Whole Grains and Cancer
Whole grain intake is associated in epidemiological studies with reduction in cancer risk from 10% to nearly 50%, depending on the type of cancer studied. For instance, postmenopausal women in the tertile eating the most whole grains had a relative risk for gastrointestinal cancers of 0.53 (31). A case-control study of cancers of the pharynx, esophagus and larynx showed that those who ate the most refined grains had an odds ratio = 5.7, whereas those who ate the most whole grains had an odds ratio = 0.5 (32). In a cohort of 35,000 Italian women, those ingesting fibre from the highest tertile of refined grains had nearly a 6-fold elevated risk of upper GI cancers, while those ingesting fibre from the highest tertile of whole grains had a reduced odds ratio = 0.5 (33).
Intake of whole grains and dietary fibre as a potential way to reduce the risk of colorectal cancer has been inconclusive, since some studies have failed to show reduced risk (34-36). It may be that the intake of fibre even in the cohort eating the highest amount of fibre was too low to reduce cancer risk, the interventions were too short or the methods were inadequate to show positive results. More recently in the Prostate, Lung, Colorectal and Ovarian Cancer Screening Trial (N=33,971), those ingesting the highest quintile of dietary fibre had a 27% lower risk of adenomas than those in the lowest quintile (37). In the European Prospective Investigation into Cancer and Nutrition (EPIC) (N~520,000), the quintile ingesting the most dietary fibre had a 25% reduced risk of large bowel cancer (38). For 61,433 women in the Swedish Cancer Registry, high intake of whole grains (4.5 or more sv/d) was associated with a 33% lower risk of colon cancer (39).
Whole grains may act to help reduce the risk of gastrointestinal cancers in a number of ways. Inulin, resistant starch and other fermentable fibres found in whole grains encourage butyrate production in the colon. Butyrate and other short chain fatty acids lower colonic pH, feed healthy colonic cells, encourage beneficial bacterial species and impair production of potentially destructive secondary bile acids. Bran and other fibrous components facilitate faster transit time and dilution of fecal material so carcinogens in the colon have reduced chance to interact with surrounding tissue. Additionally, whole grain antioxidants and phytochemicals may act to prevent destructive oxidation (3, 33).
Whole grains may help reduce the risk of sex hormone-related cancers. Postulated mechanisms include lowering of sex hormone production via phytoestrogens, such as those in rye (40-41).