FIBER - Healthy food that helps lower weight, improves cholesterol levels, and now study shows may well reduce your risk for colon cancer. What's not to like?

Whole Grain Fiber, Colorectal Cancer Risk Linked

By John Gever, Senior Editor, MedPage Today Published: November 10, 2011 Reviewed by Dori F. Zaleznik, MD; Associate Clinical Professor of Medicine, Harvard Medical School, Boston.

Individuals eating three daily servings of whole grains had a significantly, though modestly, lower risk of colorectal cancer, according to pooled data from nearly 800,000 individuals -- perhaps the largest and strongest analysis ever conducted. A 90-gram per day increase in dietary fiber from whole grains -- the amount a person would consume from three servings -- was associated with a risk reduction of 17% (95% CI 11% to 22%) for colorectal cancer, Teresa Norat, PhD, of Imperial College London, and colleagues reported online in BMJ.   The finding emerged from a meta-analysis of 25 case-control or prospective cohort studies of diet and colorectal cancer risk, with millions of participants in total. The researchers also found that "high" versus "low" intake of total dietary fiber was associated with a significant reduction in risk (12%, 95% CI 6% to 18%). For each 10-gram increase in total daily fiber intake among two million participants in 16 studies, the risk was lowered by 10% (95% CI 6% to 14%), with the benefit more apparent for colon versus rectal cancer. But fiber sources other than cereals and whole grains appeared to have less impact on risk. Norat and colleagues reported the following relative risks associated with fiber intake from fruits, vegetables, and legumes: Fruits: RR 0.93 per 10-g daily increment (95% CI 0.82 to 1.05); nine studies, 1.5 million participants Vegetables: RR 0.98 per 10-g daily increment (95% CI 0.91 to 1.06, nine studies, 1.5 million participants Legumes: RR 0.62 per 10-g daily increment (95% CI 0.27 to 1.42), four studies, 1.1 million participants "Our results provide further support for public health recommendations to increase the intake of dietary fiber in the prevention of colorectal cancer. However, they suggest a particular benefit of increasing cereal fiber and whole grain intake," Norat and colleagues wrote. They added that, on the basis of other past studies, public adoption of such recommendations would also have a range of other health benefits, including lower rates of cardiovascular disease, type 2 diabetes, obesity, "and possibly overall mortality." In an accompanying editorial, two Danish researchers concurred, and argued as well that the associations probably do represent causality. "Many of the specific mechanisms involved in the health benefits of whole grains still need to be explored and explained in detail," wrote Anne Tjonneland, PhD, and Anja Olsen, PhD, of the Danish Cancer Society. "But factors such as increased satiety, stabilized glucose homoeostasis and insulin response, and the fermentation of fiber and resistant starch in the colon to produce short chain fatty acids are probably important." They added that additional research could shed light on whether specific types of grains vary in their effects on reducing cancer risk, noting that some studies had pointed to whole grain rye as possibly better than others. Perhaps more important, the editorialists also recommended more investigation of the barriers to consumption of whole grains, which largely have to do with consumer preference. In conducting the meta-analysis, Norat and colleagues searched for studies with prospective cohort, case-cohort, or nested case-control designs that specifically examined associations between dietary fiber and colorectal cancer. Cross-sectional or retrospective case-control studies were not considered. The 25 they came up with had enrollments ranging from 8,000 to 520,000. All the individual study results were adjusted for age, with most also accounting for other major risk factors such as sex, smoking status, education level, and physical activity.  Follow-up continued for 3.5 to 17 years. The two largest studies included in the analysis -- the NIH-AARP Diet and Health Study, with 292,000 participants, and the European Prospective Investigation into Cancer and Nutrition, with 520,000 -- had medians of 4.5 and 6.2 years of follow-up, respectively. These and five other of the larger studies quantified intake of different sources of dietary fiber, which Tjonneland and Olsen indicated was especially important. However, a major limitation to the meta-analysis was that many of the studies examined diet composition only at enrollment, and the definition of fiber may have varied considerably among the studies. In most cases, the individual study investigators relied on participants to report their dietary composition. Also, not all studies adjusted for every major confounder, Norat and colleagues noted. The meta-analysis was supported by the World Cancer Research Fund. All study authors and the editorialists declared they had no conflicts of interest.

Primary source: BMJ Source reference: Aune D, et al "Dietary fibre, whole grains, and risk of colorectal cancer: systematic review and dose-response meta-analysis of prospective studies" BMJ 2011; DOI: 10.1136/bmj.d6617. Additional source: BMJ Source reference: Tjønneland A, et al "Fibre and prevention of chronic diseases" BMJ 2011; DOI: 10.1136/bmj.d6938.