Advancing the evidence base for nutritional management of boys with Duchenne muscular dystrophy

Abstract

Duchenne muscular dystrophy (DMD) is the most common and the most severe of the neuromuscular disorders. Affecting 1 in 3,500 boys, the disease causes progressive and relentless muscle weakness, limiting life to the second or third decade. The mainstay of medical management involves the use of glucocorticosteroids which has significantly altered the natural history of the disease. The nutritional issues associated with DMD are complex, yet poorly understood. To date, these issues have not been an important priority of care; but as life expectancy slowly inches forward, such concerns warrant immediate attention. This thesis aims to progress the understanding and management of nutritional issues for boys with DMD by advancing the evidence base. Five distinct lines of investigation were undertaken under this broader theme. Firstly, a comprehensive literature review highlighted many gaps in the literature specifically in the areas of energy requirements, bone health, micronutrient requirements (vitamin D), growth monitoring, and use of novel amino acids and nutritional supplements. Secondly, a retrospective case series documented the genetic, physical and clinical profile of boys with DMD living in Australia, and for the first time a relationship between genotype and function was elucidated; boys with a deletion in the dystrophin gene are more likely to stop walking before age 10 years. Whilst the relationship between growth and walking ability was unclear, a higher body mass index (BMI) may be protective against a decline in lung function. This analysis also provides evidence for commencing steroid treatment earlier, as a longer duration of steroid treatment was associated with walking past age 10 years, and increased lung function. Next, vitamin D status in boys with DMD was explored. Young, ambulatory boys with DMD who attend regular school were found to have low serum 25-hydroxyvitamin D when compared to their healthy siblings, a finding that was not explained by differences in sun exposure. Also, a series of meta-analyses demonstrated that steroid therapy is associated with suboptimal serum vitamin D concentrations, and may potentially explain the observed reduction in 25-hydroxyvitamin D. Based on data obtained by meta-analysis, recommendations for vitamin D supplementation should, at a minimum, be 1800 IU (45 µg)/d for boys with DMD receiving steroid therapy. Chapter four presents data and literature to support the use of selected outcome measures for dietary trials in DMD, specifically: StepWatch™ activity monitoring, the six minute walk test, and a three day food diary. These functional outcome measures were found to be sensitive to differences in ambulatory capacity in boys with DMD when compared to control participants. Furthermore, self-reported energy intake using food diaries was similar to total energy expenditure as determined by doubly labelled water in boys with DMD. Finally, a pilot randomised control trial is described in which a novel nutritional supplement incorporating whey protein, creatine monohydrate, glutamine and β-hydroxy-β-methylbutyrate is investigated. Results indicate that a 50 week nutriceutical intervention is feasible, and may improve functional outcome measures when used in conjunction with steroid treatment. This thesis comprises of several smaller studies that are linked in a framework of empirical research. Firstly, the evidence base for nutritional management of boys with DMD, and their nutritional status across two major paediatric clinic populations nationally is examined. The thesis then further explores confounding factors of nutritional status and determines methodology to measure these before piloting a novel nutriceutical intervention informed by the best available evidence. This body of work, with its purely nutritional focus, is the first of its kind, and will assist dietitians to provide evidence based care to this courageous group of children.