Low free 25-hydroxyvitamin D and high vitamin D binding protein and parathyroid hormone in obese Caucasians. A complex association with bone?

Abstract

BackgroundStudies have shown altered vitamin D metabolism in obesity. We assessed differences between obese and normal-weight subjects in total, free, and bioavailable 25-hydroxyvitamin D (25(OH) D, 25(OH) D-Free, and 25(OH) D-Bio, respectively), vitamin D binding protein (DBP), parathyroid hormone (PTH) and bone traits.Methods595 37-47-year-old healthy Finnish men and women stratified by BMI were examined in this cross-sectional study. Background characteristic and intakes of vitamin D and calcium were collected. The concentrations of 25(OH) D, PTH, DBP, albumin and bone turnover markers were determined from blood. 25(OH) D-Free and 25(OH) D-Bio were calculated. pQCT was performed at radius and tibia.ResultsMean +/- SE (ANCOVA) 25(OH) D-Free (10.8 +/- 0.6 vs 12.9 +/- 0.4 nmol/L; P = 0.008) and 25(OH) DBio (4.1 +/- 0.3 vs 5.1 +/- 0.1 nmol/L; P = 0.003) were lower in obese than in normal-weight women. In men, 25(OH) D (48.0 +/- 2.4 vs 56.4 +/- 2.0 nmol/L, P = 0.003), 25(OH) D-Free (10.3 +/- 0.7 vs 12.5 +/- 0.6 pmol/L; P = 0.044) and 25(OH) D-Bio (4.2 +/- 0.3 vs 5.1 +/- 0.2 nmol/L; P = 0.032) were lower in obese. Similarly in all subjects, 25(OH) D, 25(OH) D-Free and 25(OH) D-Bio were lower in obese (P<0.001). DBP (399 +/- 12 vs 356 +/- 7mg/L, P = 0.008) and PTH (62.2 +/- 3.0 vs 53.3 +/- 1.9 ng/L; P = 0.045) were higher in obese than in normal-weight women. In all subjects, PTH and DBP were higher in obese (P = 0.047 and P = 0.004, respectively). In obese women, 25(OH) D was negatively associated with distal radius trabecular density (R-2 = 0.089, P = 0.009) and tibial shaft cortical strength index (CSI) (R-2 = 0.146, P = 0.004). 25(OH) D-Free was negatively associated with distal radius CSI (R-2 = 0.070, P = 0.049), radial shaft cortical density (CorD) (R-2 = 0.050, P = 0.045), and tibial shaft CSI (R-2 = 0.113, P = 0.012). 25(OH) D-Bio was negatively associated with distal radius CSI (R-2 = 0.072, P = 0.045), radial shaft CorD (R-2 = 0.059, P = 0.032), and tibial shaft CSI (R-2 = 0.093, P = 0.024).ConclusionsThe associations between BMI and 25(OH) D, 25(OH) D-Free, and 25(OH) D-Bio, DBP, and PTH suggest that obese subjects may differ from normal-weight subjects in vitamin D metabolism. BMI associated positively with trabecular bone traits and CSI in our study, and slightly negatively with cortical bone traits. Surprisingly, there was a negative association of free and bioavailable 25(OH) D and some of the bone traits in obese women.