No Reduction in Risk for Colorectal Cancer Found with High Intake of Dietary Fiber
Colorectal Cancer and Diet
Dietary fiber is inversely associated with risk of colorectal cancer in age-adjusted analyses; however, after accounting for other risk factors, high fiber is not associated with a reduced risk of colorectal cancer. This large pooled analysis of prospective cohort studies aimed to settle the question of whether dietary fiber reduces risk for colorectal cancers. Inconsistent findings from observational studies have fueled controversy over the effects of dietary fiber, the authors note. The study's objective was to evaluate the association between fiber and risk of colorectal cancer, and the main outcome measure was incident colorectal cancer.
The Pooled Project of Prospective Studies of Diet and Cancer was established to summarize the association between dietary fiber and the risk of cancer. Data from 13 prospective cohort studies that followed 725,628 men and women for 6 to 20 years were evaluated. During the 6 to 20 years of follow-up, 8,081 cases of colorectal cancer were identified. After age adjustment, dietary fiber was significantly associated with a 16% lower risk for colorectal cancer (pooled age-adjusted RR, 0.84; 95% CI, 0.77-0.92). However, after adjusting for nondietary risk factors, vitamin use, total energy intake, dietary folate, and intake of red meat, milk, and alcohol, only a nonsignificant weak inverse association was found (pooled RR, 0.94; 95% CI, 0.86-1.03; P for trend, 0.75).
Although no major effect on the risk of colorectal cancer was found, a diet high in fiber from whole plant foods can be recommended, as this has been related to lower risk for heart disease and diabetes, the authors advise.
Comment. In the 1960s, Burkitt hypothesized that a higher intake of fiber was a major contributor to the lower rates of colorectal cancer he observed in Southern Africa compared to Western nations. His observations were supported by experimental data that suggested that fiber could bind fecal carcinogens as well as data from retrospective case-control studies. As a result, many promoted, and the lay public gradually assumed, that a reduction in colorectal cancer risk was one of fiber's many benefits. However, such enthusiasm was probably premature. Subsquent prospective studies have been inconsistent, with a majority showing no substantial effect of fiber.
The study by Park et al is a valuable contribution to the field. By pooling the data from 13 large, well-designed prospective cohort studies, Park provides compelling evidence that any inverse association between high fiber intake and colorectal cancer risk is likely due to other confounding factors associated with a high fiber diet, lifestyle factors, or other micronutrients, such as folate. After carefully accounting and controlling for the effect of these other risk factors for colorectal cancer, a specific influence of fiber was no longer apparent. The strength of the analysis is the prospective nature of the dietary assessment, the large number of participants, and the use of pooled, original data for the statistical analysis rather than a meta-analysis of published results. The findings of this study are consistent with at least 5 well-designed, randomized control trials which have shown no benefit of fiber supplementation on recurrence of adenomas, the precancerous lesions that are precursors to the vast majority of cancers.
There does remain the possibility that even higher levels of fiber intake than those examined in this study, or intake over a more extended time period or earlier in life, may have anti-cancer properties. However, for the general population, long-term intake of such substantial amounts of fiber is not likely to be realistic, and any potential benefit for colorectal cancer is likely to be modest. Nonetheless, the authors do point out that fiber has been associated with a lower risk of other chronic diseases, such as coronary heart disease, which still warrants its inclusion into a dietary program designed to promote good health.