Dairy, calcium, and vitamin D intake and postmenopausal breast cancer risk in the Cancer Prevention Study II Nutrition Cohort.

BACKGROUND: Calcium, vitamin D, and dairy products are highly correlated factors, each with potential roles in breast carcinogenesis. Few prospective studies have examined these relationships in postmenopausal women.

METHODS: Participants in the Cancer Prevention Study II Nutrition Cohort completed a detailed questionnaire on diet, vitamin and mineral supplement use, medical history, and lifestyle in 1992 to 1993. After exclusion of women with a history of cancer and incomplete dietary data, 68,567 postmenopausal women remained for analysis. During follow-up through August 31, 2001, we identified 2,855 incident cases of breast cancer. Multivariate-adjusted rate ratios (RR) were calculated using Cox proportional hazards models.

RESULTS: Women with the highest intake of dietary calcium (>1,250 mg/d) were at a lower risk of breast cancer than those reporting < or =500 mg/d [RR, 0.80; 95% confidence interval (95% CI), 0.67-0.95; P(trend) = 0.02]; however, neither use of supplemental calcium nor vitamin D intake was associated with risk. Consumption starting at two or more servings of dairy products per day was likewise inversely associated with risk (RR, 0.81; 95% CI, 0.69-0.95; P(trend) = 0.002, compared with <0.5 servings/d). The associations were slightly stronger in women with estrogen receptor-positive tumors comparing highest to lowest intake: dietary calcium (RR, 0.67; 95% CI, 0.51-0.88; P(trend) = 0.004); dairy products (RR, 0.73; 95% CI, 0.57-0.93; P(trend) = 0.0003), and dietary vitamin D (RR, 0.74; 95% CI, 0.59-0.93; P(trend) = 0.006).

CONCLUSIONS: Our results support the hypothesis that dietary calcium and/or some other components in dairy products may modestly reduce risk of postmenopausal breast cancer. The stronger inverse associations among estrogen receptor-positive tumors deserve further study.