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Objective: Vitamin D may reduce breast cancer risk through an effect on steroid hormones in cycling women.We conducted a study to determine whether there is an association between circulating 25-hydroxyvitamin D (25(OH)D) and estradiol and progesterone in young women., Methods: Volunteer women aged 18-22 and not using hormonal contraceptives were recruited during summer and winter. They provided demographic and lifestyle information and a blood sample. Women recruited in winter gave a second sample after taking vitamin D supplement for 4 weeks. There were 101 women sampled during the luteal phase (1-14 days prior to the start of the next menstrual period). Generalized estimating equation linear regression models were used to examine the relationship between 25(OH)D and estradiol and progesterone., Results: Per increase of 10 nmol/l of 25(OH)D, progesterone multiplicatively decreased by a factor of 10% (95% CI 5-14%, p < 0.001) and estradiol decreased by a factor of 3% (95% CI 0-6%, p = 0.04) after adjustment for age, body mass index, ethnicity, season, alcohol use, smoking, and physical activity., Conclusions: Higher levels of vitamin D may reduce progesterone and estradiol, providing a potential mechanism for reduction in breast cancer risk from increased vitamin D exposure in young women.

Evidence has emerged for a role of vitamin D in the development of breast cancer, and there is some suggestion that its antiproliferative effect is greater in hormone-receptor-positive cells. Few epidemiologic studies have considered the association between vitamin D and hormone-receptor-defined breast cancer, and the results are conflicting. Considering 759 cases and 1,135 controls from a case-control study (Ontario, Canada, 2003-2005), the authors examined the association between vitamin D intake at specific ages and combined estrogen-receptor- (ER) and progesterone-receptor- (PR) defined breast cancer. While increased intake of vitamin D (from the sun and diet) was most consistently associated with a significantly reduced risk of ER+/PR+ tumors (e.g., odds ratio = 0.76, 95% confidence interval: 0.59, 0.97 for use of cod liver oil during adolescence), comparable nonsignificant associations were found for receptor-negative (ER-/PR-) (odds ratio = 0.74, 95% confidence interval: 0.53, 1.04) and mixed (ER+/PR-) (odds ratio = 0.79, 95% confidence interval: 0.51, 1.22) tumors. This study suggests that vitamin D is associated with a reduced risk of breast cancer regardless of ER/PR status of the tumor. Future studies with a larger number of receptor-negative and mixed tumors are required.