Vitamin C should be tested against exercise-induced bronchoconstriction.

An ATS Clinical Practice Guideline reviewed treatments for exercise-induced bronchoconstriction (EIB) and commented briefly on vitamin C (1). The authors identified two controlled trials (2, 3) in which vitamin C halved the fall in FEV1 after exercise, but the authors did not calculate confidence intervals (CIs) for the effect. A third randomized, double-blind, placebo-controlled trial on vitamin C and EIB has also been published (4) and was included in a metaanalysis (5). The pooled relative effect estimate of three trials (2–4) indicated a 48% reduction (95% CI, 33 to 64%; P , 0.001) in the postexercise FEV1 decline when vitamin C was administered before exercise (5). The third study (4) needed imputations to include it in the metaanalysis, but it also reported that vitamin C decreased the proportion of participants who suffered from EIB by 50 percentage points (95% CI, 23 to 68; P , 0.001); this calculation did not need data imputations (5). The ATS Guideline comments that the evidence for vitamin C was limited by imprecision (1). The CIs calculated in the metaanalysis are, however, particularly narrow (5). The total number of participants in the three vitamin C trials is only 40. Nevertheless, the trials were performed in three different decades and on two different continents. The criteria for EIB differed, and the mean age of the participants was 25 and 26 years in two studies (2, 3) but 14 years in the third study (4). Still, all of the studies are consistent with a 50% reduction in the fall in FEV1 after exercise. It is not evident how far this 50% estimate can be generalized, but the close estimate in such different studies suggests that vitamin C is probably beneficial for several people who suffer from EIB. The ATSGuideline considers that the burden of dietary modification might limit the usefulness of vitamin C (1). However, two of the vitamin C studies administered the vitamin as a single dose 1 to 1.5 hours before exercise (2, 4), which is no more burdensome than administering a b2-agonist before exercise (1). Further research on the effect of vitamin C on EIB should be performed. Nevertheless, given the safety and low cost of vitamin C, and the positive findings in the three EIB studies, it seems reasonable for physically active people to test vitamin C if they suffer from EIB (5).