Nitrate in drinking water and bladder cancer: a meta-analysis.

This study examined whether exposure to nitrate in drinking water is associated with increased risk for bladder cancer by conducting a comprehensive literature research. A meta-analysis was performed with and without adjustment for confounding factors. Three groups (reference, intermediate and high groups) were established in terms of different nitrate concentrations in each included study. Separate relative risk measures were calculated for intermediate and high groups. Heterogeneity was assessed by using the Q statistics. Publication bias was evaluated by Egger's and Begg's test. Quality assessment for studies was performed by using the Newcastle-Ottawa scale. Two cohorts, two case-controls, and one ecological study were included in this study. The adjusted data showed that the combined risk ratios (RRs) were 1.13 (95% CI: 0.81 to 1.57) and 1.27 (95% CI: 0.75 to 2.15) for intermediate and high groups respectively. For unadjusted data, the corresponding RRs were 1.18 (95% CI: 0.89 to 1.57) and 1.29 (95% CI: 0.81 to 2.07). Sensitivity test indicated that results were significantly underestimated when Ward's study was included. No significant publication bias was found. There was heterogeneity among studies. The results suggested that there was no sufficient evidence that nitrate in drinking water is associated with increased risks for bladder cancer.