Urinary cadmium concentration and risk of breast cancer: a systematic review and dose-response meta-analysis.

Cadmium is a toxic and persistent heavy metal with estrogenic activities. We conducted a systematic review and meta-analysis of cohort, case-control, and cross-sectional studies of the association between urinary cadmium concentration, a biomarker of cadmium exposure, and breast cancer risk. Studies were identified by searching PubMed and Embase (to March 15, 2015) and by reviewing the reference lists of pertinent articles. Study-specific risk estimates were combined by using a random-effects model. We identified 2 cohort studies (with 67 breast cancer deaths) and 5 case-control studies and 1 cross-sectional study (with 1,416 cases and 5,083 controls) on urinary cadmium concentration in relation to breast cancer risk. The studies were published during the past 10 years (2006-2015). There was no consistent association between urinary cadmium and breast cancer mortality in the cohort studies. In case-control and cross-sectional studies, the pooled odds ratios were 2.24 (95% confidence interval: 1.50, 3.34; I(2) = 63.4%) for the highest versus lowest category of cadmium concentration and 1.66 (95% confidence interval: 1.23, 2.25) for each 0.5-µg/g creatinine increase of cadmium concentration. This meta-analysis suggests that a high cadmium exposure may be a risk factor for breast cancer, but large prospective studies are needed to confirm this finding.