Fasting metabolism is adapted to ensure the orderly mobilization of endogenous substrates and fuel for maintaining vital activity. When no exogenous energy is consumed it gives rise to a glucose deficient condition; as a result gluconeogenesis from amino acids occurs which results in a higher excretion of N and a higher heat production. When therefore fasting metabolism is adopted as the reference level of metabolism when assessing energetic efficiency, efficiency appears higher below energy maintenance (Km) than above (Kf). When enough feed is given to meet the glucogenic demand of an animal, but no more, heat production falls to a minimum, which suggests that this value, rather than fasting heat loss, would be the appropriate base for measuring the energetic efficiency of exogenous feed. In general about one-third of energy maintenance (150 kJ/kg W^0.75 daily or 20 mmol of glucose/kg W^0.75 daily) should be used for obtaining this minimum metabolic state in ruminants.