Protein and energy metabolism in starvation reconsidered

Current concepts of the metabolic changes during starvation have been heavily influenced by recent studies on grossly obese subjects undergoing therapeutic fasting. It has been suggested that during prolonged starvation the central nervous system develops the ability to utilize ketone bodies, and that the consequent reduction in gluconeogenesis leads to an adaptive sparing of protein catabolism. However, our analysis of classical studies on starvation shows that during prolonged starvation in normal subjects the fraction of total energy expenditure derived from protein (the P ratio) remained unchanged. Although urinary nitrogen was reduced during the progress of the fast, we conclude that this is secondary to the general reduction in metabolic rate, rather than to a specific sparing of protein.