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A low intake of vitamins A and C may increase the risk of developing asthma, according to 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 % increased risk of asthma. “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 Dr. Jo Leonardi-Bee from the University of Nottingham in the UK.
Dr. 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 % of the RDI. The researchers noted that people with severe asthma had significantly lower vitamin A intakes than people with mild asthma.
When Dr. 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 % 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 % lower than the RDI, said the researchers. The study is published online ahead of print in the journal Thorax.