Interaction of vitamin C with the relation between smoking and obstructive airways disease in EPIC Norfolk

Previous studies have reported an association between plasma vitamin C levels and respiratory function, but have not examined the role of vitamin C as an effect modifier of the relation between cigarette smoking and obstructive airways disease (OAD). This question was investigated in a cross-sectional analysis of a population-based study of 3,714 males and 4,256 females aged 45-74 yrs. Undiagnosed OAD was defined as a forced expiratory volume in one second (FEV1) of <80% of the predicted value and FEV1/forced vital capacity of <70% without self-reported OAD. An increase of 20 micromol x L(-1) (or 1 SD) in plasma vitamin C concentration was associated with a 13% reduction in the risk of having OAD (adjusted odds ratio (OR) (95% confidence interval) 0.87 (0.77-0.98)). The OR for current smokers relative to never smokers within the lowest quintile of plasma vitamin C concentration was 5.93 (3.03-11.61). The risk in the upper four quintiles was 2.84 (1.92-4.19). Within the lowest quintile of plasma vitamin C concentration, the risk in former smokers was strongly related to the time since quitting (interaction significant, p=0.001). These findings support a protective role for vitamin C against the risk of obstructive airways disease and support the hypothesis that vitamin C may be an effect modifier for the adverse effects of smoking on the risk of obstructive airways disease.