Utilization of endogenous and exogenous energy for protein accretion during energy undernutrition has been studied. Nine lambs nourished by intragastric infusion were given either progressively increasing or decreasing amounts of casein-N up to 2550 mg/kg metabolic weight (W0·75), with or without 250 kJ/kg W0·75 of volatile fatty acids daily. Energy balance (respiration calorimetry) and N balance were measured. While all experimental animals were in negative energy balance, N balance increased curve-linearly with the increase in casein-N infusion and attained positive N balance. Endogenous energy (presumably body fat) was found to meet the energy needs for protein accretion during energy undernutrition. It is concluded that body fat can be effectively utilized to support lean-tissue growth during energy undernutrition, so that the classical nutritional concept of dietary energy:protein ratio is only meaningful when both endogenous and exogenous energy are considered.
