Association Between Dietary/Supplementary Calcium Intake and Risk of Breast Cancer: A Systematic Review and Meta-Analysis of Cohort Studies.

Breast cancer is among the most common malignancies in women worldwide, and identifying modifiable dietary factors may support prevention efforts. Calcium has been hypothesized to influence carcinogenesis via effects on cell proliferation, differentiation, apoptosis, and estrogen-related pathways, but epidemiologic findings remain inconsistent. We performed a systematic review and meta-analysis of cohort studies to clarify the association between calcium intake and breast cancer risk. Online databases were systematically searched through December 2025. Eligible studies reported risk estimates comparing the highest versus the lowest calcium intake categories. Pooled relative risks (RRs) and 95% confidence intervals (CIs) were calculated using fixed-effect models. Fourteen cohort studies (38 effect sizes) were included. Overall, higher calcium intake was associated with a significantly lower risk of breast cancer (RR: 0.93; 95%CI: 0.90-0.96; p < 0.001; I2 = 35%). In subgroup analyses, inverse associations were observed in both premenopausal and postmenopausal women, across shorter and longer follow-up periods, and in European and American cohorts. Dietary calcium intake, alone or combined with supplements, was associated with a reduced risk, whereas supplemental calcium alone was not. These findings suggest that higher calcium intake, particularly from dietary sources, may modestly reduce breast cancer risk. Further research is needed to confirm causality and clarify mechanisms.