Energy partitioning, tissue growth and appetite control

UMetabolizable energy (ME) and protein contained within the food consumed by an animal during growth are directed (almost) entirely towards heat production and the deposition of body protein and fat.The partition of nutrients between these three compartments or between the various organs and tissues in the growing animal is obviously determined by the amount of ME available for growth, the availability of other nutrients (protein, minerals, etc.) relative to ME and the animal's intrinsic 'growth plan' which is a function of its age, genotype and its physiological state.I wish to propose that for most practical purposes the partition of ME and protein during growth into body protein, fat and heat can be described by a model of extreme simplicity (Fig. 1).When a growing animal is kept in a thermoneutral environment and given access to a diet or choice of diets that it can 'understand' in a metabolic sense (this definition excludes cafeteria diets) then regulation of nutrient supply is dominated by the impetus for lean tissue growth, or protein accretion.In Fig. 1 the rate of protein accretion is shown as a regulator for both ME and protein intake.This, in itself, is neither original nor controversial.The more interesting feature of Fig. 1 is what it leaves out.It contains (in the circumstances defined previously) no regulator for heat production or fat deposition or any mechanism whereby the rates of these processes may be transduced and fed back into the control of nutrient supply.These omissions are harder to defend.Fig. 1.A simple, hypothetical model to describe the regulation of dietary energy and protein during growth.