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

Elisa Saarnio, Minna Pekkinen, Suvi T Itkonen, Virpi Kemi, Heini Karp, Kaisa K Ivaska, Juha Risteli, Marja-Kaisa Koivula, Merja Kärkkäinen, Outi Mäkitie, Harri Sievänen, Christel Lamberg-Allardt

Journal: PloS one 2018;13(2):e0192596

PMID: 29489840

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Plain Language Summary

Obese people are at a higher risk of vitamin D deficiency, and this could have an impact on bone mineral density. This study examined whether there were any differences between obese, overweight and normal-weight adults in levels of vitamin D, vitamin D binding protein (DBP), parathyroid hormone (PTH) and bone density. More than 500 healthy Finnish men and women aged 37-47 years took part in the study. Vitamin D levels were found to be lower in obese than in normal weight people. Levels of DBP and PTH were higher in obese than normal weight women. Some measures of bone density were higher in obese women. Markers of bone turnover were lower in obese people than those of normal weight. The findings of this study suggest that obese people may differ from normal weight subjects in vitamin D metabolism. It is not yet clear what impact this has on bone health.

Abstract

BACKGROUND

Studies 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)DFree, and 25(OH)DBio, respectively), vitamin D binding protein (DBP), parathyroid hormone (PTH) and bone traits.

METHODS

595 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)DFree and 25(OH)DBio were calculated. pQCT was performed at radius and tibia.

RESULTS

Mean±SE (ANCOVA) 25(OH)DFree (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)DFree (10.3±0.7 vs 12.5±0.6 pmol/L; P = 0.044) and 25(OH)DBio (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)DFree and 25(OH)DBio 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 = 0.047and P = 0.004, respectively). In obese women, 25(OH)D was negatively associated with distal radius trabecular density (R2 = 0.089, P = 0.009) and tibial shaft cortical strength index (CSI) (R2 = 0.146, P = 0.004). 25(OH)DFree was negatively associated with distal radius CSI (R2 = 0.070, P = 0.049), radial shaft cortical density (CorD) (R2 = 0.050, P = 0.045), and tibial shaft CSI (R2 = 0.113, P = 0.012). 25(OH)DBio was negatively associated with distal radius CSI (R2 = 0.072, P = 0.045), radial shaft CorD (R2 = 0.059, P = 0.032), and tibial shaft CSI (R2 = 0.093, P = 0.024).

CONCLUSIONS

The associations between BMI and 25(OH)D, 25(OH)DFree, and 25(OH)DBio, 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.

Address: Calcium Research Unit, Department of Food and Nutrition, University of Helsinki, Helsinki, Finland.; Folkhälsan Institute of Genetics, Folkhälsan Research Center, Helsinki, Finland.; Children's Hospital, Helsinki University Central Hospital and University of Helsinki, Helsinki, Finland.; Department of Cell Biology and Anatomy, Institute of Biomedicine, University of Turku, Turku, Finland.; Department of Clinical Chemistry, University of Oulu, Oulu, Finland.; Northern Finland Laboratory Centre Nordlab, Oulu, Finland.; Medical Research Center, Oulu, Finland.; Department of Clinical Chemistry, University of Oulu, Oulu, Finland.; Northern Finland Laboratory Centre Nordlab, Oulu, Finland.; Medical Research Center, Oulu, Finland.; Tykslab, the Hospital District of Southwest Finland, Turku, Finland.; Department of Clinical Chemistry, University Hospital Turku, Turku, Finland.; The UKK Institute for Health Promotion Research, Tampere, Finland.

DOI: 10.1371/journal.pone.0192596