Does dietary fiber reduce the risk of colon cancer?

When I was growing up in the 1970's, there were a lot of commercials for cereals containing fiber. Dietary fiber (i.e. roughage) refers plant components that resist human digestive enzymes in the gut; these include cellulose, lignin, pectins, and fructans. Some foods rich in fiber are beans (legumes), dark-green vegetables, and whole grains. The physiology of dietary fiber on the digestive tract is not fully understood, but one main benefit is to normalize bowel movements by adding bulk to stool.

As a result, while I yearned for Frosted Flakes or Cocoa Puffs, my Mom favored cereals in which Euell Gibbons was the spokesperson (Video 1).

During that era, doctors believed that consuming fiber would reduce the risk for colon cancer (colorectal cancer or CRC). The evidence came from observations that countries with higher fiber intake tended to have reduced rates of colon cancer than countries with lower fiber intake. More specifically, colon cancer was more prevalent in Western developed countries; many of these countries consumed less fiber. It turns out other factors that contribute to CRC also correlate with the diet of advanced Western societies (e.g. red meat).

In the 1980's and 1990's more extensive medical studies were performed to explore the possible protective effects of fiber. First there were observational (cohort) studies that followed people (who initially did not have colon cancer) for their fiber consumption and for their incidence of colon cancer. An influential paper from the Journal of American Medical Association (JAMA) pooled the results from 13 such studies. The subjects were placed into five groups (quintiles) based on their fiber intake and tracked over 6 to 20 years. The authors found that

"In this large pooled analysis, dietary fiber intake was inversely associated with risk of colorectal cancer in age-adjusted analyses. However, after accounting for other dietary risk factors, high dietary fiber intake was not associated with a reduced risk of colorectal cancer."

The key point was that people who consumed more fiber also had healthier lifestyles in general, and it was necessary to disentangle these other health factors. Thus, the initial analysis showed a reduced relative risk (RR = 0.84, i.e. high fiber groups had 16% less CRC) compared to the normal diet group that was statistically significant. However when adjusting for additional factors that co-varied with fiber intake, then the relative risk for the fiber group statistically insignificant (RR = 0.94). These co-varying factors included red meat consumption, alcohol intake, education, physical activity, and smoking.

Finally, the nail in the coffin with respect to this issue was provided by randomized control trials (RCTs) considered the gold-standard of clinical trials. One such study took people who had precancerous polyps removed during a colonoscopy, and then randomly divided them into two groups. One group was put on a high fiber diet, while the other group followed a lower fiber diet. After 4 years, all participants were subjected to another colonoscopy and polyps were counted.

There were a total of 1905 randomized subjects. Of the 958 subjects in the high-fiber group, 39.7% had at least one recurrent polyp; of the 947 subjects in the control (low-fiber) group, 39.5% had at last one polyp. Thus, the risk ratio was 1.00 i.e. no difference.

The authors concluded:

"Adopting a diet that is low in fat and high in fiber, fruits, and vegetables does not influence the risk of recurrence of colorectal adenomas."

Should you eat a diet high in fiber? The above data suggest that dietary fiber does not protect you against colon cancer, but it turns out there is evidence that fiber reduces risks for other conditions such as heart disease and diabetes (the subject of a future post).

Video 1. Euell Gibbons convinced countless Mothers to buy their kids Grape Nuts cereal.