Among the branched-chain amino acids, leucine is known to decrease food intake via mTORC1-dependent mechanisms. We now hypothesize that leucine protects against diet-induced obesity (DIO) by having a beneficial effect on inflammation and insulin sensitivity. We postulate that these effects are due to leucine’s ability to affect intracellular pathways that modulate energy balance as well as protein synthesis, inflammation and mitochondrial function. To study leucine’s effects on the development of DIO, in Aim 1 C57BL/6 mice will receive either low-fat chow or high-fat diet (HFD), and half will receive leucine in the drinking water. After 12 weeks, we will evaluate glucose tolerance and insulin sensitivity, in vivo body composition and energy expenditure. Food intake will be assessed in these mice after receiving acute leptin, to verify whether leucine supplementation halts the development of DIO-associated leptin resistance. At sacrifice, tissues and blood will be collected for assessment of inflammatory cytokines. Activation of the mTOR pathway and of the inflammatory pathway involving nuclear factor-kappa B and SOCS-3 will be studied in the hypothalamus. mRNA levels of hypothalamic neuropeptides regulating food intake and of genes involved in thermogenesis and mitochondrial activity will be measured. In Aim 2, we will study leucine’s effects once the DIO phenotype has developed. Half a group of DIO mice will continue on HFD for 12 weeks; the other half will be switched to chow to verify that (a) leucine supplementation facilitates body weight loss, (b) leucine’s effects in DIO mice resemble those obtained by returning mice to chow. Animals will then undergo the metabolic and molecular analysis described earlier. This novel project will explore leucine’s actions during DIO and body weight loss. Our findings will clarify whether and in which way this nutrient might ease the obese condition and help halt its epidemic.