Serum Vitamin D: Correlates of Baseline Concentration and Response to Supplementation in VITAL-DKD.

Context: The effect of daily vitamin D supplementation on the serum concentration of vitamin D (the parent compound) may offer insight into vitamin D disposition.

Objective: To assess the total serum vitamin D response to vitamin D3 supplementation and whether it varies according to participant characteristics. To compare results with corresponding results for total serum 25-hydroxyvitamin D [25(OH)D], which is used clinically and measured in supplementation trials.

Design: Exploratory study within a randomized trial.

Intervention: 2000 International Units of vitamin D3 per day (or matching placebo).

Setting: Community-based.

Participants: 161 adults (mean ± SD age 70 ± 6 years; 66% males) with type 2 diabetes.

Main Outcome Measures: Changes in total serum vitamin D and total serum 25(OH)D concentrations from baseline to year 2.

Results: At baseline, there was a positive, nonlinear relation between total serum vitamin D and total serum 25(OH)D concentrations. Adjusted effects of supplementation were a 29.2 (95% CI: 24.3, 34.1) nmol/L increase in serum vitamin D and a 33.4 (95% CI: 27.7, 39.2) nmol/L increase in serum 25(OH)D. Among those with baseline 25(OH)D < 50 compared with ≥ 50 nmol/L, the serum vitamin D response to supplementation was attenuated (15.7 vs 31.2 nmol/L; interaction P-value = 0.02), whereas the serum 25(OH)D response was augmented (47.9 vs 30.7 nmol/L; interaction P-value = 0.05).

Conclusions: Vitamin D3 supplementation increases total serum vitamin D and 25(OH)D concentrations with variation according to baseline 25(OH)D, which suggests that 25-hydroxylation of vitamin D3 is more efficient when serum 25(OH)D concentration is low.