Researchers found that among more than 24,000 middle-aged and older Japanese women, those with the highest levels of the soy compound, genistein, were only one-third as likely as other women to develop breast cancer over 10 years.
Genistein is one of the major isoflavones, a group of plant compounds found in soybeans, chick peas and other legumes that are structurally similar to the hormone estrogen, and are believed to bind to estrogen receptors on body cells and displace strong estrogenic hormones. The stronger estrogenic hormones, such as the ones metabolized through certain liver pathways, can contribute to cancer, while the weaker estrogens metabolized through other hepatic pathways can be protective against cancer. It is believed that certain soy compounds like genistein can displace the stronger estrogens with the protective less-estrogenic compounds.
The new findings, reported in the Journal of Clinical Oncology, suggest that past speculations about soy contributing to tumor development and growth is, in fact, not the case in women.
"This finding suggests a risk-reducing rather than a risk-enhancing effect of isoflavones on breast cancer, even at relatively high concentrations within the range achievable from dietary intake alone," write the researchers, led by Dr. Motoki Iwasaki of the National Cancer Center in Tokyo.
The study included 24,226 women ages 40 to 69 who gave blood samples and completed a dietary assessment, and then were followed for an average of 10 years. In that period of time, 144 women were diagnosed with breast cancer. When Iwasaki's team separated the women based on their blood levels of genistein at the beginning of the study, they found that the one-quarter with highest levels were 65 percent less likely to develop breast cancer compared to the quarter of women with the lowest genistein levels.
Iwaski stressed that most past studies on soy isoflavones and breast cancer have used dietary questionnaires. "In contrast, our study used a direct measurement of plasma isoflavone levels, which provides not only an index of intake but also of the absorption and metabolism of isoflavone," he said.
Together with past studies, the findings suggest that a high isoflavone intake may help lower breast cancer risk.
SOURCES: Journal of Clinical Oncology, April 1, 2008, and Reuters Health.