Thirty years later Dr Dennis Burkitt, while working as a doctor in Africa, discovered that there was a much lower incidence of cancer of the colon among rural black Africans than among Europeans and Americans. He attributed this low incidence to their relatively crude diet. The theory was that fibre hastened the passage of the bowel contents thus allowing less time for cancer-inducing agents to form. This, of course, presupposed that food became carcinogenic in the gut and there was no evidence that it did. Neither was there any evidence that moving food through the intestine at a faster rate decreased the risk of cancer.