HEALTH NEWS

Study Title:

Fiber and Colon Cancer

Study Abstract

Objective To investigate the association between intake of dietary fibre and whole grains and risk of colorectal cancer.

Design Systematic review and meta-analysis of prospective observational studies.

Data sources PubMed and several other databases up to December 2010 and the reference lists of studies included in the analysis as well as those listed in published meta-analyses.

Study selection Prospective cohort and nested case-control studies of dietary fibre or whole grain intake and incidence of colorectal cancer.

Results
25 prospective studies were included in the analysis. The summary relative risk of developing colorectal cancer for 10 g daily of total dietary fibre (16 studies) was 0.90 (95% confidence interval 0.86 to 0.94, I2=0%), for fruit fibre (n=9) was 0.93 (0.82 to 1.05, I2=23%), for vegetable fibre (n=9) was 0.98 (0.91 to 1.06, I2=0%), for legume fibre (n=4) was 0.62 (0.27 to 1.42, I2=58%), and for cereal fibre (n=8) was 0.90 (0.83 to 0.97, I2=0%). The summary relative risk for an increment of three servings daily of whole grains (n=6) was 0.83 (0.78 to 0.89, I2=18%).

Conclusion
A high intake of dietary fibre, in particular cereal fibre and whole grains, was associated with a reduced risk of colorectal cancer. Further studies should report more detailed results, including those for subtypes of fibre and be stratified by other risk factors to rule out residual confounding. Further assessment of the impact of measurement errors on the risk estimates is also warranted.

Our results indicate a 10% reduction in risk of colorectal cancer for each 10 g/day intake of total dietary fibre and cereal fibre and a about a 20% reduction for each three servings (90 g/day) of whole grain daily, and further reductions with higher intake. These findings thus have important public health implications. Our results provide further support for public health recommendations to increase the intake of dietary fibre in the prevention of colorectal cancer. However, they suggest a particular benefit of increasing cereal fibre and whole grain intake. Increasing the intake of dietary fibre and whole grains is also likely to reduce the risk of cardiovascular disease,76 77 78 type 2 diabetes,74 75 overweight and obesity,72 73 and possibly overall mortality,76 78 thus there are several health benefits by increasing fibre intake and replacing refined grains with whole grains.

In summary, our meta-analysis suggests that a high intake of dietary fibre, particularly from cereal and whole grains, is associated with a reduced risk of colorectal cancer. Further studies should report more detailed results, including those for subtypes of fibre, stratify the results by subsites within the colorectum, and stratify the results by other risk factors to be able to rule out residual confounding. Further assessment of the impact of measurement errors on the risk estimates is also warranted.

Mechanisms
A protective effect of dietary fibre and whole grain consumption on risk of colorectal cancer is biologically plausible. Whole grain foods are important sources of dietary fibre and may decrease the risk of colorectal cancer by increasing stool bulk, diluting faecal carcinogens, and decreasing transit time, thus reducing the contact between carcinogens and the lining of the colorectum.7 In addition, bacterial fermentation of fibre results in the production of short chain fatty acids, which may have protective effects against colorectal cancer.66 Other components of whole grains may also protect against colorectal cancer, including antioxidants, vitamins, trace minerals, phytate, phenolic acids, lignans, and phytoestrogens.66 67 68 Whole grains have a high content of folate and magnesium, which have been associated with a reduced risk of colorectal cancer.69 70 71 Higher intakes of dietary fibre and whole grain also protect against weight gain72 73 and type 2 diabetes,74 75 and it is possible that part of the potential effect of fibre intake is mediated through improved weight control and reduced insulin resistance, although these may not be the main mechanisms. However, the results persisted in studies that adjusted for both folate and body mass index, suggesting an association independent of folate and body mass index.

Study Information

Dagfinn Aune, Doris S M Chan, , Rosa Lau, Rui Vieira, , Darren C Greenwood, , Ellen Kampman, , Teresa Norat.
Dietary fibre, whole grains, and risk of colorectal cancer: systematic review and dose-response meta-analysis of prospective studies.
BMJ
2011 November
Department of Epidemiology and Biostatistics, School of Public Health, Imperial College London, St Mary’s Campus, London W2 1PG, UK