A relatively low dietary intake of vitamins A and C boosts the risk of asthma, suggests a systematic analysis of the available evidence, published ahead of print in the journal Thorax.

These findings clash with a large review of the evidence, which was published last year.

Observational studies in recent years have pointed to a link between dietary antioxidant vitamins (A,C, and E) and asthma, but the results of clinical trials have proved inconclusive, claim the authors.

In a bid to try and clear up the confusion, they conducted a comprehensive search of major databases of peer reviewed research, abstracts of conference proceedings on asthma and wheeze, and bibliographies from 1980 to the end of 2007.

They found 40 relevant studies, which had looked at the association between vitamin intake and asthma and wheezing, out of more than 2,600.

The pooled results showed that dietary intake of vitamin A was significantly lower among those with asthma than in those who had not been diagnosed with the disease.

The average intake of 182 ug a day was equivalent to between a quarter and a third of the recommended daily amount of vitamin A.

Vitamin A intake was also significantly lower - equivalent to around half of the recommended daily amount - in those with severe disease than in those with mild asthma.

Low circulating levels of vitamin C in the bloodstream and lower dietary intake of foods containing vitamin C were also associated with a 12% heightened risk of asthma.

Vitamin E intake was not associated with a diagnosis of asthma, but blood levels were significantly lower among those with severe disease compared with those whose symptoms were mild.

This translates into an intake that is 2 mg/day or 20% lower than the recommended daily amount of vitamin E.

The findings for a link between dietary antioxidants and wheeze were less consistent.