Serum calcium and breast cancer risk: results from a prospective cohort study of 7,847 women

Experimental and epidemiological studies suggest that calcium-regulating hormones—parathyroid hormone (PTH) and vitamin D—may be associated with breast cancer risk. No prospective cohort study has investigated the association between pre-diagnostic calcium levels and subsequent risk of breast cancer. We have examined this in a cohort of 7,847 women where serum calcium levels and established risk factors for breast cancer had been assessed at baseline. During a mean follow-up of 17.8 years, 437 incident breast cancer cases were diagnosed. Incidence of breast cancer was calculated in different quartiles of serum calcium levels and a Cox’s proportional hazards analysis was used to obtain corresponding relative risks (RR), with a 95% confidence interval (CI), adjusted for potential confounders. In premenopausal women, serum calcium levels were inversely associated with breast cancer risk in a dose-response manner. The adjusted RR (95% CI) of breast cancer was in the 2nd calcium quartile 0.91 (0.65–1.30), in the 3rd quartile 0.89 (0.60–1.31), and in the 4th quartile 0.56 (0.32–0.98), as compared to the 1st calcium quartile. In postmenopausal overweight women (BMI > 25), breast cancer risk was higher in calcium quartiles 2–4 as compared to the 1st quartile. Our findings may have implications for primary prevention of breast cancer and for the management of asymptomatic primary hyperparathyroidism.