Level 2: Mid-level evidence
According to the 2002 American Cancer Society Guidelines on Nutrition and Physical Activity for Cancer Prevention, diets high in fruits and vegetables may reduce the risk for colorectal cancer (CRC) (CA Cancer J Clin. 2002;52:92-119). It is unclear what components of fruits and vegetables may be chemoprotective, but dietary fiber has been suggested.
However, dietary fiber was not associated with a reduced risk of CRC in a pooled analysis of 13 prospective cohort studies with 725,628 men and women who were followed for 6-20 years (JAMA. 2005;294:2849-2857). Among participants, 8,081 developed CRC. Increased dietary fiber intake was associated with a lower risk of CRC in unadjusted analyses and analyses that did not adjust for other dietary factors. This association was not significant after adjusting for dietary folate intake and intake of red meat, total milk, and alcohol.