The association of oral contraceptive use with plasma 25-hydroxyvitamin D levels.

OBJECTIVE This analysis was conducted to compare wintertime 25-hydroxyvitamin D (25OHD) levels of young women who did and did not use oral contraceptives (OC). METHODS The subjects were 66 Caucasian women aged 20 through 40 recruited from the Boston area. Plasma 25OHD was measured in February or March and again 1 year later. Other measurements included height, weight and vitamin D intake from diet and supplements. RESULTS The initial mean 25OHD level of the 26 OC users was 41% higher than those of nonusers before adjustment for age and vitamin D intake (83 +/- 40 (sd) nmol/L compared with 59 +/- 22), and 39% higher after adjustment (p = 0.003). Five women who discontinued OC use during the year following their initial measurement all had decreases in their 25OHD levels (mean change was -25.5 +/- 17.7 (SD) nmol/L), whereas levels in women whose OC use or non-use was constant did not change. CONCLUSION OC use increases circulating levels of 25OHD, and should be considered when interpreting values obtained for clinical evaluation or nutrition research.