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 have shown that, in Mexican American women, higher intake of dietary fiber is associated with lower circulating estrogen levels.
Because high estrogen levels have been linked to breast cancer, this finding could provide a significant step toward preventing breast cancer.
Although there have been several studies that have looked at the relationship between dietary fiber and breast cancer, they are equivocal. Researchers have yet to connect the two, to show a true and unequivocal cause-and-effect relationship between fiber and breast cancer risk. However, there is scientific evidence that dietary fiber may play an important role in the metabolism of estrogens and may therefore be an important determinant of circulating estrogen levels in the body.
"There's been so much research on this subject, and yet the jury's still out," says Kristine Monroe, Ph.D., a postdoctoral fellow in the Keck School's Department of Preventive Medicine and first author on the study. "Latinas enrolled in the Multiethnic Cohort Study have lower breast cancer rates than any major racial/ethnic group in the U.S. Even after adjusting for known risk factors, their incidence rate is still 20 percent less than white women, who have been the focus of the majority of earlier research and whose dietary fiber intake is generally not that high."
To rectify that situation, Monroe decided to use data collected by the National Institutes of Health-funded Multiethnic Cohort Study of Diet and Cancer to begin looking at the relationship between dietary fiber and estrogen levels in Mexican-American women, an ethnic group in which dietary fiber intake is higher on average than in most other populations. Even then, says Monroe, she selected for the study those who had the highest and lowest intake levels among all the Mexican-American women.
In the end, Monroe and her colleagues--including Laurence N. Kolonel and Suzanne Murphy, professors at the Cancer Research Center of Hawaii, Brian E. Henderson and Malcolm Pike, professors of preventive medicine at the Keck School of Medicine of USC--looked at blood hormone levels of 252 Latina women participating in the Multiethnic Cohort Study in relation to dietary intakes. Dietary fiber intake was quantified in two ways: (1) from a food frequency questionnaire administered at the time of the blood draw; and (2) from biomarkers of dietary fiber intake found in the blood samples.
What they found was that, as dietary fiber intake increases, levels of estrone and estradiol, two female hormones measured in blood, drop sharply. In addition, the researchers say, they found that as dietary fat intake increased in the women studied, so did the hormone levels. "However, when dietary fiber and fat are both included in the statistical model, only dietary fiber remains a significant predictor of hormone levels," Monroe notes.
The next step, Monroe says, is to see if a higher intake of dietary fiber in these women leads to a lower incidence of breast cancer.
"This study provides clear evidence of an association between dietary fiber intake and circulating hormone levels in postmenopausal Latina women," the researchers note, "and potentially provides a dietary means for lowering a woman's risk of breast cancer."