Aim: The study aims to assess the a priori hypothesis that regular supplementation with vitamin C or magnesium will permit a reduction in the corticosteroid dose required to maintain asthma control in adults.
Methods: We invited all participants recruited from primary care centres who completed a parallel-group, randomised, placebo-controlled, 16-week supplementation trial of 1g/day vitamin C or 450 mg/day magnesium to continue and participate in a structured corticosteroid reduction protocol over 10 weeks.
Results: A total of 92 participants (29 vitamin C, 31 magnesium and 32 placebo) entered the study. Assuming no reduction in corticosteroid dose in the 10 who subsequently withdrew, the geometric mean reductions in inhaled corticosteroid dose achieved with vitamin C, magnesium and placebo were 49, 13 and 11 microg, respectively. Relative to placebo, the unadjusted effect of vitamin C was significant, and remained at borderline significance after adjustment for baseline corticosteroid dose (relative reduction ratio=4.03, 95% CI 0.95 to 17.1, P=0.06).
Conclusions: We conclude that while vitamin C supplements may have modest corticosteroid sparing effects and hence the potential to reduce exposure to their side effects, magnesium supplements have no effect on the inhaled corticosteroid dose required to maintain asthma control.