Vitamin D binding protein is not affected by high-dose vitamin D supplementation: a post hoc analysis of a randomised, placebo-controlled study

OBJECTIVES: Vitamin D binding protein (VDBP) is the main transporter of 25-hydroxyvitamin D(3) (25-OHD) in the circulation. The aim of this study was to investigate if VDBP is affected by high dose vitamin D supplementation and if VDBP-levels correlate with free 25-OHD. Correlation between free 25-OHD measured with ELISA and total 25-OHD in the circulation was also analysed. Plasma samples from a randomized, controlled trial in which persistent MRSA-carriers were randomized to treatment with vitamin D, 4000 IE/day, (n = 27) or placebo (n = 32) for 12 months were used. Plasma from baseline and after 6 months of treatment were analysed for VDBP, 25-OHD and free 25-OHD. RESULTS: VDBP levels were not affected by vitamin D treatment, although the 25-OHD levels increased significantly in the vitamin D treated subjects. There was a strong correlation between 25-OHD and free 25-OHD (r(2) = 0.68, p < 0.0001), while there was no correlation between VDBP and free 25-OHD. Thus, our data shows that VDBP are not affected by vitamin D supplementation and the levels of VDBP are not associated with the free fraction of 25-OHD. Since there was a strong correlation between free 25-OHD and total 25-OHD it appears to be sufficient to measure only total 25-OHD. Trial registration http://www.clinicaltrials.gov; NCT02178488. Date of registration: June 30, 2014; Date of enrolment of the first participant: Dec 1, 2014