In a human intervention study, changes in physiological, metabolic, and neuroendocrine functions in response to weight cycling will be investigated under controlled conditions in normal weight subjects. The mechanisms of the catch-up fat phenomenon are analysed starting from stable energy balance followed by overfeeding, weight loss and weight regain following weight loss (refeeding). Changes in body composition (including ectopic fat), metabolism (resting energy expenditure, substrate oxidation rates, insulin resistance) and plasma hormone concentrations will be assessed. Fat tissue probes will be used to characterise key enzymes and signalling pathways, redox status and whole genome expression. Modulation of the hormonal response to weight cycling is brought about by varying macronutrient content and glycemic index of the diets. We hypothesize that, insulin and leptin resistance are explained by increased insulin secretion during the refeeding period. Both, adaptive thermogenesis as well as insulin and leptin resistance can be ameliorated by attenuation of the increase in insulin and leptin secretion during refeeding a low GI diet after weight loss.