December 17, 2005
Dietary Fiber Does Not Lower Colorectal Cancer Risk

The role of dietary fiber as a preventive against colon cancer remains very unproven.

In an analysis combining data from 13 studies, high intake of dietary fiber was not associated with reduced risk of colorectal cancer, according to a study in the December 14 issue of JAMA.

Dietary fiber has been hypothesized to reduce the risk of colorectal cancer, according to background information in the article. However, the results of numerous epidemiological studies have been inconsistent. Ecological correlation studies and many case-control studies have found an inverse association between dietary fiber intake and risk of colorectal cancer. But most prospective cohort studies have found no association between dietary fiber intake and risk of colorectal cancer or adenomas (precursors of colorectal cancer), and randomized clinical trials of dietary fiber supplementation have failed to show reductions in the recurrence of colorectal adenomas.

Yikyung Park, Sc.D., formerly of the Harvard School of Public Health, Boston, and colleagues evaluated the association between dietary fiber intake and risk of colorectal cancer by reanalyzing the primary data from 13 prospective cohort studies (Pooling Project of Prospective Studies of Diet and Cancer). The pooled analysis included 725,628 men and women who were followed-up for 6 to 20 years across studies.

During the follow-up, 8,081 colorectal cancer cases were identified. Among the studies, median (midpoint) energy-adjusted dietary fiber intake ranged from 14 to 28 g/d in men and from 13 to 24 g/d in women. The major source of dietary fiber varied across studies with cereals as a major contributor to dietary fiber intake in the European studies, and fruits and vegetables as the main sources in the North American studies.

Note the factors here that influenced risk. These are the things you want to get more or less of to lower your risk.

In the age-adjusted model, dietary fiber intake was significantly associated with a 16 percent lower risk of colorectal cancer in the highest quintile compared with the lowest. This association was attenuated slightly but still remained statistically significant after adjusting for nondietary risk factors, multivitamin use, and total energy intake. Additional adjustment for dietary folate intake further weakened the association. In the final model, which further adjusted for other dietary factors, such as red meat, total milk, and alcohol intake, only a nonsignificant weak inverse association was found. Fiber intake from cereals, fruits, and vegetables was not associated with risk of colorectal cancer.

The factors that reduced the association between fiber and risk are the factors that probably really do reduce risk. So get less red meat and milk for example. Did alcohol intake reduce or increase the risk of colon cancer? Probably the fruits and vegetables decreased risk due to other compounds (notably vitamins, minerals, and anti-carcinogen compounds) and not due to fiber. The fiber just happened to always be there.

One theory on why fiber should lower colon cancer risk is that it dilutes toxins and speeds the passage of toxins through the intestinal tract. Maybe adjusting for red meat consumption factors out the value of eating the fiber. If a person does eat a lot of red meat then maybe fiber really does protect agianst toxins formed in the intestines from red meat. Though I'm speculating.

But consumption of whole plant food like vegetables that are high in dietary fiber is associated with lower risk of heart disease and diabetes. So you could eat high fiber fruits and vegetables anyway.

"The association between dietary fiber intake and risk of colorectal cancer has been inconsistent among observational studies and several factors may explain the disparity: potential biases in each study, the failure to adjust for covariates in the multivariate models, and the range of dietary fiber intake," the authors write.

"In conclusion, we did not find support for a linear inverse association between dietary fiber intake and risk of colorectal cancer in a pooled analysis of 13 prospective cohort studies. Although high dietary fiber intake may not have a major effect on the risk of colorectal cancer, a diet high in dietary fiber from whole plant foods can be advised because this has been related to lower risks of other chronic conditions such as heart disease and diabetes," the researchers write.

Just eating a high fiber food extract is probably a waste of time. But vegetables continue to be good for you. Most people (and myself) continue to not eat enough veggies.

Share |      Randall Parker, 2005 December 17 09:27 AM  Aging Diet Cancer Studies

carl said at December 17, 2005 10:39 AM:

So the argument seems to be that even though "In the age-adjusted model, dietary fiber intake was significantly associated with a 16 percent lower risk of colorectal cancer in the highest quintile compared with the lowest", this isn't actually due to fiber since as one starts peeling off other factors this effect goes away. Isn't it also possible that fiber is the ONE factor that is at play here and all the other factors are just along for the ride. Causation does not imply correlation goes both ways.

Anyone know much about the controlled experiments? Did they have enough power to detect an effect?

Don't get me wrong, I love the idea that fiber intake is not important and that instead all I have to do is take a pill.

Marvin said at December 17, 2005 11:18 AM:

This is a good illustration of the folly of much meta-analysis. By failing to discriminate between types of fiber, the authors could not possibly achieve a meaningful result. By combining studies with different definitions of "fiber" and different types of underlying diet, any power to achieve significance is completely lost, if you know anything about nutrition.

Carl, there is no controlled experiment involved here. This is meta-analysis of thirteen observational studies. No experiment. It is very hard to achieve significance in dietary observational studies. Diluting the differences by poor classification makes the whole thing meaningless. The footnote to the report admits the shortcoming, and as always, "more studies will be needed."

In other words, any excitement over this report is rather premature.

peter Andonian said at December 17, 2005 5:45 PM:

One problem with all the data regarding diet and supplements is that the time interval for protective effects to show up can be very long, sometimes over 15 years. The nurses heath study realeased in 1999 showed folic acid giving almost no protection for the first 10 years, but after 15 years the incidence of colon cancer in women using 400 mcg. of folic acid have a relative risk of only .25 vs. the lower folic acid group. This is probably due to the fact that cancer is a result of cumulative mutations in cells that take many years to develop. Here is a link to the study:

Jonathan said at February 9, 2008 6:24 PM:

Dr. Denis Burkitt came up with the fiber theory in the early 70's. He was trying to explain why colon cancer is confined to the western world. He reported that colon cancer is nearly 15 times as common among African-Americans as among Africans.

He had another theory, which has been ignored, but which is a much more likely explanation. In Africa, as in most of the world, they use squat toilets. Man, like all primates, was designed to squat for complete evacuation of the colon.

To learn more about why this theory makes more sense than the fiber theory please visit my website,

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