Protein energy relationships with particular references to energy undernutrition: A review

Abstract

Presently the energy-protein interrelationship of nonruminant and ruminant animals is presumed to be additive, i.e. the output from protein intake depends on the level of energy supplied and vice versa. However, when some of the earlier works on energy-protein relationships in ruminants were reevaluated and expressed in terms of protein supply at the intestinal level, it was more closely related to protein availability at the intestinal level than the level of energy intake per se. In sheep and cattle, based on the intragastric infusion technique, adequate protein with little or no energy supply permitted an animal to gain protein even with negative energy balance, presumably by oxidizing body fat. Increasing the protein supply, even without any exogenous energy, increased N retention curvilinearly. At a very high level of protein supply, protein retention was equivalent to an amount needed to approach 0.8kg liveweight gain daily. Similar results have been observed in sheep from intragastric infusion or fishmeal supplemented straw-based diets. Thus, during energy undernutrition, the use of the dietary protein-energy ratio to estimate tissue protein requirements has limited value for ruminants, even when based on absorbed amino acids, because endogenous fat can serve as a source of energy to fuel protein accretion. Normal tissue gain can be achieved with no exogenous energy when plenty of endogenous energy, e.g. fat, is available.