Low dietary intakes of vitamins A and C may increase the risk of developing asthma, suggests a review of 40 studies and 30 years of research.
Low blood levels of vitamin C and lower dietary intake of vitamin C-containing foods were associated with a 12 per cent heightened risk of asthma, say findings published online in Thorax.
Our findings from [the current] systematic review and meta-analysis indicate that low levels of vitamin C intake, and to a lesser extent vitamin A, are consistently associated with asthma risk to a degree that, if causal, would be sufficient to be clinically relevant, wrote the researchers, led by Jo Leonardi-Bee from the University of Nottingham in the UK.
According to the European Federation of Allergy and Airway Diseases Patients Association (EFA), over 30m Europeans suffer from asthma, costing Europe 17.7bn every year. The cost due to lost productivity is estimated to be around 9.8bn.
The condition is on the rise in the Western world and the most common long-term condition in the UK today
According to the American Lung Association, almost 20m Americans suffer from asthma. The condition is reported to be responsible for over 14m lost school days in children, while the annual economic cost of asthma is said to be over $16.1bn.
Leonardi-Bee and his co-workers note that the new findings are plausible since vitamin A and C have well-known anti-inflammatory and antioxidant actions. However, the new findings are at odds with a recent Cochrane review concluded that there is no appreciable effect of vitamin C on asthma.
One explanation for this discrepancy would be that the observational data are systematically flawed by biases leading to spurious results from meta-analyses, and particularly publication bias, they said.
The Nottingham researchers searched the literature for peer reviewed research, abstracts of conference proceedings on asthma and wheeze, and vitamin intakes. A total of 40 studies were identified.
The pooled results showed that vitamin A dietary intakes were significantly lower among asthmatics than in those who had not been diagnosed with the disease. The average intake of 182 micrograms of the vitamin was equivalent to between 25 and 33 per cent of the RDI.
The researchers noted that people with severe asthma had significantly lower vitamin A intakes than people with mild asthma.
When Leonardi-Bee and his co-workers considered vitamin C they found low blood levels of the vitamin and low dietary intakes of vitamin-C foods were associated with a 12 per cent increase in the risk of asthma.
With regards vitamin E, intakes were not associated with asthma, but severe asthmatics were found to have significantly lower blood levels than mild asthmatics, and 20 per cent lower than the RDI, said the researchers.
Correlation or causality?
The authors point out that their research does not prove cause and effect, but they suggest that an earlier large review, which found no association between antioxidants and asthma risk, was limited in its scope (Cochrane Database Syst Rev., 2004; (3): CD000993).
Published online ahead of print, doi: 10.1136/thx.2008.101469
Association between antioxidant vitamins and asthma outcome measures: systematic review and meta-analysis
Authors: S. Allen, J.R. Britton, J.A. Leonardi-Bee