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

We found that vitamin D deficiency was very common in Norwegian young to middle-aged adults (J Epidemiol Community Health 2014). Winter season and obesity were the two most important determinants of vitamin D deficiency, followed by current smoking. Cod liver oil intake and physical activity were associated with a better serum levels of vitamin D. Regarding the relationship between vitamin D and incident asthma, we observed that low vitamin D status may have played an important role in increasing the risk of asthma among a subgroup of men without allergy, who seemed to have a severe type of asthma (Am J Epidemiol 2013). The relationship of vitamin D with lung function and lung function changes was evaluated in a general population of Canadian and Norwegian adults respectively (Am J Clin Nutr 2013 and Eur Respir J 2015). The two studies demonstrated that low vitamin D level was associated with lung dysfunction, especially for overweight or obese individuals and smokers. We are also one of the few to investigate the association of vitamin D with lung function and lung function changes in asthma patients (Eur Respir J 2015 and Am J Epidemiol 2016). In addition, low vitamin D status was a risk factor for obesity development in young to middle-aged adults (Am J Epidemiol 2012). There seems to be a bidirectional association between vitamin D and obesity. Obesity leads to low serum vitamin D levels and low serum vitamin D levels were associated with development of obesity. We also found that general obesity (reflected by body mass index) increased the risk of asthma in both women and men. In women, abdominal obesity (reflected by waist circumference) was another significant risk factor for asthma development in addition to general obesity (Eur Respir J 2013). Obesity is one of the components to define metabolic syndrome. Metabolic syndrome was significantly associated with an increased risk of asthma in adults (Eur Respir J 2013), and obesity and elevated circulating glucose levels seemed to play more important roles in increasing the risk than other metabolic components. Further, adults with obesity and also mental distress had a substantially increased risk of asthma compared with adults with obesity or mental distress alone (Int J Epidemiol 2013).

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.