Table 3 Findings of the included trials

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 files2 and3 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 files2 and3 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 file2 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 PC_20, (2.0 to 5.1 fold)

Vit C increased histamine PC₂₀ geometric mean from 7.8 to 25.1 mg/ml. See Additional files2 and3 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 file2 for the calculations.