Fiber and Colon Cancer

A very large pooled analysis (Park's Pooled Project) from JAMA 2005 found a statistically significant association between fiber intake and colon cancer. The project had an advantage over a meta-analysis in that it reanalyzed data from included studies, instead of using published results. The Nurses' Health Study, Womens' Health Study, and other large prospective cohorts were included.

"In their combined analysis of 13 prospective studies [involving 725,628 men and women followed for 6 to 20 years], Park et al found evidence of an increased risk of colorectal cancer among individuals with very low intake of total dietary fiber (about the lowest 11%). After adjustment for measurement error, the relative risk for intakes of less than 10 g per day vs 10 or more g per day increased from 1.22 to 2.16."
- Dietary Fiber and Colorectal Cancer, An Ongoing Saga, JAMA December 2005
- Dietary Fiber Intake and Risk of Colorectal Cancer, A Pooled Analysis of Prospective Cohort Studies, JAMA December 2005

That's a 2.2 times increased risk for colon cancer (220% increase) for fiber intakes 10 g or lower.

Park found smoking was the strongest confounder. (That doesn't bode well for smokers on low fiber diets.) The risk above was adjusted for smoking. He also said that risk may be underestimated: "A true association between dietary fiber intake and risk of colorectal cancer may be underestimated in our study," because of errors in reporting and measuring.
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The EPIC Study, a large prospective analysis of 519,978 individuals from 10 European countries also found a statistically significant association between fiber intake and colon cancer.

"Dietary fibre in foods was inversely related to incidence of large bowel cancer (adjusted relative risk 0.75 [95% CI 0.59-0.95] for the highest versus lowest quintile of intake)."

"In populations with low average intake of dietary fibre, an approximate doubling of total fibre intake from foods could reduce the risk of colorectal cancer by 40%."
- Dietary Fibre In Food And Protection Against Colorectal Cancer In The European Prospective Investigation Into Cancer And Nutrition (EPIC): An Observational Study., Lancet 2003

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Here's further study of the EPIC population, not only confirming but also strengthening the link:

"The inclusion of an additional 656 cases confirmed our previously published results, with a strong and significant reduction in colorectal cancer risk of ~9% for each uncalibrated quintile increase in fiber (P linear trend < 0.001) compared with an 8% reduction in our previous report."
- Is The Association With Fiber From Foods In Colorectal Cancer Confounded By Folate Intake?, Cancer Epidemiology Biomarkers & Prevention, 2005

Such that ... in populations with low average intake of dietary fiber, an approximate doubling of total fiber intake from foods could reduce the risk of colorectal cancer by 45%.

In 1.2 million people studied, the link between fiber and colon cancer continues to bear scrutiny.
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