People with diets low in amounts of insoluble fibre have a greater risk of colon-rectal cancer. Nearly all cases of this type of cancer are believed to begin with the formation of benign growths call polyps in the colon or rectum. Over a period of years, the polyps can enlarge and some may gradually become malignant. Diets rich in cereals, such as bran, reverse the usual progression by inhibiting a pre-malignant lesion.