Vitamin C reduces exercise induced asthma aka broncho constriction or the decline of 10% or greater forced exercise volume within 1 second (FEV1). Dr. Harri Hemila from the University of Helsinki, Finland did a meta-analysis of Vitamin C studies and found that exercise induced wheezing, coughing and shortness of breath, known exercise induced asthma can be reduced by taking Vitamin C at a minimum dose of 200mg.
About 10% of the general population suffers from exercise-induced bronchoconstriction, but among some fields of competitive winter sports the prevalence can be up to 50%. In sports such as marathon running and skiing there is a 48% reduction in FEV1.
Vitamin C also halves the incidence of common cold episodes in people enduring heavy short-term physical stress.
Dr. Hemila concludes that given the low cost and safety of vitamin C and the consistency of positive findings in three randomized trials on EIB, it seems reasonable for physically active people to test vitamin C on an individual basis if they have respiratory symptoms such as cough associated with exercise.
Vitamin C may alleviate exercise-induced bronchoconstriction: a meta-analysis. Harri Hemilä. BMJ Open 2013;3:6 e002416 doi:10.1136/bmjopen-2012-002416