Low vitamin A and C levels may boost asthma risk

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.

The researchers note that the new findings are plausible since vitamin A and C have well-known anti-inflammatory and antioxidant actions. 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).

Steve - we posted this because it shows how meta-analyses can be interpreted differently, based upon the researchers motivations. The Thorax review was positive, yet the Cochrane Review was not. Using meta analyses are not the ideal way to make definitive conclusions.