Differences in natural estrogen levels may explain why higher rates of breast cancer are found among post-menopausal women in some racial and ethnic groups, according to a study released on Wednesday.

Of women in five racial/ethnic groupings studied, native Hawaiians had the highest risk of breast cancer -- 65 percent greater than Caucasians, according to the study by researchers at the University in Southern California (USC).

The Hawaiians also had some of the highest levels of circulating estrogens.

Post-menopausal Japanese-Americans were found to have the second-highest breast cancer risk of the five groups. They also have comparatively high levels of natural estrogen.

Latinas had similar hormone profiles to whites, but their risk of breast cancer was the lowest among the groups studied, according to the study.

Post-menopausal African-American women were found to have a slightly lower breast cancer risk than whites, despite having higher hormone levels.

In the study, researchers at USC's Keck School of Medicine researched the hormone levels in 739 post-menopausal women who were not on hormone replacement therapy.

A woman's natural hormone level can affect her risk of developing breast cancer.

"We had observed that some groups, such as native Hawaiians, have higher breast cancer rates compared to white women," said V. Wendy Setiawan, an assistant professor of preventive medicine at Keck. "We knew hormones are a factor, so we decided to test them. The research seems to support that."

The study appears in the October issue of Cancer Epidemiology, Biomarkers & Prevention.

Source: Xinhua