Dietary fibres may protect or enhance carcinogenesis.

Dietary fibre (DF) is widely considered to protect against cancer, especially colorectal cancer. However, a large prospective epidemiological study has shown no apparent effect of DF intake on the development of colorectal cancer. We suggest that this may be because the term DF represents a wide range of materials, some able to protect, but some able to enhance carcinogenesis. This is consistent with data from animal carcinogenesis experiments. Most of the DF in western diets is in the form of plant cell walls, but these vary in their composition and it is unlikely that all types are protective. The few data available indicate that plant cell walls containing suberin or lignin may be the most protective, although they are present in only small amounts in food plants. DFs are also added to foods. These include components obtained from plant cell walls, such as pectins, as well as soluble DFs from other sources. In general, animal carcinogenesis experiments indicate that soluble DFs do not protect and some may enhance carcinogenesis. Few human intervention studies have been done on DF or sources of DF, with the exception of wheat bran, a good source of DF, which has been shown to protect. Possible mechanisms whereby DF may enhance carcinogenesis are discussed. In addition to DFs, resistant starches and non-digestible oligosaccharides are added to foods; these, like DF, escape digestion in the small intestine. However, so far only a few animal carcinogenesis experiments have been reported using these materials, and no human intervention studies. We believe caution should be exercised in the addition of such materials to food.