Association of free vitamin D concentrations and asthma treatment failures in the VIDA Trial.

BACKGROUND

Use of vitamin D serum concentrations as a biomarker of vitamin D status is questionable because of variation in vitamin D binding protein.

OBJECTIVE

To determine associations between free vitamin D3 concentrations and rates of treatment failure and exacerbations in patients with asthma participating in the Vitamin D Add-on Therapy Enhances Corticosteroid Responsiveness in Asthma (VIDA) trial.

METHODS

Free concentrations were directly measured by enzyme-linked immunosorbent assay and stratified into low, medium, and high groups: less than 5pg/mL (n = 65), 5 to 9pg/mL (n = 84), and greater than 9pg/mL (n = 48) after 12 weeks of supplementation with oral vitamin D3 and associated with outcomes.

RESULTS

Outcomes did not associate with free concentrations: overall treatment failure rates were 0.60 (95% confidence interval [CI] 0.46-0.78), 0.53 (95%CI 0.40- 0.70), and 0.69 (95%CI 0.54-0.90)/person-year (P = .51), respectively; overall exacerbation rates were 0.28 (95%CI 0.17-0.48), 0.15 (95%CI 0.08-0.30) and 0.42 (95%CI 0.27-0.66)/person-year (P = .22). Mean (standard deviation) baseline free concentrations were lower in non-Hispanic blacks and Hispanics compared with non-Hispanic whites: 4.10 (1.33) and 4.38 (1.11) pg/mL vs 5.16 (1.65) pg/ml, (P < .001 and P = 0.038), respectively. Mean (standard deviation) baseline free concentrations differed between females and males: 4.57 (1.58) and 5.08 (1.41) (P = .026); and between non-overweight (body mass index [BMI] < 25) and overweight (BMI > 25): 5.45 (1.86) vs 4.54 (1.39) (P < .001). The free fraction differed by race and sex but not by BMI.

CONCLUSION

The use of free concentrations was inferior to total concentrations as a biomarker of efficacy of vitamin D supplementation in VIDA trial participants. Future studies of vitamin D status in patients with asthma should measure both free and total concentrations to better understand which marker of vitamin D function is most informative.