In the May 28 issue of the Archives, Lin and colleagues1 showed that higher intakes of total calcium and vitamin D were moderately associated with a lower risk of premenopausal breast cancer. However, intakes of both nutrients unexpectedly were not inversely associated with the risk of breast cancer among postmenopausal women. The authors stated that the protective effects of both nutrients occur only when intakes of calcium and vitamin D are substantially high, since inadequacy of both nutrients is very common in postmenopausal women. The importance of estrogen in the development and progression of breast cancer has been recognized for some time. The main source of estrogen in postmenopausal women is catalyzed by P450 aromatase, encoded by the CYP19 gene.2 Interestingly, it has been demonstrated that vitamin D is a potent stimulator of CYP19 (P450 aromatase gene) transcription.3 Therefore, vitamin D intake in postmenopuasal women may increase circulating estrogen levels by increasing aromatase expression. This action may attenuate the protective effect of vitamin D intake in postmenopausal women.
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