Currently Japan is in the midst of satiation, under which condition, together with genetic predisposition, problems of obesity and related adult diseases will arise unless proper care is taken to positively expedite energy expenditure. In this article, the author reviews the present understanding of the mechanism of energy expenditure and its practical application in humans as well as in animals, focusing on the following four lines of evidence including his own results: (1) the importance of the sympathetic nervous system in controlling energy expenditure of the body, particulaly of the slow-twitch oxidative fibers of skeletal muscles, (2) the function and physiological roles of the mitochondrial uncoupling-protein family (UCP-1-3) in heat-producing organs, particularly in brown adipose tissues, (3) the central effects of leptin and its novel signaling pathway on the glucose and fatty acid metabolism in skeletal muscles, the major working organ involved in the resting-energy metabolism, and (4) an exploratory investigation on the effective components of flavor or spices such as zingerone that stimulate oxygen consumption and decrease respiratory quotient.