Dietary fiber, especially from whole grains and cereals, is associated with a decreased risk of colorectal cancer.

Diet, especially red and processed meat, and alcohol, have been shown to be associated with colorectal cancer. On the other hand, dietary fiber lowers the risk of colorectal cancer. Dietary fiber could protect against colorectal cancer by increasing bulk of stools and diluting any cancer causing substances in the digestive tract. It also enhances clearance, thus reducing the contact time of any cancer causing substances with the digestive tract. Dietary fiber gets converted into short chain fatty acids, which protect against colorectal cancer. However, which fibers are likely to provide benefit is not clearly known.
Whole grains are not only a good source of dietary fiber, but also contain antioxidants, vitamins, minerals, phytate, phenolic acids, lignans and phytoestrogens which may contribute to their anticancer effect. They also contain large amounts of folate and magnesium, which have known anticancer effects. Their ability to reduce weight and improve insulin resistance may also play a role in reducing cancer risk.
A group of reviewers analyzed 21 previously published studies with more than 14,500 cases to establish the role of dietary fiber and whole grains in preventing the development of cancer. They found that there was a 10% reduction in risk of developing colorectal cancer for each 10 g/day intake of total dietary fiber and cereal fiber and about a 20% reduction for each three servings (90 g/day) of whole grain daily. Further reductions in risk were observed with higher intake.
The study thus found that dietary fiber, cereal fiber and whole grains do reduce the risk for developing colorectal cancer.
However, fibers from other sources like fruits, vegetables and legumes do not appear to show the same beneficial effect.
Reference:
Source-Medindia