Table 1

Trials on vitamin C supplementation and exercise-induced bronchoconstriction

Study		Descriptions
Schachter and Schlesinger20	Methods	Randomised, double-blind, placebo-controlled crossover trial
	Participants	12 asthmatic participants, selected from among workers of Yale University in the USA: “all 12 participants gave a characteristic description of EIB.” All included participants had at least 20% reduction in maximal expiratory flow 40% after exercise 5 Males, 7 females; mean age of 26 years (SD 5 years)
	Type of exercise	Exercise by using a cycloergometer was begun at a constant speed of 20 km/h against a zero workload. At the end of each 1 min interval, the workload was increased by 150 kpm/min, keeping the pedalling speed constant throughout the experiment. Exercise against progressively larger workloads was continued until either the heart rate reached 170 bpm or the participants fatigued
	Intervention	On 2 subsequent days, the participants ingested 0.5 g of vitamin C or sucrose placebo in identical capsules 1.5 h before the exercise. Washout overnight
	Outcome	Change in FEV₁ was calculated as: (preexercise vs 5 min postexercise)
	Notes	See online supplementary file 2 for the calculation of the vitamin C effect from the individual-level data
Cohen et al 21	Methods	Randomised, double-blind, placebo-controlled crossover trial
	Participants	20 asthmatic participants in Israel. All of them had demonstrated EIB by having a ‘decline of at least 15%’ in FEV₁ after a standard exercise test 13 Males, 7 females; mean age of 14 years (range 7–28 years)
	Type of exercise	A 7 min exercise session using a motorised treadmill. Each participant exercised to submaximal effort at a speed and slope to provide 80% of the motional oxygen consumption as adjudged by a pulse oximeter
	Intervention	2 g of vitamin C or placebo 1 h before the exercise. Washout 1 week
	Outcomes	Change in FEV₁ was calculated as: (preexercise vs 8 min postexercise). Secondary outcome: proportion of participants who suffered from EIB after the exercise session (decline in FEV₁ at least 15%)
	Notes	Individual-level data on the FEV₁ levels was reported only for 11 of the 20 participants (Cohen et al, table 2). Dr Cohen was contacted, but he no longer had the data. Therefore, a conservative ‘no vitamin C effect’ was imputed for the 9 participants for whom experimental data were not available; see online supplementary file 2
Tecklenburg et al11	Methods	Randomised, double-blind, placebo-controlled crossover trial
	Participants	8 participants from a population of university students and the local community, Indiana, USA, with physician-diagnosed mild-to-moderate asthma. All participants had documented EIB as indicated by a ‘drop greater than 10%’ in postexercise FEV₁. They also had a history of chest tightness, shortness of breath and intermittent wheezing following exercise. 2 Males, 6 females; mean age of 24.5 years (SD 5 years)
	Type of exercise	Participants ran on a motorised treadmill, elevated by 1% per min until 85% of the age-predicted maximum heart rate and ventilation exceeding 40–60% of the predicted maximum voluntary ventilation. Participants maintained this exercise intensity for 6 min. Following the 6 min steady state exercise, the grade of the treadmill continued to increase at 1% per min until volitional exhaustion
	Intervention	1.5 g vitamin C or sucrose placebo was administered as capsules matched for colour and size daily for 2 weeks. Washout 1 week. Participants were advised to avoid high vitamin C foods during the study
	Outcome	Change in FEV₁ was calculated as: (preexercise vs the lowest value within 30 min postexercise)
	Notes	Dr Tecklenburg kindly made the mean and SD for the paired FEV₁ decline available. For the decline in FEV₁ level, the mean difference was +6.5 percentage points (paired SD 7.4) in favour or vitamin C

EIB, exercise-induced bronchoconstriction; FEV₁, forced expiratory volume in 1 s.