By Alison McCook
NEW YORK (Reuters Health) June 17 - Japanese women who are frequent eaters of miso soup, a soy-filled staple of Japanese cuisine, and soy ingredients called isoflavones appear to be less likely to develop breast cancer, researchers reported Tuesday.
Women in Asian countries have only a fraction of the risk of breast cancer seen in Western countries, and the current findings add to a growing body of evidence that suggests isoflavone intake might help explain why.
In Japan, for instance, women typically consume approximately 700 times more isoflavones than U.S. whites.
Still, more studies are needed to determine whether the soy ingredient does, in fact, reduce breast cancer risk, study author Dr. Seiichiro Yamamoto of the National Cancer Center Research Institute in Tokyo told Reuters Health.
"The evidence level of isoflavone/soy and breast cancer is elevated from 'possible' to 'probable' by our study," Yamamoto said. "But it is still not convincing."
However, eating a little extra soy couldn't hurt, the researcher added.
"Because no harmful evidence about soy intake is reported and Asian people eat a lot of soy, it is not bad to recommend to eat soy and isoflavone in Western countries," Yamamoto said.
Many researchers have investigated the link between eating soy and developing breast cancer, but previous studies have shown mixed results, with some suggesting that soy and isoflavones offer no benefits in protecting women against breast cancer, according to the Journal of the National Cancer Institute report.
In the new study, the researchers asked more than 21,000 middle-aged women living in Japan how much soy and soy-containing products they ate, then followed them for 10 years and noted who developed breast cancer.
During the study period, 179 women developed breast cancer, the authors write. Women who reported eating miso soup and foods that contain isoflavones were less likely to be diagnosed with the disease than others.
Those women who consumed the most isoflavones typically drank at least two to three cups of miso soup daily and also ate soy-containing foods such as soybeans and tofu almost every day.
These soy-containing foods alone, however, did not influence breast cancer risk in the same way as miso soup or total isoflavone amount.
Interestingly, women who ate the least amount of isoflavones still consumed around 250 times more of a type of isoflavone called genistein than U.S. white women.
And the highest rate of breast cancer -- seen in women who ate the least amount of isoflavones -- was still lower than that seen in similarly aged women living in Western countries, the authors report.
Yamamoto and colleagues suggest that conflicting reports of the influence of isoflavones on breast cancer risk may have resulted from errors in measuring how much of the compound women ate, or from comparisons involving non-Asian women, who may show only small differences in the amount of isoflavones consumed by breast cancer patients and those who are cancer-free.
SOURCE: Journal of the National Cancer Institute 2003;95:906-913