Combined Oral Contraceptives: Association with Serum 25-Hydroxyvitamin D and Calcium and Bone Homeostasis

Background: Use of combined oral contraceptives (COCs) has been found to increase serum 25-hydroxyvitamin D [25(OH)D] but effects on calcium and bone homeostasis are unclear. Materials and Methods: Serum 25(OH)D, parathyroid hormone (PTH), alkaline phosphatase (ALK) and estradiol, dietary intake of bone-related nutrients and foods, bone mineral density (BMD), and body fat were compared in adult women (20-35 years; body mass index 21.5 ± 2.3 kg/m²) users (+COC, n = 32) and nonusers (-COC, n = 20) of COC. Biochemical markers were measured by automated assays. BMD at total body (TB), lumbar spine (LS), femoral neck (FN) and trochanter (TR), and body fat, were measured by dual-energy X-ray absorptiometry. Dietary intake was assessed by a food frequency questionnaire. Results: Intake of calcium, dairy foods, and fruits and vegetables, were adequate and did not differ by COC. Mean 25(OH)D was 35% higher in +COC (110.4 ± 27.3 nmol/L, 44.2 ± 1.8 ng/mL) compared with -COC (81.7 ± 22.8 nmol/L, 32.7 ± 2.3 ng/mL; p < 0.001). Mean PTH, ALK, and estradiol were 28%, 12%, and 62% lower, respectively, in +COC compared with -COC (p ≤ 0.05). Mean BMD z-scores (all sites) were adequate and did not differ by COC. There were no correlations between 25(OH)D and dietary, biochemical, and body composition variables. PTH was inversely correlated with TR-BMD z-score in -COC (r = -0.47; p = 0.04), and ALK was inversely correlated with TB-, TR-, and LS-BMD z-scores in -COC (r ≤ -0.43; p ≤ 0.04), but not in +COC. Conclusions: Increased serum 25(OH)D with COC use was paralleled by expected physiologic adjustments in calcium and bone homeostasis, and adequate bone mass status, in nonobese young adult women consuming bone-healthy diets.