They say their findings, which offer direct evidence of the association between fibre and the hormone, could lead to a dietary strategy for lowering a woman's risk of breast cancer.
Breast cancer rates have risen in recent decades to become the most common cancer among women in the European Union and US. Britain has one of the highest breast cancer death rates in the world, according to Breast Cancer Research, with one woman in nine developing the disease during her lifetime.
There have already been several studies investigating the relationship between dietary fibre and breast cancer but researchers have not been able to show a true and unequivocal cause-and-effect relationship between fibre and breast cancer risk.
In a new study, researchers from the Keck School of Medicine of the University of Southern California in Los Angeles, the University of Hawaii in Honolulu, and the University of Helsinki in Finland examined blood oestrogen levels in around 250 Mexican-American women, an ethnic group in which dietary fibre intake is higher on average than in most other populations.
"Latinas enrolled in the Multiethnic Cohort Study have lower breast cancer rates than any major racial/ethnic group in the US. Even after adjusting for known risk factors, their incidence rate is still 20 per cent less than white women, who have been the focus of the majority of earlier research and whose dietary fibre intake is generally not that high," explained the study first author Kristine Monroe, a postdoctoral fellow in the Keck School's department of Preventive Medicine.
Dietary fibre intake was quantified by a food frequency questionnaire administered at the time of the blood draw and by using biomarkers of dietary fibre intake found in the blood samples.
Speaking yesterday at the American Association for Cancer Research (AACR) conference, 'Frontiers in Cancer Prevention Research', the researchers said they found the two female hormones estrone and estradiol dropped sharply as dietary fibre intake increased.
In addition, as dietary fat intake increased in the women studied, so did the hormone levels.
"However, when dietary fibre and fat are both included in the statistical model, only dietary fibre remains a significant predictor of hormone levels," said Monroe.
The next step is to see if a higher intake of dietary fibre in these women leads to a lower incidence of breast cancer, she added.
"This study provides clear evidence of an association between dietary fibre intake and circulating hormone levels in postmenopausal Latina women and potentially provides a dietary means for lowering a woman's risk of breast cancer," concluded the researchers.
Earlier this year a Swedish team reported that postmenopausal women in the highest quintile of fibre intake had a 40 per cent lower risk of breast cancer than those with the lowest. Combining high fibre with a low fat diet reduced the risk even further.