The Relationship Between Oral Contraceptive Use With Total Hydroxyvitamin D And Its Metabolites In Young Adult Women

Abstract

Vitamin D deficiency, defined as total hydroxyvitamin D (25(OH)D) <50 nmol/L, is associated with poor bone health, impaired muscle function and increased risk of some diseases. The biologically active form of vitamin D is dihydroxyvitamin D (1,25(OH)2D), but there is emerging evidence that the relative proportion of 1,25(OH)2D to the catabolic metabolite (24,25(OH)2D) determines biological activity. In women, higher 25(OH)D concentrations have been reported in oral contraceptive pill (OCP) users, but the influence of OCP and other hormonal contraceptives (HC) on vitamin D metabolites and their ratio is unknown. PURPOSE: To examine the relationship between vitamin D metabolites, the vitamin D metabolite ratio (VMR) and HC use in young adult women. METHODS: 512 female Army recruits, mean (SD) age 23 (3.2) years, height 1.66 (0.06) m, and body mass 64.8 (7.9) kg, volunteered to provide a venous blood sample at the start of basic military training. Samples were analysed for 25(OH)D, 1,25(OH)2D, 24,25(OH)2D, intact parathyroid hormone (iPTH), and albumin adjusted calcium (aCa). Participants had passed Army selection including medical screening and physical performance testing. Hormonal contraceptive use was assessed by questionnaire, and later verified from medical records. RESULTS: 163 women using vitamin D supplements were excluded from the analysis. Remaining participants were subdivided into groups: no-HC (n=157); OCP (n=95); progesterone-only pill (POP, n=25); injection (n=13); and, implant (n=59). There were no differences in participant characteristics between groups (p>0.05). Compared with no-HC, OCP users had higher 25(OH)D (61 (28) vs 73 (31) nmol/L), 1,25(OH)2D (159 (38) vs 177 (50) pmol/L), 24,25(OH)2D (6 (3) vs 7 (4) nmol/L), and lower iPTH (3.64 (1.04) vs 3.22 (1.05) pmol/L) (p<0.01). There were no differences in the VMR (25(OH)D:24,25(OH)2D and 1,25(OH)2D:24,25(OH)2D), or in aCa, between groups (p>0.05). iPTH and 25(OH)D:24,25(OH)2D VMR were lower, and aCa was higher, in POP than in no-HC users (p<0.05). CONCLUSION: Oral contraceptive pill users have higher 25(OH)D, possibly from the stimulation of vitamin D binding protein and increased bioavailability of 25(OH)D, but the balance between the active and catabolic vitamin D metabolites is not dependent on HC use.