LEP and breast cancer: In addition, the subgroup analysis of control source indicated that there were significantly higher serum leptin levels in BC cases in hospital-based control (SMD = 0.51, 95% CI = 0.31–0.71, P <.0001) or population-based control (SMD = 0.85, 95% CI = 0.55–1.14, P <.0001) (Fig. 4).