Chapter one provides an overview of the regulation of insulin secretion/action, details normal insulin action, and, describes the concept of insulin resistance and its assessment. It specifically focuses on obesity (as the most important contributor to insulin resistance) and reviews dietary effects on insulin resistance and plasma lipids. Chapter two describes methods which are common to chapters 3, 4 and 5: assessment of habitual diet; formulation/practical administration of intervention diets; blood sampling; and assessment of body composition, insulin sensitivity, and vascular compliance. Chapter three compares the effects of a high versus low sucrose diet (25 vs. 10 %, respectively, oftotal energy intake) in 13 healthy subjects, in a randomised crossover design with sequential 6 week dietary interventions separated by a 4 week washout. Diets were weight maintaining with identical macronutrient and fibre content. The results demonstrated that a high sucrose intake, as part of an isocaloric weight maintaining diet, had no detrimental effect oninsulin resistance. Chapter four investigates the effects of a low carbohydrate versus low fat weight reduction diet (0.5 kg/week) on insulin sensitivity and cardiovascular risk in overweight/obese subjects (n = 24) using a parallel group randomised controlled trial design. Following similar weight loss, both diets were equally effective in improving insulin sensitivity. The low fat diet also had beneficial effects on augmentation index (a measure of arterial stiffness), a finding which was not evident within the low carbohydrate group. Chapter five explores whether adipokines (leptin, adiponectin and retinol binding protein 4) potentially mediate the effects of weight loss (induced by hypocaloric dieting in study 2) on insulin sensitivity. Results demonstrated that diet induced changes in adipokines did not correlate with the change in insulin sensitivity. Chapter six provides a general discussion and concluding remarks for the main body of the thesis.