The main organs that produce heat when the body is quiet are the internal organs and the brain. Of the viscera, the liver produces the most heat. Forms of heat production in the body include basal metabolic heat production, skeletal muscle heat production during exercise, special kinetic effects of food, and war and non-war heat production. The body’s heat is mainly provided by the metabolism of the three major nutrients (sugar, fat, and protein), and the stronger the metabolism, the greater the heat production capacity of the organ. In quiet, the body’s heat production is mainly visceral heat production, and among the various organs of the viscera, the liver produces the most heat, because the temperature of the blood flow in the liver is 0.4 to 0.8 degrees Celsius higher than that of the blood flow in the aorta. Heat production is increased in cold conditions by warbling (simultaneous contraction of skeletal muscle extensors and flexors) and by nonwarbling heat production (by increasing the metabolism of brown fat distributed in the great vessels of the neck, groin, and subscapular region). Brown fat is particularly important for thermogenesis in newborns, which contain high levels of brown fat, due to the imperfect development of thermoregulation in newborns. In addition, the main organ that generates heat during exercise is skeletal muscle.