The energy lost from an animal system in a form other than as a combustible compound. Heat production may be measured by either direct or indirect calorimetry. In direct calorimetry, heat production is measured directly by physical methods, whereas indirect calorimetry involves some indirect measure of heat such as the measurement of oxygen uptake and CO2 production using thermic equivalent of oxygen based on the respiratory quotient (RQ) and theoretical considerations. The commonly accepted equation for indirect computation of heat production from the respiratory exchange is HE = 3.866 (liters oxygen) = 1.200 (liters CO2) – 1.431 (g urinary nitrogen) – 0.518 (liters methane). Heat production may also be measured as the difference from the determination of total carbon and nitrogen balance or from a comparative slaughter experiment. These methods arrive at total heat production be different calculations and are subject to systematic errors of measurement.