The energetic efficiency of metabolism

Abstract

The efficiency with which an animal can utilize the metabolizable energy (ME) contained in the food it eats is determined by the amount of heat (H) it produces in metabolism. This paper is devoted to an analysis of the factors which affect H in animals. The topic is of perennial interest to students of both animal and human nutrition concerned respectively with the efficient use of animal feedstuffs by livestock and with problems experienced by man in maintaining energy balance through adult life. There are three ways by which one can approach the analysis of metabolic heat production. (I) Analysis by external inputs. Analysis of H according to measurable variables in the whole animal and its environment, namely size of animal, quantity and quality of food intake, behaviour and activity and the thermal environment. (2) Analysis by internal inputs. Analysis of H according to substrate kinetics and the enthalpy changes incurred thereby. (3) Analysis by internal output. Analysis according to the amount of work done by different organs and tissues in different circumstances. These forms of analysis require a little further explanation. The analysis according to external input is the classic approach of the calorimetrist measuring energy flow in the whole animal in response to changes, e.g. in food intake or ambient temperature. Measurements of this sort are very numerous and while they may not be very profound in a metabolic sense, they are usually very precise and the limited conclusions that can be drawn from them can usually be drawn with confidence. Analysis according to internal input refers to the 'lower level' of the modeller who uses his knowledge of stoichiometry and the power of his computer to derive a more elegant and comprehensive picture of the flow of energy through an animal and the likely consequence to the animal of any perturbation of the inputs to the system. The disadvantage of this approach is that, at present, the uncertainty attached to some of the estimates can be large. The philosophy behind the approach to analysis by internal output is that the metabolism of the animal is not so much driven by the amount of energy flowing into the system but pulled along by the requirement of different organs and tissues for energy substrates to regenerate (principally) ATP from ADP produced as a direct consequence of the work done by those tissues in support of maintenance, and, for example, growth or lactation. This form of analysis can be further subdivided into analysis by form or function. Analysis by form involves the measurement or estimation of H in different organs (e.g. the gut and liver). Analysis by function involves measurement of the contribution to total metabolic rate of energetically expensive processes such as protein synthesis. 0 0 ~ ~-6 6 ~ 1 / 8 1 / ~ 0 1 ~-q 8 o ~ bo~.oo C 1981 The Nutrition Society at