Studies have shown increased oxidative stress in patients with chronic airflow limitation (Ochs-Balcom et al 2005) and accumulating evidence suggests that dietary antioxidant vitamins are positively associated with lung function (Schunemann et al 2001), with serum beta-carotene levels being associated with improved FEV-| (Grievink et al 2000). Thus it has been suggested that antioxidant protection is important for protecting the lungs against high oxygen levels and that oxidative stress may contribute to respiratory pathology such as asthma (Rahman et al 2006, Wood et al 2003).
This is supported by the finding of a significant association between serum vitamin C, vitamin E, beta-cryptoxanthin, lutein/zeaxanthin, beta-carotene, and retinol with FEV-, (Schunemann et al 2001), together with a study involving a subset from the Beta-carotene 98
CARET study of 816 asbestos-exposed men with a high rate of current and former
cigarette smoking, which found that serum beta-carotene was associated with a significant improvement in FEV-, and FVC (Chuwers et al 1997).
Studies on the correlation between serum beta-carotene levels and asthma, however, have produced mixed results. One small study of 1 5 asthmatic subjects and 16 healthy controls found that despite similar dietary intake, whole blood levels of total carotenoids, including beta-carotene, lycopene, lutein, beta-cryptoxanthin and alpha-carotene, were significantly lower in the asthmatics with no differences in plasma or sputum carotenoid levels (Wood et al 2005). Another small study found that beta-carotene, alpha-tocopherol and ascorbic acid were significantly lower in asthmatics at remission compared to controls and that beta-carotene was significantly lower and lipid peroxidation products significantly higher during attacks than periods of remission (Kalayci et al 2000). A further small study found that increased dietary consumption of beta-carotene was associated with better QOL (Moreira et al 2004).
These results are supported by an analysis of 7505 youths (4-16 years) from the Third National Health and Nutrition Examination Survey, which found that increased serum beta-carotene was associated with reduced asthma prevalence (Rubin et al 2004). Another analysis involving 4093 children from the same study, 9.7% of whom reported a diagnosis of asthma, found that asthma diagnosis was associated with lower levels of serum beta-carotene, vitamin C, alpha-carotene and beta-cryptoxanthin (Hank-Khan et al 2004).
In contrast to these findings, a much larger study involving 771 persons with self-reported asthma, 352 persons with former asthma and 15,418 persons without asthma, asthma status was not significantly associated with serum antioxidant concentrations (Ford et al 2004). Similarly, in a study of 15 mild asthmatics and 1 5 age-and sex-matched controls, oxidative stress was found to be increased in the asthmatics, with no difference in plasma dietary antioxidant vitamins (Wood et al 2000).
Although the role of antioxidant vitamins in prevention and/or treatment of asthma remains to be determined (Kalayci et al 2000), the results of one intervention study suggest that there may be a role for beta-carotene in exercise-induced asthma. This randomised, double-blind, placebo-controlled trial, involving 38 subjects with proven exercise-induced asthma, found that supplementation with 64 mg/day of natural algal beta-carotene for 1 week protected against post-exercise reduction in FEV, (Neuman etal 1999).
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