Vitamin D plays an important role in musculoskeletal health due to its involvement in the regulation of calcium and phosphorus⁽Reference Holick¹⁾. The two forms of vitamin D – vitamin D₂ and vitamin D₃ – are both metabolised into 25-hydroxyvitamin D (25OHD) in the liver⁽Reference Henry²⁾, which is the marker of vitamin D status. 25OHD is then transported to the kidney via the serum, bound to vitamin D binding protein (VDBP)⁽Reference Bikle³⁾, where 25OHD is converted to 1,25-dihydroxyvitamin D (1,25(OH)₂D) which is the active form of vitamin D⁽Reference Tripkovic, Wilson and Hart⁴⁾. VDBP has a greater affinity for vitamin D₃ than vitamin D₂ due to the differences in their side chains⁽Reference Tripkovic, Wilson and Hart⁴⁾. The aims of this analysis were to examine the effect of vitamin D supplementation on VDBP & 1,25(OH)₂D and to assess differences in VDBP & 1,25(OH)₂D in those with suboptimal (<50 nmol/L) vitamin D status compared to those with optimal (>50 nmol/L) vitamin D status.

Healthy Caucasian (CA) and South Asian (SA) women, aged 20–64yrs, were recruited from the county of Surrey onto the D2-D3 study⁽Reference Tripkovic, Wilson and Hart⁴⁾. The D2-D3 study was run over two consecutive winters and the participants took 15μg vitamin D₂, 15μg vitamin D₃ or a placebo daily for 12 weeks. A sub-set of participants (n73: n54 CA and n19 SA) were used in this analysis to explore the effects on VDBP and 1,25(OH)₂D. Anthropometric measurements and blood samples were taken at baseline, week 6 and week 12. Serum 25OHD was measured using liquid chromatography tandem mass spectrophotometry (ANSciex 5500 tandem mass spectrophotometer and MassChrom^® 25OHD3/D2 kit), serum 1,25(OH)₂D was measured using an in vitro chemiluminescent immunoassay (DiaSorin Liaison XL 1,25-dihydroxyvitamin D), and VDBP was measured using a monoclonal Human VDBP Immunoassay Quantikine ELISA (R&D Systems Europe). Statistical analysis was carried out using IMB SPSS24^® (2016).

There were no significant differences in VDBP or 1,25(OH)₂D between CA or SA women at baseline (p > 0·5), therefore CA and SA were combined for these analyses. The data are presented in Table 1. In a mixed between-within analysis of variance, a significant interaction was found between time and intervention group on 1,25(OH)₂D (p < 0·001, partial eta squared = 0·26). There was a significant main effect of time on 1,25(OH)₂D (p = 0·007, partial eta squared = 0·14). No significant interaction was found between time and intervention group on VDBP (p = 0·295). Within the placebo group, there was no significant interaction between time and baseline vitamin D status (categorised as suboptimal <50 nmol/L or sufficient >50 nmol/L) on VDBP or 1,25(OH)₂D (p = 0·275, p = 0·766 respectively).

Table 1. VDBP and 1,25(OH)₂D concentrations at each visit (mean ± SD)

In conclusion, the 12-week vitamin D supplementation intervention had a significant effect on 1,25(OH)₂D concentrations, with concentrations increasing in those taking vitamin D₂ or D₃. There was no significant effect on VDBP concentrations, However there is some debate as to the reliability of the kit that was used to measure VDBP, which is important to consider⁽Reference Bouillon, Jones and Schoenmakers⁵⁾ and further analysis is certainly warranted.