Dietary fibre, whole grains, and risk of colorectal cancer: systematic review and dose-response meta-analysis of prospective studies OPEN ACCESS

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, I=0%), for fruit fibre (n=9) was 0.93 (0.82 to 1.05, I=23%), for vegetable fibre (n=9) was 0.98 (0.91 to 1.06, I=0%), for legume fibre (n=4) was 0.62 (0.27 to 1.42, I=58%), and for cereal fibre (n=8) was 0.90 (0.83 to 0.97, I=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, I=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. Introduction Colorectal cancer is the third most common type of cancer, with 1.2 million new cases diagnosed in 2008 worldwide, accounting for about 9.7% of all cases of cancer.1 Evidence from ecological studies, migrant studies, and secular trend studies suggest that environmental risk factors are of major importance in the cause of colorectal cancer.2-4 Dietary factors have been suspected as important, but only intakes of red and processed meat and alcohol are considered to be convincing dietary risk factors for colorectal cancer.5 In the 1970s, Burkitt proposed the hypothesis that dietary fibre reduces the risk of colorectal cancer, based on the observation of low rates of such cancer among rural Africans who ate a diet with a high fibre content.6 Several plausible mechanisms have been proposed to explain the hypothesis, including increased stool bulk and dilution of carcinogens in the colonic lumen, reduced transit time, and bacterial fermentation of fibre to short chain fatty acids.7 However, although many epidemiological studies have investigated the association between fibre intake and risk of colorectal cancer, the results have not been consistent and the possibility of residual confounding by folate intake remains a controversial issue.8 Case-control studies have generally shown a protective association,9 10whereas the results from cohort studies have been mixed.8 11-31 In addition, it is not clear whether only specific types or sources of fibre are associated with the risk. Although initial cohort studies generally reported no significant association between fibre intake and risk of colorectal cancer, the hypothesis regained interest when the Correspondence to: D Aune [email protected] Extra material supplied by the author (see http://www.bmj.com/content/343/bmj.d6617/suppl/DC1) High versus low intake of dietary fibre and whole grains and risk of colorectal cancer High versus low intake of fruit, vegetable, legume, and cereal fibre and risk of colorectal cancer No commercial reuse: See rights and reprints http://www.bmj.com/permissions Subscribe: http://www.bmj.com/subscribe BMJ 2011;343:d6617 doi: 10.1136/bmj.d6617 (Published 10 November 2011) Page 1 of 20