Investigating the link between corticosteroids and diet in adults with asthma

Research Doctorate - Doctor of Philosophy (PhD)Asthma is public health problem, affecting adults and children of all ages globally. As there is no cure for asthma, optimum management of the condition is essential through appropriate pharmacotherapy and modification of environmental and lifestyle risk factors. Dietary intake and weight gain are important environmental exposures that may contribute to pathogenesis and progression of asthma, given the proposed association of asthma and obesity and evidence that some dietary nutrients affect asthma outcomes beneficially or adversely. Furthermore it is unknown whether asthma treatment medication, such as oral or inhaled corticosteroids impact upon dietary intake or cause weight gain and whether biological markers such as leptin are important in this association. Further investigation into these risk factors is required to improve asthma management. The current thesis aims to describe the dietary intake of adults with asthma; to investigate the association of dietary nutrient intake with corticosteroids and asthma outcomes; and to examine whether oral corticosteroids affect appetite, dietary intake, body weight and body composition in adults with asthma. Chapter 2 presents the current evidence for the effect of oral corticosteroids on appetite, dietary intake, body weight and body composition. In this systematic literature review we found it was unlikely that oral corticosteroids have a significant effect on dietary intake, appetite or body weight and composition when used to treat inflammatory conditions or in healthy subjects. There is limited objective evidence for the association of oral corticosteroids and these adverse effects, particularly with respect to short term oral corticosteroid therapy in inflammatory conditions, including asthma. In Chapter 3 the dietary intake of adults with asthma was examined; compared to healthy subjects; and evaluated for associations with biological markers and asthma outcomes. Subjects with severe asthma were found to have a different dietary intake to healthy subjects, with an increased intake of fat and sodium and decreased intake of fibre and potassium. In all subjects with asthma increased fat and decreased fibre intake were associated with greater airway inflammation and poorer lung function. This finding suggests that these nutrients may be important to address in nutritional counselling of adults with asthma. There was no relationship between dietary intake and inhaled or oral corticosteroids. Serum leptin was increased in subjects with asthma compared to healthy controls, and while it was not directly related to dietary intake, it was associated with inhaled corticosteroid use in males. This finding is an indication that leptin may be important to investigate further in the relationship between appetite and corticosteroid use in asthma. Chapter 4 explored the effects of a short term oral corticosteroid intervention on appetite, dietary intake, leptin, body weight and body composition in adults with asthma. This 10 day, randomised, placebo-controlled cross-over study was designed to emulate therapy for a moderate to severe asthma exacerbation and assess outcomes objectively. The intervention did not increase leptin, appetite or dietary intake, and had no significant adverse effects on body weight or body composition. The study highlighted the need for management of adverse effects such as sleep and gastrointestinal disturbance during oral corticosteroid therapy. The research conducted as part of this thesis has contributed to the understanding of the relationship between diet and asthma; found key nutrients to target in dietary interventions in asthma; and provided new evidence on the adverse effects of oral corticosteroid use in asthma