Understanding low childhood muscular fitness as a risk factor for impaired glucose homeostasis and type 2 diabetes in adulthood

Background: Physical activity guidelines have been updated to recognise the importance of children participating in muscle-strengthening activities in addition to aerobic activities. These changes reflect a growing evidence base highlighting the health benefits of childhood muscular fitness, with an emerging area of research examining whether these health benefits span childhood to adulthood. As interest into childhood muscular fitness grows, how levels have changed over time and what factors determine higher levels is unclear. By better understanding these components of muscular fitness, a greater insight into childhood muscular fitness as a risk factor for future health outcomes can be gained. Also unknown, is whether low childhood muscular fitness is a risk factor for type 2 diabetes in later life. Aims: This thesis aimed to firstly gain a better understanding of childhood muscular fitness by quantifying the 30-year secular changes in childhood muscular fitness levels and identifying factors associated with childhood muscular fitness phenotypes. Secondly, this thesis aimed to determine if childhood muscular fitness was associated with measures of glucose homeostasis in young-adulthood and explore if these associations persist despite changes in muscular fitness into young-adulthood. Factors associated with the persistence or change in muscular fitness between childhood and young-adulthood were identified and the association between muscular fitness measured across the life course (childhood, young- and mid-adulthood) and prediabetes and type 2 diabetes in mid-adulthood was explored. Methods: This thesis used data from the Childhood Determinants of Adult Health (CDAH) Study and the Child Health CheckPoint Study (CheckPoint), two Australian-based cohort studies that assessed the muscular fitness of Australian children. Baseline muscular fitness data, in addition to a range of modifiable and environmental factors, for the CDAH Study were collected for children aged 7‚Äö-15 years in 1985. In 2015, the CheckPoint study examined the health and muscular fitness of Australian children aged 11‚Äö-12 years. Therefore, muscular fitness data on two broadly representative samples of Australian children were available 30- years apart. As part of the CDAH Study, participants were followed up in young- (aged 26‚Äö-36 years) and mid-adulthood (36‚Äö-49 years) where muscular fitness was remeasured, and cardiometabolic risk factor information was collected including a fasting blood sample tested for fasting glucose, glycated haemoglobin (HbA1c) and insulin. Results: A better understanding of childhood muscular fitness was gained by identifying that contemporary Australian children had lower muscular fitness compared with their counterparts from 30-years ago, independent of changes in body mass, and that adiposity, cardiorespiratory fitness, fat-free mass, flexibility and speed capability were determinants of muscular fitness preceding the secular decline. Higher childhood muscular fitness was associated with lower levels of measures of glucose homeostasis in young-adulthood and males with persistently high muscular fitness between childhood and young-adulthood had the most favourable glucose homeostasis profile in young-adulthood. Strategies aimed at obesity prevention and promoting physical activity, cardiorespiratory fitness, healthy dietary choices and increased education could have additional benefits of maintaining or improving muscular fitness from childhood to adulthood. Lastly, muscular fitness measured in childhood, young-adulthood and mid-adulthood contributed approximately equally to prediabetes and type 2 diabetes risk in midadulthood. Discussion: These findings suggest that given childhood muscular fitness levels have declined over the past 30-years and that low childhood muscular fitness is a risk factor for impaired glucose homeostasis in adulthood, more contemporary children could be at risk of developing impaired glucose homeostasis in later life. Risk reduction strategies are therefore essential, and childhood muscular fitness levels could be a potential target. Furthermore, as cumulative exposure to higher levels of muscular fitness across the life course was associated with less impairments in measures of glucose homeostasis and lower odds of developing prediabetes and type 2 diabetes in adulthood, strategies aimed at increasing muscular fitness could be implemented in childhood and maintained across the life course to reduce future development of type 2 diabetes.