The quality of evidence is downgraded by study limitations (unclear allocation concealment), by inconsistency (variability in results), and by imprecise results (few patients and wide confidence intervals).
A Cochrane review [Abstract] 1 included 11 studies with a total of 419 subjects (10 studies in adults and 1 in children). Eight studies (n=379) included people with asthma and 3 studies (n=40) included subjects with exercise-induced bronchoconstriction. Six studies used regular regimes with differing dosages varying from 1 g (4 studies) and 1.5 g (1 study) to 5 g once daily (1 study) with a duration of treatment ranging from 2 weeks to 6 months.Five studies reported results using single-dose regimes prior to bronchial challenges or exercise tests (dosages were 500 mg in 2 studies and 2 g in 3 studies).
One study with 201 adults with asthma reported no significant difference in health-related quality of life (HRQL). There were no data available to evaluate the effects of vitamin C supplementation on exacerbations in adults. One small study reported data on asthma exacerbations in children and there were no exacerbations in either the vitamin C or placebo groups. In lung function and symptom outcomes there was no significant difference between vitamin C and placebo. No adverse events at all were reported.Studies in exercise-induced bronchoconstriction suggested some improvement in lung function measures with vitamin C supplementation but theses studies were few and very small.
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