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 2,000 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.