Table 1 Characteristics of the included trials

Anah et al. 1980[27]

Participants

41 asthmatic subjects attending an asthma clinic in Nigeria. All had had asthma for at least 4 yrs. The participants had histories of increased asthma attacks during the rainy season. In all cases their attacks were precipitated by respiratory infections, which started with a sore throat and a dry cough. The trial was conducted during the rainy season. Patients with bronchitis were excluded. 22 M, 19 F; age 15 to 46 y (mean 27 y); 22 vit C 19 placebo.

Duration

14 wk

Intervention

1 g/d vit C or a placebo for 14 wk.

Outcome

Frequency of asthma attacks. “Severe attacks” indicate those that needed emergency attendance at the hospital; “moderate attacks” those that necessitated the use of inhalers more frequently, and “mild attacks” those that caused some increase in wheezing and breathlessness.

Notes

See calculations in Additional files2 and3.

Schertling et al. 1990[30, 31]

Participants

29 Participants with a diagnosis of infection-related asthma in former East-Germany. Patients with acute and serious purulent infections were excluded. 18 M, 11 F; age 18 to 60 y.

Duration

Total duration 5 wk, composed of 2 periods of 2 wk intervention and a 1 wk washout between them.

Intervention

5 g/d vit C or placebo for 1 wk before the histamine sensitivity test in the middle of the 2 wk intervention. Washout 1 wk between the 2-wk intervention phases.

Outcomes

1) Sensitivity to histamine: positive result indicates that exposure to <1 μmol histamine increased respiratory tract resistance by 50%. 2) Asthma symptom score, 3) PEF

Notes

See calculations in Additional file2. The histamine sensitivity data are reported for 23 participants. There is no description for the missing data.

Bucca et al. 1989[25, 26]

Participants

9 members of hospital staff in Italy with a negative history of asthma and atopy. All suffered from the common cold with cough on the first vit C test day, and all had recovered on the second vit C test day 6 wk later. 5 M, 4 F; age 18 to 48 y (mean 29 y).

Duration

Two study days separated by 6 wk.

Intervention

Single dose 2 g of vit C.

Outcome

PC₂₀ was measured at baseline and 1 h after vit C administration on both study days.

Notes

See calculations in Additional files2 and3.