Association between vitamin D and respiratory outcomes in Canadian adolescents and adults.

Background: Asthma is one of the most prevalent chronic diseases worldwide, affecting more than 200 million people. Vitamin D deficiency has been reported among individuals with asthma and might play a role in asthma exacerbations. In this cross-sectional study, we investigated the association of serum 25-hydroxy vitamin D [25(OH)D] levels and current asthma, ever asthma, and lung function. Methods: Data from 3937 subjects aged 13-69 years who participated in the Canadian Health Measures Survey - Cycle 1 were considered in this study. Serum 25(OH) D levels were categorized into <= 49 nmol/L (low), 50-74 nmol/L (moderate) and >= 75 nmol/L (high). Results: The proportion of subjects with current and ever asthma was greater in the lower 25(OH) D category than in moderate and high categories. After adjusting for potential confounders, subjects in the low 25(OH) D levels were more likely to have current asthma than those in the moderate levels (OR: 1.54, 95% CI: 1.01-2.36). Low 25(OH) D levels were also associated with ever asthma (OR: 2.12, 95% CI: 1.40-3.21) among those with a family history of asthma and this association was stronger in those with asthma onset before 20 years of age. High 25(OH) D levels were associated with lower mean value of FEV1/FVC ratio. No significant association was observed between 25(OH) D levels and other lung function measurements. Conclusion: In this study, 25(OH) D levels below 50 nmol/L were associated with an increased risk of current and ever asthma. Further exploration of this relationship is needed to determine the optimal level of vitamin D in the management of asthma in adolescents and adults.