Obesity is a serious global health problem (WHO, 2009) and an independent risk factor for a number of chronic diseases (Quilliot, Petit et al. 2002). Obesity is caused by a mismatch between energy intake and energy expenditure leading to storage of surplus energy and expansion of fat stores which causes metabolic disturbances (Speakman, 2004). Diet and exercise interventions remain the cornerstone of treatment for obesity but they have been largely ineffective (Franz et al. 2007). The purpose of this thesis was to identify components of a lifestyle intervention that are most likely to improve body composition, insulin sensitivity, and novel biomarkers of insulin resistance in obese individuals. This was achieved by investigating 4 different short term interventions. In study 1, isocaloric diet and exercise interventions were compared, while in study 2 concurrent training (regular resistance training combined with aerobic exercise) and concurrent training incorporating an eccentric component were compared. Briefly, the key findings of this PhD thesis were that isocaloric diet and exercise interventions lead to similar reductions in body weight, but exercise training may lead to greater improvements in body composition and metabolic health. Importantly, aerobic fitness was the single best predictor of improvements in metabolic health in this population. Resistance training is important for improving lean tissue mass, fat oxidation and resting metabolic rate. The novel biomarkers of insulin resistance are differentially regulated by diet, exercise and different modes of exercise training. Improvements in body composition and fitness drive improvements in insulin sensitivity and the circulating concentration of the biomarkers, and there is a cyclical relationship between the biomarkers and metabolic health. It is important to study the biomarkers for better understanding of metabolic processes but larger scale studies may be required to determine their role. A combination of calorie restriction, aerobic exercise and resistance training will optimise improvements in insulin resistance and body composition in obese individuals.