Use of hormonal contraceptives (HC) may influence total plasma concentrations of vitamin D metabolites. A likely cause is an increased synthesis of vitamin D binding protein (VDBP). Discrepant results are reported on whether the use of HC affects free concentrations of vitamin D metabolites. Aim
: In a cross-sectional study, plasma concentrations of vitamin D metabolites, VDBP, and the calculated free vitamin D index in users and non-users of HC were compared and markers of calcium and bone metabolism investigated. Results:
75 Caucasian women aged 25–35 years were included during winter season. Compared with non-users (n =
23), users of HC (n =
52) had significantly higher plasma concentrations of 25-hydroxyvitamin D (25OHD) (median 84 interquartile range: [67–111] vs.
70 [47–83] nmol/L, p =
0.01), 1,25-dihydroxyvitamin D (1,25(OH)2
D) (198 [163–241] vs.
158 [123–183] pmol/L, p =
0.01) and VDBP (358 [260–432] vs.
271 [179–302] µg/mL, p <
0.001). However, the calculated free indices (FI-25OHD and FI-1,25(OH)2
D) were not significantly different between groups (p >
0.10). There were no significant differences in indices of calcium homeostasis (plasma concentrations of calcium, parathyroid hormone, and calcitonin, p >
0.21) or bone metabolism (plasma bone specific alkaline phosphatase, osteocalcin, and urinary NTX/creatinine ratio) between groups. In conclusion:
Use of HC is associated with 13%–25% higher concentrations of total vitamin D metabolites and VDBP. This however is not reflected in indices of calcium or bone metabolism. Use of HC should be considered in the interpretation of plasma concentrations vitamin D metabolites.