Background Vitamin D binding protein (DBP) is the main carrier of vitamin D in circulation and plays an important role in regulating vitamin D concentration and its bioavailability for target tissues. Vitamin D has been shown to relate to risk factors for diabetes including adiposity and insulin resistance. Vitamin D deficiency is also reported to be associated with features of polycystic ovary syndrome, a common condition characterised by obesity and insulin resistance. No previous studies have examined the concentrations of DBP, bioavailable and free 25-hydroxyvitamin D (25OHD) in women with PCOS and their associations with metabolic parameters. 

Objectives We compared DBP, bioavailable and free 25OHD concentrations in women with PCOS and controls and investigated their relationships with BMI, body composition, markers of insulin resistance (homeostatic model assessment of insulin resistance: HOMA-IR and glucose infusion rate (GIR) from hyperinsulinaemic euglycaemic clamp), and lipid profile. 

Methods We measured 25OHD and DBP and albumin concentrations in bio-banked plasma samples from a physically and biochemically well-characterised population of 90 women with PCOS and 59 controls. Bioavailable and free 25OHD were calculated using validated formula. Participants of the study had BMI, body composition (DEXA), HOMA-IR, GIR and lipid profile data available.

Results DBP concentrations were lower in women with PCOS compared to controls (p= 0.02). Bioavailable and free 25OHD were not different between PCOS and control group (both p>0.1). There was no relationship between DBP and BMI, % body fat and markers of insulin resistance. HDL was the main determinant of DBP in the whole population after adjusting for covariates including PCOS/control status. Total and free 25OHD were related to markers of insulin resistance and lipids only in PCOS group. However, the relationships except for free 25OHD and HDL were not significant after adjusting for age and BMI. Furthermore, DBP, bioavailable and free 25OHD and not total 25OHD were significantly related to BMD after adjusting for age and BMI in controls. Among women with PCOS, those with obesity (BMI≥30 kg/m2) had lower total and free 25OHD but not significantly different bioavailable 25OHD concentrations compared to non-obese (BMI< 30). DBP concentration was higher in obese group compared to non-obese in women with and without PCOS. However, the difference did not reach a statistical significance. 

Conclusion In our study, women with PCOS had lower DBP but not significantly different bioavailable and free 25OHD concentrations compared to controls. The difference in DBP was not mediated by insulin resistance or BMI. Further studies are needed to investigate the underlying reason for lower DBP in women with PCOS and implications of our findings.

Bioavailable vitamin D and vitamin D binding protein in polycystic ovary syndrome: correlations with insulin resistance