The effect of vitamin C on bronchoconstriction and respiratory symptoms caused by exercise: a review and statistical analysis

Physical activity increases oxidative stress and therefore the antioxidant effects of vitamin C administration might become evident in people undertaking vigorous exercise. Vitamin C is involved in the metabolism of histamine, prostaglandins, and cysteinyl leukotrienes, all of which appear to be mediators in the pathogenesis of exercise-induced bronchoconstriction (EIB). Three studies assessing the effect of vitamin C on patients with EIB were subjected to a meta-analysis and revealed that vitamin C reduced postexercise FEV₁ decline by 48% (95% CI: 33% to 64%). The correlation between postexercise FEV₁ decline and respiratory symptoms associated with exercise is poor, yet symptoms are the most relevant to patients. Five other studies examined subjects who were under short-term, heavy physical stress and revealed that vitamin C reduced the incidence of respiratory symptoms by 52% (95% CI: 36% to 65%). Another trial reported that vitamin C halved the duration of the respiratory symptoms in male adolescent competitive swimmers. Although FEV₁ is the standard outcome for assessing EIB, other outcomes may provide additional information. In particular, the mean postexercise decline of FEF₅₀ is twice the decline of FEV₁. Schachter and Schlesinger (1982) reported the effect of vitamin C on exercise-induced FEF₆₀ levels in 12 patients suffering from EIB and their data are analyzed in this paper. The postexercise FEF₆₀ decline was greater than 60% for five participants and such a dramatic decline indicates that the absolute postexercise FEF₆₀ level becomes an important outcome in its own right. Vitamin C increased postexercise FEF₆₀ levels by 50% to 150% in those five participants, but had no significant effect in the other seven participants. Thus, future research on the effects of vitamin C on EIB should not be restricted to measuring only FEV₁. Vitamin C is inexpensive and safe, and further study on those people who have EIB or respiratory symptoms associated with exercise is warranted.