It’s always a challenge to take a single, isolated nutrient and try to prove a health benefit within a research study. Unlike drugs, which mostly have a clear mode of action on their own, nutrients generally usually work synergistically with other nutrients and lifestyle factors to generate health benefits. So when a meta-analysis (review of multiple studies) of one vitamin all show a similar clinical outcome, it is a significant finding and offers some clarity on the use of a nutrient in isolation as well as in combination with others.
A 2014 meta-analysis of nine studies on vitamin C for lung health (specifically exercise-induced bronchoconstriction or EIB) showed a positive correlation between vitamin C and lung health. The studies focused on EIB, a specific condition when the airways narrow during vigorous exercise, and are more common when the air temperature is low. It’s a relatively common event in people with asthma and endurance athletes.
The test to determine exercise-induced bronchoconstriction evaluates “forced expiratory volume” (FEV1), measuring the amount of air the lungs can exhale. When the FEV1 is decreased by 10% or more, it is determined to qualify as bronchoconstriction. In this review it was found that vitamin C doses as modest as 200 mg daily (and up to 1,500 mg) consistently reduced the decline of FEV1 and supported healthy breathing. The scientists reviewing the studies concluded that vitamin C supplementation is worth exploring for physically-active people to support healthy lung function in situations where they have EIB or for general lung improvement – albeit that this benefit has yet to be established.
 Hemila H. The effect of vitamin C on bronchoconstriction and respiratory symptoms caused by exercise: a review and statistical analysis. Allergy, Asthma & Clinical Immunology 2014, 10:58 View Full Paper