Obesity and colon and rectal cancer risk: a meta-analysis of prospective studies

Background: Whereas obesity has been associated with an increased risk of colon cancer in men, a weak or no association has been observed in women. Results for rectal cancer have also been inconsistent. Objective: The objective was to perform a meta-analysis to summarize the available evidence from prospective studies on the associations of overall and abdominal obesity with the risk of colon and rectal cancer. Design: We searched MEDLINE (1966-April 2007) and the references of the retrieved articles. Study-specific relative risks (RRs) were pooled by using a random-effects model. Results: Thirty prospective studies were included in the metaanalysis of body mass index (BMI; in kg/m(2)). Overall, a 5-unit increase in BMI was related to an increased risk of colon cancer in both men (RR: 1.30; 95% CI: 1.25,1.35) and women (RR: 1.12; 95% CI: 1.07, 1.18), but the association was stronger in men (P < 0.001) . BMI was positively associated with rectal cancer in men (RR: 1.12; 95 % CI: 1.09,1.16) but not in women (RR: 1.03; 95 % CI: 0.99,1.08). The difference in RRs between cancer sites was statistically significant (P < 0.001 in men and P = 0.04 in women). Colon cancer risk increased with increasing waist circumference (per 10-cm increase) in both men (RR: 1.33; 95% CI: 1.19, 1.49) and women (RR: 1.16; 95% CI: 1.09, 1.23) and with increasing waist-hip ratio (per 0.1-unit increase) in both men (RR: 1.43; 95% CI: 1.19, 1.71) and women (RR: 1.20; 95% CI: 1.08, 1.33). Conclusions: The association between obesity and colon and rectal cancer risk varies by sex and cancer site.