In reply
We thank Dizdar and colleagues for offering an alternative explanation to our finding of no risk reduction of postmenopausal breast cancer with higher vitamin D intake.1 They suggest that the positive interplay between vitamin D and estrogens might have attenuated the protection of vitamin D against breast cancer development among postmenopausal women.
We have tested the potential interaction between vitamin D intake and use of exogenous hormones in relation to the risk of postmenopausal breast cancer and found that the interaction was not statistically significant (P value for interaction, .36). Among never users of postmenopausal hormone therapy, the hazard ratios of breast cancer in the higher 4 quintile groups of vitamin D intake relative to those in the lowest group were 1.45 (95% confidence interval [CI], 0.91-2.31), 1.69 (95% CI, 1.07-2.67), 1.68 (95% CI, 1.05-2.70), and 1.39 (95% CI, 0.80-2.42) (P value for trend, .37). Among the postmenopausal women who were currently receiving hormone therapy, the hazard ratios of breast cancer in the higher 4 quintiles of vitamin D intake compared with those in the lowest intake group were 1.19 (95% CI, 1.18-2.26), 1.83 (95% CI, 1.00-1.95), 1.59 (95% CI, 0.93-1.87), and 0.73 (95% CI, 0.95-2.00) (P value for trend, .57).