BACKGROUND: Brown adipose tissue (BAT) is a thermoregulatory tissue that may have a positive influence on metabolic health by improving glucose homeostasis, reducing adiposity and increasing energy expenditure. It is enriched with uncoupling protein-1 (UCP1) and therefore produces heat by uncoupling oxidative phosphorylation from ATP production. It has long been known that infants are born with BAT, however, only recently has BAT been reported in children and adults. In humans, BAT is predominantly located in the supraclavicular (SCV) region, however there are smaller depots in the peri-renal and mediastinal areas. BAT has primarily been studied in humans using Positron Emission Tomography/Computed Tomography (PET/CT). Such studies have demonstrated that children appear to have a higher prevalence of BAT than adults, however this methodology is not suitable for widespread research in healthy children. Therefore non-invasive methods that accurately measure BAT are required. The factors influencing BAT activity are of interest as this tissue may act as a desirable therapeutic target for metabolic diseases such as obesity and diabetes. PURPOSE: The purpose of this thesis project was two fold; Part 1 involved the examination of SCV skin temperature as a measure of BAT activity in children and the objective of Part 2 was to determine if SCV skin temperature had a relationship to body composition and physical activity in children. METHODS: We recruited 33 pre-pubertal boys (ages 8-10) to this cross-sectional study. The study included 3 visits, in which we measured lean mass and fat mass with dual-energy X-ray absorptiometry, resting energy expenditure (REE) before and after a 30 minute 12ºC cold exposure with indirect calorimetry, objective physical activity with an accelerometer and SCV temperature (measured every 2.5 minutes during a 30 minute, 12ºC cold exposure) with an Infrared Thermal Camera. Lean mass and Fat mass were quantified as lean mass index (LMI) and fat mass index (FMI). Physical activity was quantified as total accelerometer counts per minute (CPM) and minutes of moderate to vigorous physical activity (MVPA) per day. For Part 1, we assessed the precision of IR-thermal imaging of SCV skin temperature by examining the reproducibility of eight skin temperature outcomes over two trials. Furthermore, we assessed the accuracy of these eight skin temperature outcomes by investigating their association with energy expenditure. For Part 2 we evaluated the relationship of FMI, LMI and physical activity (MVPA) with SCV skin temperature in a multivariate analysis. RESULTS: Following these analyses, post-cooling skin temperature had the highest reproducibility of the eight skin temperature outcomes (intraclass correlation coefficient of 0.95, p<0.001) and was also significantly correlated with energy expenditure (Pearson correlation=0.392 p=0.032). Therefore, we used this outcome measure when examining the relationship between SCV skin temperature, body composition and physical activity. Fat mass index (FMI) was inversely related to post-cooling SCV skin temperature (β= -0.125, p<0.001). Minutes of moderate to vigorous physical activity and lean mass index (LMI) were not related to post-cooling SCV skin temperature. CONCLUSION: This study determined that post-cooling SCV skin temperature may be useful for detecting BAT in children and it is inversely related to adiposity. / Thesis / Master of Science (MSc)