Background/objectives: Vitamin D modifies airway inflammation and appears to be important in building immunity against respiratory infections, and so is potentially beneficial in asthma. However, to date results from studies investigating the relationship of vitamin D with respiratory outcomes are inconsistent. The objective of this study was to determine the association of vitamin D with current wheeze, ever asthma and lung function.
Subjects/methods: Data on 1213 children aged 6-12 years, from the Canadian Health Measures Survey, were considered for the study. Serum 25-hydroxy vitamin D (25(OH)D) levels were categorized into three categories: ⩽49 nmol/l (low); 50-74 nmol/l (moderate); and ⩾75 nmol/l (high).
Results: Children in the low and high 25(OH)D categories were more likely to report current wheeze in comparison to those in the moderate category (odds ratio (OR): 3.26, 95% confidence interval (CI): 1.16-9.17 and OR: 2.14, 95% CI: 1.07-4.28, respectively). The results for the association between 25(OH)D levels and ever asthma were similar to those observed for current wheeze, except that a significant association was observed only in the lower category. The rate of increase in forced expiratory volume in one second and forced vital capacity with age was greater in the moderate category.
Conclusions: Children in both low and high 25(OH)D categories had increased risk of current wheeze and reduced rate of change in lung function with age in comparison to the moderate category, raising the possibility of a U-shaped association between vitamin D levels and respiratory health. Clearer guidelines for optimal vitamin D levels are required to improve respiratory health in children.