Abstract

Using the model for energy regulation proposed by PAYNE and DUGDALE (1977), a quantitative relationship between energy metabolism, i.e., basal metabolic rate (BMR) and protein metabolism, i.e., fasting urinary nitrogen loss (FUNL) is presented. An analysis of classical human studies on starvation shows that the fraction of total energy expenditure derived from protein (i.e., the P ratio) remains unchanged, confirming a close association between protein and energy metabolism. The propensity to use protein as a fuel appears to be fixed in an individual regardless of whether the synthesis of protein is limited by energy or protein intake. During starvation, obese individuals appear to mobilise about 5% of total energy from protein breakdown, while normal weight subjects about 20%. Body composition exerts a profound influence on protein metabolism. A quantitative relationship between BMR, FUNL and obligatory nitrogen loss (ONL) is also presented. It is thus proposed that body composition may significantly influence ONL and hence human protein requirements.