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Higher levels of the vitamin, as well as the antioxidant beta-carotene and trace mineral selenium, were associated with a lower risk of asthma in a large study by Cornell researchers published earlier this year.
The new study by a team at Johns Hopkins University School of Medicine in Baltimore shows that children with asthma tended to have lower blood levels of vitamin C than children with healthy lungs.
But the team cautioned that antioxidant levels may be surrogate markers for other factors such as race, poverty, tobacco exposure, or general nutritional status.
The study, published in the February 15 issue of the American Journal of Epidemiology, is based on data from the Third National Health and Nutrition Examination Survey (NHANES III), conducted in the United States between 1988 and 1994. The authors selected 4,093 children (aged 6-17 years), with 9.7 per cent of these diagnosed with asthma.
Initial analyses showed that asthma was associated with lower levels of serum vitamin C, beta-carotene, and beta-cryptoxanthin. But in logistic models that included age, body mass index, socioeconomic variables, antioxidant levels, parental asthma, and household smoking, the only antioxidants significantly associated with asthma were vitamin C and beta-carotene.
The odds ratio for asthma in the highest quintile of serum vitamin C relative to the lowest was 0.65, whereas it was 0.74 for beta-carotene.
Incidence of asthma in developed countries is rising, thought by some to be triggered by modern lifestyles, including pollution and hygiene. In the UK, which has the highest prevalence of severe wheeze in children aged 13-14 years worldwide, 1.4 million children (one in 8) are currently receiving treatment for asthma, compared to one in 13 adults.
Studies into dietary protection have produced conflicting results. Fruit and vegetable intake, as well as fish fatty acids, have shown some effect on reducing risk of the disease. Researchers in the Netherlands have also found that eating foods containing full milk fat protects against asthma among young children.