Fri Apr 11, 2008 11:12am EDT
By Amy Norton
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.
SOURCE: Journal of Clinical Oncology, April 1, 2008.