or placebo. Isoflavones are components of soy foods thought to have anti-estrogen activity (estrogen is "fuel" for many breast cancers).
After six months, the researchers examined levels of Ki-67 -- a protein marker of cancer cell growth -- in certain breast cancer cells taken from the women. Overall, there were no differences in Ki-67 levels between women who took the soy supplement and those who took the placebo.
However, the level of Ki-67 increased from 1.71 to 2.18 in premenopausal women taking the soy supplement, which suggests the supplement might even have a negative effect, according to the study published in February issue of the journal Cancer Prevention Research.
"This was a small finding," Khan stressed in a news release from the American Association for Cancer Research, "but one that should suggest caution."
"Simply put, supplements are not food. Although soy-based foods appear to have a protective effect, we are not seeing the same effect with supplementation using isolated components of soy, so the continued testing of soy supplements is likely not worthwhile," Kahn concluded.
But one expert said the study, while valuable, had limitations.
Dr. Patrick Borgen, director of Breast Cancer Care Services at the Maimonides Cancer Center in New York City, called the study "thought provoking and well-executed."
But he added that uncertainties remain. For example, the area of the breast from which the cells were taken and studied matters, because cancer develops in different ways across the geography of the breast. Furthermore, other potential risk factors, such as diet, exercise, alcohol intake and stress, could play a role in the women's breast cancer risk as well and "are extremely hard to control for in this kind of study," Borgen said.
Finally, Borgen said, it is still difficult to predict the "long-term consequences" of the cell changes captured by Ki-67 testing.
For all of those reasons, "the conclusions -- that further study may not be warranted or that use of these supplements in premenopausal women may be dangerous -- should therefore be taken in the context of the limitations of the study," Borgen said.
-- Robert Preidt
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