Intake of dairy products, calcium, and vitamin d and risk of breast cancer.

BACKGROUND Laboratory data suggest that calcium and vitamin D, found at high levels in dairy products, might reduce breast carcinogenesis. However, epidemiologic studies regarding dairy products and breast cancer have yielded inconsistent results. We examined data from a large, long-term cohort study to evaluate whether high intake of dairy products, calcium, or vitamin D is associated with reduced risk of breast cancer. METHODS We followed 88 691 women in the Nurses' Health Study cohort from the date of return of their food-frequency questionnaire in 1980 until May 31, 1996. Dietary information was collected in 1980 and updated in 1984, 1986, 1990, and 1994. We identified 3482 women (premenopausal = 827, postmenopausal = 2345, and uncertain menopausal status = 310) with incident invasive breast cancer. We used pooled logistic regression to estimate multivariable relative risks (RRs) using 2-year time increments. The RRs and 95% confidence intervals (CIs) were calculated for each category of intake compared with the lowest intake group. All statistical tests were two-sided. RESULTS Intakes of dairy products, calcium, or vitamin D were not statistically significantly associated with breast cancer risk in postmenopausal women. In premenopausal women, however, consumption of dairy products, especially of low-fat dairy foods and skim/low-fat milk, was inversely associated with risk of breast cancer. The multivariable RRs comparing highest (>1 serving/day) and lowest (800 mg/day versus 500 IU/day versus <or=150 IU/day; RR = 0.72, 95% CI = 0.55 to 0.94), and lactose (quintile 5 versus quintile 1; RR = 0.68, 95% CI = 0.54 to 0.86] also had inverse associations with premenopausal breast cancer risk. By taking into account supplemental calcium and vitamin D intake, we found that association with calcium was due mainly to dairy sources whereas the association with vitamin D may be independent of dairy intake. CONCLUSIONS We found no association between intake of dairy products and breast cancer in postmenopausal women. Among premenopausal women, high intake of low-fat dairy foods, especially skim/low-fat milk, was associated with reduced risk of breast cancer. Similar inverse associations were seen with components (calcium and vitamin D) of dairy foods, but their independent associations with breast cancer are difficult to distinguish.