Contrary evidence

More recent studies have cast doubt on the claim that dietary fiber reduces the risk of colon cancer.[20][21] Regarding prostate cancer, a major 2002 study concluded that "A low-fat, high-fiber diet heavy in fruits and vegetables has no impact on PSA levels in men over a four-year period, and does not affect the incidence of prostate cancer."[22]
The belief that dietary fiber prevents these cancers was based on epidemiological evidence showing a very low incidence in the developing world. Dr. Denis Burkitt was the main proponent of this theory, and his 1979 best-selling book, "Don't Forget Fibre in Your Diet," was translated into nine languages. Less well-known is his theory, mentioned in the book, that the squatting defecation posture used in the developing world where Ottoman toilets are more widespread than the Western world is also a preventative factor, especially for colon diseases.