Differences in estrogen levels may partially explain the ethnic disparities in breast cancer rates among U.S. women, new research suggests.

In a study of more than 700 postmenopausal women, researchers found that participants’ blood levels of estrogen and “male” hormones, called androgens, varied by race and ethnicity. And the differences in estrogen, which fuels breast tumor growth, often paralleled ethnic differences in breast cancer risk.

Native Hawaiians, for example, had the highest levels of estrogen and androgen, and the highest rate of breast cancer. On average, their estrogen levels were about one-quarter higher than those of white women, according to findings published in the journal Cancer Epidemiology, Biomarkers & Prevention.

Similarly, the study found, women of Japanese descent had higher estrogen levels than white women did, and their breast cancer rate followed suit.

These findings fit the theory that racial or ethnic differences in estrogen levels account for some of the differences seen in breast cancer rates, according to study leader Dr. Veronica Wendy Setiawan of the University of Southern California in Los Angeles.

An exception, she told Reuters Health, was the finding that African-American women had lower rates of breast cancer than white women did, despite having higher estrogen levels.

“We can’t explain that yet,” Setiawan said. It’s known that before menopause, black women have a higher risk of breast cancer than white women, she noted. So it’s a puzzle as to why their risk doesn’t remain elevated after menopause, even though their estrogen levels remain relatively high.

Another finding with no clear explanation is the fact that Japanese-American women had higher estrogen levels and a higher rate of breast cancer than white women did - a stark reversal of what’s been previously observed.

This rise in estrogen levels among Japanese Americans may be driving the rise in breast cancer, according to Setiawan, but no one knows what factors - such as diet or other lifestyle changes - are affecting estrogen levels in these women.

Excess body fat raises estrogen levels, but Japanese Americans in the study were generally thinner than other women.

The findings are based on 739 women who are part of a larger study that has followed an ethnically diverse group of adults from California and Hawaii for more than a decade.

Setiawan notes that more research is needed to figure out why black women differ from other women when it comes to the relationship between estrogen levels and breast cancer - and why estrogen levels appear to be changing among Japanese women.

SOURCE: Cancer Epidemiology, Biomarkers & Prevention, October 2006.