The Cancer Research
Whole grains provide dietary fiber and nutrients beyond what you can get from refined grains. Emerging research is exploring whether intact whole grains (such as brown rice, farro, bulgur, sorghum and quinoa) offer additional benefits beyond bread, cereals and pasta made from milled, minimally processed whole grain flour.
Accurately assessing people’s consumption of whole grains is challenging, but crucial to study their influence on cancer risk. One indicator is body levels of alkylresorcinol compounds, a measure gaining popularity with researchers. But even this may not capture all forms of whole grains consumed.
Interpreting the data
After a systematic review of the global scientific literature, AICR/WCRF analyzed how whole grains and their nutrients affect the risk of developing cancer.
- Evidence categorized as “convincing” or “probable” means there is strong research showing a causal relationship to cancer—either decreasing or increasing the risk. The research must include quality human studies that meet specific criteria and biological explanations for the findings.
- A convincing or probable judgement is strong enough to justify recommendations.
- There is probable evidence that whole grains DECREASE the risk of:
- Colorectal cancer
- There is probable evidence that foods with dietary fiber DECREASE the risk of
- Colorectal cancer
- Evidence categorized as “limited suggestive” means results are generally consistent in overall conclusions, but it’s rarely strong enough to justify recommendations to reduce the risk of cancer.
Ongoing Areas of Investigation
- In animal studies, resistant starch and fermentable types of dietary fiber support growth of healthful bacteria in the colon.
- In cell and animal studies, phenolic acids increase cells’ antioxidant and anti-inflammatory defenses against damage that could lead to cancer. Emerging evidence in animal studies suggests they may also improve glucose metabolism and decrease insulin resistance, and alter the gut microbiota (microbes living in the colon), creating an environment in the body less likely to support cancer.
- Phytic acid may reduce damage to colon cells from free radicals produced there. However, the effects of whole grains as a source of this protection are not yet known.
- In cell studies, lignans increase antioxidant, anti-inflammatory and carcinogen-deactivating enzymes. They also decrease growth and increase the self-destruction of cancer cells. In studies with mice, lignans decrease cancer development and growth.
Analysis combining six observational cohort studies found 15% lower overall cancer risk with the equivalent of three standard servings (90 grams) per day. One example of how you can accumulate this amount is with two slices of whole-grain bread plus a one-ounce portion of whole-grain cereal.
- Colorectal cancer: Whole grains are identified in the AICR Third Expert Report as a dietary choice that probably protects against colorectal cancer. Analysis combining six studies found risk reduced 17% for the equivalent of three standard servings (90 grams) per day. A similar reduction in risk of colorectal cancer risk related to whole grain consumption was reported in an analysis commissioned by the World Health Organization.
- Whole grains might also reduce risk of other cancers, but data is very limited. Analysis for the AICR Third Expert Report showed a trend for lower breast cancer risk with increased dietary fiber from grains.
Limited evidence from controlled trials and observational population studies suggests that whole grains may lower cancer risk by reducing inflammation and insulin resistance, but study results are inconsistent.
Weight: Since excess body fat and weight gain increase the risk of at least 12 cancers, an influence on weight and body fat are important to consider. Analysis for the AICR Continuous Update Project (CUP) concluded that whole grains and other foods categorized as low-calorie density (meaning they are not concentrated in calories and help people eat a level of calories appropriate for them) probably reduce risk of weight gain, overweight and obesity. The report summarizes evidence showing that overall, controlled intervention trials and observational population studies in adults link greater whole grain consumption with less long-term weight gain, although some studies show no effect.
Dietary Fiber: Observational population studies link high dietary fiber consumption with reduced risk of colorectal cancer. One meta-analysis of 16 prospective studies also links dietary fiber with lower risk of breast cancer. However, analysis for the AICR/WCRF Third Expert Report considered potential for an association of dietary fiber and this and several other cancers, and found the evidence too limited to support a conclusion.
- Some whole grains provide fermentable fibers that gut bacteria can use to produce butyrate and other short-chain fatty acids (SCFAs). These SCFAs reduce markers of inflammation and oxidative stress in human clinical trials, and show effects on gene expression that could reduce cancer development.
Resistant Starch: Digestive enzymes can’t completely digest resistant starch, so it passes to the colon where gut bacteria use it like fermentable fiber to form SCFAs. More research is needed, since limited human trials suggest that not all forms of resistant starch are the same.
Alkylresorcinols: Researchers can use body level of alkylresorcinols, compounds in whole-grain wheat and rye, as a marker of whole grain consumption. Higher alkylresorcinol levels were associated with lower risk of colorectal cancer risk in the European Prospective Investigation into Cancer (EPIC). This could reflect effects of the compounds themselves, but current evidence is stronger in supporting whole-grain foods themselves as protective.
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