Women with high blood levels of an estrogen-like compound found in soy seem to have a lower risk of developing breast cancer, a new study suggests.
Researchers found that among more than 24,000 middle-aged and older Japanese women, those with the highest levels of the compound, called genistein, were only one-third as likely as other women to develop breast cancer over 10 years.
Genistein is one of the major isoflavones, 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.
While some studies have linked soy consumption with a lower risk of breast cancer, others have found no protective effect. Some animal research, in fact, has suggested that genistein might spur tumor development and growth. The new findings, reported in the Journal of Clinical Oncology, suggest that this is not the case in women, at least when genistein is consumed through food alone.
"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, then were followed for an average of 10 years. During that time, 144 women were diagnosed with breast cancer.
When Iwasaki's team separated the women based on their blood levels of genistein at the study's start, they found that the one-quarter with highest levels were 65 percent less likely to develop breast cancer than the quarter of women with the lowest genistein levels.
There was no risk reduction seen among women with moderate levels of the isoflavone, however.
Most past studies on soy isoflavones and breast cancer have used dietary questionnaires, Iwasaki noted. "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," the researcher told Reuters Health.
Together with past studies, Iwasaki said, the findings suggest that a high isoflavone intake from food may help lower breast cancer risk.
Whether the findings necessarily extend to women in Western countries is not clear, however. Japanese women, Iwasaki noted, typically consume soy isoflavones on a regular basis starting from a young age, which may influence the compounds' effects on breast cancer development.
