Table 3.

Findings of the included trials

Study	Outcome	P (2-tail) for the difference	Estimate of vitamin C effect (95% CI)	Notes
Anah et al. 1980 [27]	Incidence of all asthma attacks:	0.019	RR = 0.22 (0.06 to 0.81)	All asthma attacks: 9/22 and 35/19 (attacks/persons) in vit C and placebo groups, respectively. See Additional files 2 and 3 for the calculations.
	Incidence of severe and moderate asthma attacks:	0.003	RR = 0.11 (0.02 to 0.48)	Severe and moderate asthma attacks: 3/22 and 23/19 (attacks/persons) in vit C and placebo groups, respectively. See Additional files 2 and 3 for the calculations.
Schertling et al. 1990 [30,31]	Proportion of participants who were sensitive to histamine:	0.0005	52 percentage points decrease (25 to 71)	The P-value was calculated from the discordant observations: 12 were sensitive to histamine in the placebo phase but not in the vit C phase; 0 were sensitive to histamine in the vit C phase but not in the placebo phase. See Additional file 2 for the calculations.
	Asthma symptom score:	0.12	Placebo: 0.72, Vit C: 0.65	Scale 0 to 3; 0 indicates no symptoms. The P-value was calculated by Schertling et al. [30,31].
	PEF:	0.12	Placebo: 400 L/min, Vit C: 409 L/min	The P-value was calculated by Schertling et al. [30,31].
Bucca et al. 1989 [25,26]	Histamine PC20:	0.0003	3.2 fold increase in PC20, (2.0 to 5.1 fold)	Vit C increased histamine PC20 geometric mean from 7.8 to 25.1 mg/ml. See Additional files 2 and 3 for the calculations.
	Interaction between the vitamin C effect and the common cold:	0.003		Interaction test for the vit C effect on PC20 (before/after vit C) and the presence of the common cold (yes/no). See Additional file 2 for the calculations.

Abbreviations:

CI: Confidence Interval.

PC₂₀: Concentration of histamine needed for a 20% FEV₁ decrease.

PEF: Peak Expiratory Flow.

RR: Rate Ratio.