Diabetes mellitus and risk of bladder cancer: a meta-analysis

Epidemiological evidence indicates that individuals with diabetes mellitus have an increased risk of several cancers. We performed a systematic review with meta-analysis to evaluate the association between diabetes and risk of bladder cancer. Pertinent studies were identified by searching MEDLINE (from January 1966 to July 2006) and by reviewing the reference lists of retrieved articles. We included case-control and cohort studies reporting relative risk (RR) estimates with 95% CIs (or data to calculate them) of bladder cancer associated with diabetes. Studies of type 1 diabetes were not included. Summary RRs were calculated using a random-effects model. A total of 16 studies (seven case-control studies, three cohort studies and six cohort studies of diabetic patients) fulfilled the inclusion criteria. Analysis of all studies showed that diabetes was associated with an increased risk of bladder cancer, compared with no diabetes (RR=1.24, 95% CI 1.08-1.42). There was strong evidence of heterogeneity among these studies (p < 0.0001). Stratification by study design found that diabetes was associated with an increased risk of bladder cancer in case-control studies (RR=1.37, 95% CI 1.04-1.80, p(heterogeneity)=0.005) and cohort studies (RR=1.43, 95% CI 1.18-1.74, p(heterogeneity)=0.17), but not in cohort studies of diabetic patients (RR=1.01, 95% CI 0.91-1.12, p(heterogeneity)=0.35). Findings from this meta-analysis suggest that individuals with diabetes may have a modestly increased risk of bladder cancer.