The brown adipose tissue, where the uncoupling protein 1 (UCP1) is located, is a thermogenetic tissue only present in small mammals and neonates, with the function of keeping the temperature and body weight stable when exposed to either cold or a hypercaloric diet. Since the UCP1 is located exclusively in brown adipose tissue, and this tissue has little expression in human adults, the studies have emphasized the UCP2 and UCP3, homologous proteins to the UCP1, expressed in multiple tissues and skeletal muscle, respectively. Physical activity increases UCP2 and UCP3 mRNA but it is now being questioned if this is due to changes in lipid metabolism or energy metabolism. During food restriction and fasting, there is a decrease in the amount of body adipose tissue and increase in the plasmatic free fatty acid concentration, with up-regulation of the UCP2 and UCP3 in the muscle, and an increase in lipid oxidation. The increase in free fatty acids represents an important intracellular indication of the induction of the UCP expression in the muscle, which can be connected to its use as fuel until there is an increased demand by the organism to dissipate energy. However the discussion continues about whether the UCP2 and UCP3 in the skeletal muscle function to mediate thermogenesis or in the regulation of lipid oxidation.
diet; protein-restricted; gene expression; exercise; brown fat; fasting; UCP2; UCP3
