There have been a number of reviews of the relationship between the consumption of whole grains and cancer risk which all conclude that whole grain foods are associated with lower cancer risk. In one review of 40 case-control studies of 20 cancers, the pooled odds ratio for high versus low whole grain intake was 0.66 (95%CI: 0.60-0.72).
The evidence suggests whole grain and fibre rich grain foods may protect against colorectal cancers, gastric cancers and possibly also breast, endometrial and prostate cancers.
Regular consumption of whole grain foods has been linked to a variety of positive health outcomes with respect to specific cancers:
- In case-control and cohort studies whole grains have been shown to reduce the risk of developing cancer of the colon by as much as 30% and a meta-analysis of 11 cohort studies estimated a risk reduction of 6% for the development of colorectal cancer comparing the highest with lowest quintiles of whole grain consumption. A meta-analysis of the largest two intervention studies with wheat bran concluded that there was a significant 19% reduction in risk of recurrence of adenomas in men, but not in women.
- One prospective cohort study in 10 European countries with 312 incident gastric cancers found that intake of cereal fibre was protective (with a hazard ratio of 0.69 in the highest quartile). This is consistent with Italian case-control studies which estimate an odds ratio of 0.5 for risk of stomach cancer with the highest whole grain intake.
- In a prospective cohort study of more than 400,000 people, intake of wholegrain foods and fibre from grains was inversely associated with risk of cancer of the small intestine, with an odds ratio of 0.59 in those with the highest quintile of whole grain intake.
- Some case-control studies have reported significant protection against breast cancer with high fibre intakes.
- Rodent studies with chemically-induced mammary cancer have shown a significant protective effect of wheat bran when provided at 12% to 15% of fibre by weight against a high fat (40% of energy) diet. Wheat bran was shown to be capable of inhibiting mammary cancer in a way not shared by alpha-cellulose
- A small Finnish cross-sectional pilot study investigated the association between whole rye and whole wheat consumption and risk of breast cancer. Alkylresorcinols, phenolic compounds found in rye and wheat bran, have been shown to be valid biomarkers of whole rye and whole wheat consumption. Rye and wheat fibre intake, as well as blood and urine levels of alkylresorcinol metabolites, all were found to be significantly lower in the breast cancer survivors compared to the vegetarian and non-vegetarian groups.
A limited number of studies have also indicated whole grains may have a protective effect on prostate, endometrial, pancreatic and respiratory tract cancers.
The mechanisms by which whole grain foods may be cancer protective are not yet clear. The evidence indicates a protective effect of cereal fibre with the World Cancer Research Fund upgrading the level of evidence for the protection of dietary fibre from probable to convincing in 2011.
Other potential protective components within whole grains include flavonoids, henolics, phytoestrogens, lignans, protease inhibitors, saponins and selenium. Chan et al (2007) suggested that another plausible biological mechanism for the reduction in cancer risk could be the decreased levels of insulin and/or inflammation observed with consumption of whole grain foods.
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