Vitamin D association with estradiol and progesterone in young women.

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.