NEW YORK (Reuters Health) – Low dietary intake of vitamin A and C is associated with an increased risk of asthma and wheeze, according to a report in the July Thorax.

Although epidemiological studies have shown an association between asthma and low antioxidant vitamin intake, the authors explain, controlled trials of vitamin supplementation in asthma have been inconclusive.

Dr. S. Allen and colleagues from University of Nottingham, Nottingham, UK conducted a systematic review and meta-analysis to provide pooled quantitative estimates of the likely magnitude of the effect of dietary intake and blood levels of antioxidant vitamins on a range of measures of asthma and asthma severity.

Low self-reported dietary intake and serum levels of vitamin A and vitamin C were associated with increased odds of having asthma, the authors report.

Patients with severe asthma had lower dietary intakes of vitamin A (but not vitamin C) than did patients with mild asthma.

Low serum levels of vitamin A and low dietary intake of vitamin C were also associated with increased odds of wheeze, the researchers note.

There was no consistent association between vitamin A and C levels and airway reactivity.

Dietary intake and serum levels of vitamin E were not consistently associated with the diagnosis or severity of asthma, the presence of wheeze, or airway reactivity.

“The epidemiological evidence thus suggests that vitamins A and C are linked to asthma,” the authors conclude. “Further investigations are necessary to account for the observed associations using well-designed randomized controlled trials of vitamin supplementation in asthma.”

“Trials of vitamin C supplementation to date have been disappointing,” the investigators say. “Whether the effect of vitamin A will prove more important to clinical management or whether the observed associations with diet are due to confounding effects will only be resolved by further clinical trials.”

Thorax 2009;64:610-619.