The incidence of type 2 diabetes mellitus in children and adolescents has increased rapidly in recent decades, with insulin resistance a key risk factor. High levels of sedentary time (ST) and low levels of moderate-to-vigorous physical activity (MVPA) and cardiorespiratory fitness (CRF) are associated with higher insulin resistance. It is not well-established, however, how an individual’s body composition mediates these relationships. Furthermore, due to bias often presented when ratio scaling CRF for body weight, there is debate regarding the most appropriate scaling method. This thesis utilised a sample of healthy Finnish 6-8 year olds (Chapter 4, N= 418 (206 girls); Chapter 5, N= 451 (216 girls)) from the Physical Activity and Nutrition in Children study. Chapter 4 sought to contribute to the existing body of knowledge on the relationships between MVPA, ST, body composition, and insulin resistance in children. Furthermore, this chapter aimed to assess the mediating role of body composition in the relationships of MVPA and ST with insulin resistance. Chapter 5 sought to contribute to the existing body of knowledge on the relationships between CRF,
body composition, and insulin resistance in children, and to explore the influence scaling method has on the associations between CRF and insulin resistance. Furthermore, Chapter 5 aimed to assess the mediating role of body composition in the relationships of CRF with insulin resistance. Chapter 4 showed in both girls and boys MVPA was inversely, and ST positively, associated with fat mass and insulin resistance. In boys, ST was positively associated with lean mass. The relationships of MVPA and ST with insulin resistance were mediated by fat mass, but not lean mass in both girls and boys. Chapter 5 showed with appropriately scaled CRF the associations with fat mass and insulin resistance were attenuated, or no longer significant, in comparison to ratio scaled CRF for body weight, such that the mediating role of body composition could not be analysed. Future studies should ensure CRF is appropriately scaled to account for body size and composition. It should be noted, however, that these are cross-sectional associations; Determining the longitudinal mediating role of fat mass in the relationship of MVPA, ST, and CRF with insulin resistance is warranted.