Long-term dietary calcium intake and breast cancer risk in a prospective cohort of women.

BACKGROUND: Calcium may potentially influence the risk of breast cancer because of its role in regulating cell proliferation, differentiation, and apoptosis. However, prospective studies of calcium intake in relation to breast cancer incidence are sparse.

OBJECTIVE: The objective of this study was to prospectively examine and show the association, if any, of dietary calcium intake with risk of breast cancer by estrogen receptor (ER) and progesterone receptor (PR) status of the tumor.

DESIGN: The Swedish Mammography Cohort is a population-based prospective cohort of 61,433 women who were cancer-free at enrollment in 1987-1990. Dietary calcium intake was assessed with a food-frequency questionnaire at baseline and again in 1997. Cox proportional hazards models were used to estimate rate ratios (RRs) and 95% CIs, adjusted for breast cancer risk factors.

RESULTS: During an average of 17.4 y of follow-up, 2,952 incident cases of invasive breast cancer were ascertained. Dietary calcium intake was not associated with risk of overall breast cancer; the multivariate RR for the highest compared with the lowest quintile of calcium intake was 0.97 (95% CI: 0.87, 1.09; P for trend: 0.49). There was a statistically significant inverse trend for ER-negative/PR-negative (ER-/PR-) breast cancer (P for trend: 0.02); the multivariate RR for the comparison of extreme quintiles of calcium intake was 0.66 (95% CI: 0.44, 0.99). Calcium intake was not associated with ER-positive/PR-positive (ER+/PR+) or ER+/PR- tumors.

CONCLUSIONS: Our findings do not support an association between dietary calcium intake and overall breast cancer risk. The inverse relation between calcium intake and ER-/PR- breast cancer requires confirmation in other studies.