Observational and mechanistic data suggest a role for vitamin D in childhood asthma. However, subsequent interventional trials have been inconsistent. We aimed to assess the effect of 15 weeks of vitamin D3 supplementation compared to placebo in Irish asthmatic children. We conducted a double-blind, randomized, placebo-controlled trial of vitamin D supplementation (2,000IU/d) in 44 urban, Caucasian children at high latitude. Assessments were completed at baseline and after 15 weeks of supplementation. Outcome measures lung function, subjective asthma control and biochemical parameters of total vitamin D, allergy, immunity, airway inflammation and systemic inflammation. Finally, parents/gaurdians completed a weekly diary during the trial. There was no significant difference in baseline 25(OH)D levels but there was a significant increase in median 25(OH)D in the vitamin D3 group (57.5 to 105nmol/L) compared to the placebo group (52.5 to 57.5nmol/L) (p <0.0001). There was no significant difference between groups regarding subjective asthma control. Compared to placebo, there was as a significant decrease in school days missed due to asthma (1 vs. 5 days, p = 0.04) and alkaline phosphatase (-3.4 vs. +16; p = 0.037) in the vitamin D3 group but there were no beneficial effects regarding several other secondary endpoints. However, there were non-significant, advantageous changes in the placebo group compared to the vitamin D3 group in subjective asthma control and lung function, particularly percentage of predicted forced expiratory volume in 1 second (+2.5 vs. -4; p = 0.06). Vitamin D3 supplementation led to a significant increase in serum 25(OH)D and decreased school days missed (p = 0.04) but no other advantageous changes in asthma parameters compared to placebo. The potential adverse effect of vitamin D deficiency on growth and the potential negative effect of high serum 25(OH)D on pulmonary function warrant further investigation. This article is protected by copyright. All rights reserved.
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