Meta-analysis of prospective cohort studies of cigarette smoking and the incidence of colon and rectal cancers.

Although the American College of Gastroenterology colorectal cancer screening guidelines highlight cigarette smoking as a risk factor, cigarette smoking is still an arguably underappreciated risk factor for this disease, especially for its subsites: colon cancer (CC) and rectal cancer (RC). A literature search of MEDLINE and EMBASE was performed up to 30 April 2013 for prospective cohort studies. A random-effects meta-analysis was carried out to estimate the summary relative risks (SRRs) and 95% confidence intervals (CIs) for the associations. A total of 24 prospective studies, which reported data for cigarette smoking and incidence of CC and RC separately, were included. Our analysis showed that, compared with never-smokers, current smokers had a higher risk of RC than CC (CC: SRR=1.09, 95% CI, 1.01-1.18; RC: SRR=1.24, 95% CI, 1.16-1.39; PRC vs. CC=0.034), whereas former smokers had a similar risk of CC and RC. Current smokers had a significantly higher risk of proximal CC than distal CC (P=0.035). This meta-analysis suggests that cigarette smoking is associated with an increased risk of both CC and RC, and that the magnitude of the association is stronger for RC than that for CC.