Background: Polycystic ovary syndrome (PCOS) is common and characterised by reproductive and metabolic features. Women with PCOS have lower vitamin D levels compared to healthy controls. Vitamin D binding protein (DBP) is the main carrier of vitamin D and plays an important role in regulating vitamin D concentration and bioavailability. To our knowledge, no previous studies have examined DBP, bioavailable and free 25-hydroxyvitamin D (25(OH)D) in women with PCOS. Our aim was to 1) compare DBP, bioavailable and free 25(OH)D concentrations in women with PCOS and controls; 2) to investigate relationships between DBP, bioavailable and free 25(OH)D and metabolic features.
Methods: In a cross-sectional study using bio-banked samples, we measured 25(OH)D, DBP, albumin, and calculated bioavailable and free 25(OH)D. BMI, body composition(DXA), insulin resistance (HOMA-IR and glucose infusion rate from hyperinsulinaemic euglycaemic clamp) and serum lipids were also measured in 90 women with PCOS and 59 controls.
Results: DBP concentrations were lower in PCOS compared to controls (median[IQR]:443.40[314.4] vs 482.4[156.8]μg/ml, p=0.02). No significant differences were found in bioavailable or free 25(OH)D concentrations between groups. DBP was not associated with BMI, %body fat or insulin resistance. HDL cholesterol was the main determinant of DBP in the overall cohort (β=-0.12, p=0.02), after adjusting for covariates including PCOS/control status, age, BMI, total 25(OH)D and HOMA-IR. In PCOS, total and free 25(OH)D were related to markers of insulin resistance and lipids. Only the associations between total 25(OH)D and HDL (p=0.001), free 25(OH)D and triglycerides (p=0.02), and HDL (p<0.001) remained significant after adjusting for age and BMI.
Conclusion: Women with PCOS had lower DBP, but similar bioavailable or free 25(OH)D concentrations compared to controls, independent of BMI and age. DBP was not associated with insulin resistance or BMI in PCOS. Further studies are needed to investigate the pathophysiology and clinical implications of reduced DBP in PCOS.