A new study presented at the American Society of Clinical Oncology Meeting has reported that high circulating vitamin D levels are associated with higher estrogen levels among postmenopausal aromatase inhibitor users but not among women using tamoxifen or not using any anti-estrogen treatment. The study was designed to investigate the relationship between vitamin D and estrogen levels in postmenopausal women with breast cancer treated with anti-estrogen therapy. Vitamin D supplementation is common among those with breast cancer. However, it has been reported that vitamin D influences aromatase activity (in which androgens are converted into estrogens in the body).

The study included 125 postmenopausal women within two years of diagnosis of early stage breast cancer. The women were recruited in Toronto and Los Angeles between March 2009 and January 2010. Blood was drawn and was analyzed for vitamin D (25(OH)D), estradiol (E2) and estrone (E1). A total of 56 participants used aromatase inhibitors, 30 used tamoxifen, and 39 did not use any anti-estrogen therapies. The women using tamoxifen were not found to differ for important variables from the no hormone group, and these two groups were combined into a non-aromatase inhibitor group. Average age was 62 years, body mass index (BMI) was 27 kg/m2, and vitamin D supplementation was 1,244 IU/day - these attributes were similar in both the aromatase inhibitor and non-aromatase inhibitor groups.

Not surprisingly, given that aromatase inhibitors are designed to reduce circulating estrogen, average estradiol and estrone levels were significantly lower in aromatase inhibitor users (19.9 pmol/L and 26.2 pmol/L) than in non-users (39.6 pmol/L and 123.7 pmol/L). However, the relationship between vitamin D and estradiol was different in aromatase inhibitor users than non-users. In aromatase inhibitor users, higher levels of vitamin D (as measured by 25(OH)D) were found to be correlated with higher levels of estradiol, whereas in non-aromatase inhibitor users higher levels of vitamin D were correlated with lower levels of estradiol. Circulating vitamin D was not found to be associated with estrone in either group. In analyses taking account age and BMI, vitamin D was found to be positively correlated with estradiol in aromatase inhibitor users and negatively in non-aromatase inhibitor users. In addition, higher BMI was associated with higher estrone levels in non-aromatase inhibitor users. The authors conclude that in this group of postmenopausal breast cancer survivors, circulating vitamin D was associated with higher estradiol among aromatase inhibitor users but not among non-aromatase inhibitor users. If replicated, this finding has implications for breast cancer survivors receiving aromatase inhibitors.

Please see our article on breast cancer diet during aromatase inhibitor treatment for more information on how to optimize treatment with aromatase inhibitors and reduce side effects.