title: Long-term dietary calcium intake and breast cancer risk in a prospective cohort of women.
abstract: 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.
  
  
  METHODS
  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.
authors.affiliation: Division of Nutritional Epidemiology, National Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden. susanna.larsson@ki.se
authors.forename: Susanna C
authors.initials: SC
authors.lastname: Larsson
authors.forename: Leif
authors.initials: L
authors.lastname: Bergkvist
authors.forename: Alicja
authors.initials: A
authors.lastname: Wolk
mesh_terms: D001943:Breast Neoplasms / Q000453:epidemiology* / Q000378:metabolism / Q000517:prevention & control; D002136:Calcium, Dietary / Q000008:administration & dosage*; D015331:Cohort Studies; D016001:Confidence Intervals; D004032:Diet; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008875:Middle Aged; D015999:Multivariate Analysis; D015596:Nutrition Assessment; D016017:Odds Ratio; D016016:Proportional Hazards Models; D011446:Prospective Studies; D011960:Receptors, Estrogen / Q000378:metabolism*; D011980:Receptors, Progesterone / Q000378:metabolism*; D012307:Risk Factors; D011795:Surveys and Questionnaires; D013548:Sweden / Q000453:epidemiology
chemical_list: D002136:Calcium, Dietary; D011960:Receptors, Estrogen; D011980:Receptors, Progesterone