Obesity, vitamin D insufficiency, and asthma in Norwegian and Canadian populations

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Background: Obesity, vitamin D insufficiency and asthma are uprising and serious public health problems. Vitamin D is postulated to play a role in asthma development due to its regulating effect on the immune system. However, previous studies show quite c onflicting results. Obesity has been reported to be associated with both vitamin D insufficiency and asthma. Nevertheless, the interrelationship between obesity, vitamin D insufficiency and asthma has not systematically been studied in a prospective setti ng. The primary objective of this project is to investigate serum 25(OH)D levels in relation to incident asthma, prevalent asthma and asthma severity/control. The secondary objectives of this project: a) To investigate the predictive effect of obesity o n asthma incidence and severity/control; b) To explore if vitamin D insufficiency is a plausible explanation for obesity-asthma association. Methods: Two populations are included in the project, i.e. a defined prospective cohort Norwegian population (n = 25500) and a cross-sectional Canadian population (n = 5000). Of the cohort Norwegian population, a nested case-control study including all new asthma cases (n = 700) and random non-asthma controls (n = 1100) will be carried out to evaluate the relationsh ip between serum 25(OH)D levels and incident asthma. In the project, there are two main exposure variables, i.e. obesity expressed by measured body mass index and waist circumference, and serum 25(OH)D levels which will be measured by HPLC assays. There a re three main outcome variables including incident asthma, prevalent asthma and asthma severity/control. Implication: It will provide high scientific value of enhancing our understanding about the interrelationship between obesity, vitamin D insufficienc y and asthma. If the relationships are found to be true, we will have a potentially cheap, safe and easy means of control and prevention for asthma, particularly in obesity-related difficult asthma.