An older term for diet-induced thermogenesis, the heat the body produces while digesting, absorbing, and metabolizing food. Every meal raises metabolic rate for several hours: protein costs the most energy to process, raising expenditure by roughly a fifth to a third of its calories, while carbohydrates and fats cost less. This obligatory heat production is one reason the thermic effect of food figures in energy balance equations, and it explains the warmth felt after a large meal. The quaint phrase animal heat dates from nineteenth-century physiology, when scientists first measured the warmth generated by living bodies and sought its sources in food combustion. Modern nutrition expresses the same phenomenon in kilojoules and metabolic rates, but the underlying observation, that eating literally warms the eater, remains unchanged.