January 20, 1999
New study finds no link between high-fiber diet and reduced risk for colorectal cancer in women
In a study that contradicts the conventional wisdom about the cancer-preventing effects of dietary fiber, researchers at Dana-Farber and Brigham and Women's Hospital have found no association between fiber intake and the development of colorectal (colon or rectum) cancer in women.
The findings, derived from data in the landmark BWH-based Nurses' Health Study, were published in the Jan. 21 issue of the New England Journal of Medicine.
"Certainly other research has found there are health benefits to consuming dietary fiber for conditions such as coronary heart disease, hypertension, and certain types of diabetes. However, our study found that reducing the risk of colorectal cancer is not a benefit, even among participants who consistently consumed high levels of fiber," says study leader Charles Fuchs, M.D., M.P.H., chief of Ambulatory Services on the Longwood campus of Dana-Farber/Partners CancerCare.
"In contrast to former studies that resulted in different findings, our study involved vast numbers of healthy participants and validation of questionnaires and medical history, and it had asignificantly longer follow-up period," he continues.
The study was based on an analysis of more than 88,000 women, 34 to 59 years of age, who had no history of cancer and completed a detailed questionnaire in 1980 about their dietary patterns and other risk factors for cancer. During a 16-year follow-up period, information on diet and medical diagnoses was tracked. In 1996, 787 cases of colorectal cancer had been documented.
It was found that participants' fiber intake (total intake and intake separately of cereals, fruits, or vegetables) had no significant influence on the incidences of colorectal cancer. Similarly, no evidence of an inverse association between fiber and colorectal cancer was witnessed in any subgroup defined by such factors as age, family history, other dietary patterns, or lifestyle.
In an accompanying editorial, John Potter, M.D., Ph.D., of the Fred Hutchinson Cancer Research Center in Seattle, speculates on why the results of this study seem to diverge from those of previous studies. One reason might be differences in the way that dietary fiber intake is measured, he suggests. Other reasons might have to do with the many unknowns that surround dietary fiber: Which components of fiber might be responsible for reducing cancer risk? Are those components canceled out by other dietary substances? Does it matter whether one starts a high-fiber diet early or late in life?
Potter also points to some of the uncertainties inherent in epidemiological research. These often relate to the group of people studied: If participants do not fully represent people in the general population, the results of the study may not be broadly applicable. To answer some of the questions surrounding the relationship (or lack of it) between dietary fiber and cancer, Potter calls for more research on fundamental human biology and for better databases for epidemiological studies involving nutrition.
Fuchs notes that even though the new study is at odds with the "legend" of the anti-cancer benefits of high-fiber diets, the study confirms what every other "prospective" study of the issue has found. (Retrospective studies ask participants to recall an aspect of their behavior dietary habits, for example over a previous period of years. Prospective studies track participants' actual behavior over time, and are therefore considered to be more reliable.)
Researches say the study's results underscore the importance of early detection and screening for colo-rectal cancer.
The Nurses' Health Study was launched in 1976 at Brigham and Women's. It is the longest-running major women's health study ever undertaken and has resulted in hundreds of journal articles, many containing groundbreaking findings on how to prevent some of the major causes of disease and death in women.
