Cover Image
close this bookProtein-Energy Requirements of Developing Countries: Evaluation of New Data (UNU, 1981, 268 p.)
close this folderDiscussions and recommendations of the task forces
close this folderA note on energy utilization and its efficiency
View the document(introduction...)
View the documentReferences



The chemical energy in foodstuffs, manifested by the liberation of heat during combustion, is used primarily for doing work (internal and external work). External work is that performed by the body in its environment; internal work is mechanical and chemical Synthesis of compounds in reactions that would not proceed spontaneously, transport of ion against electrochemical gradients, etc.) In the steady state, i.e., when there is no change in chemical composition and body mass, all the internal work performed is dissipated as heat as prescribed by the first law of thermodynamics.

Energy utilization should be taken to mean the amount of work (external and internal) performed in or by the body by a unit change (decrease) in body energy. The latter change is equal to the heat output in thermal steady state. The relation between the two is subject to the restriction imposed by the second law of thermodynamics, which can be expressed as DH = G-TDS, where the entropy term, TDS, is almost always so small that it can be neglected. The change in Gibbs free energy, DG, is equal to the maximum work that can be harvested in the process. This amount of work can never be realized lit is the theoretical case where all processes proceed reversibly). The actual work performed is always less than the maximum work and depends on the degree of coupling that the body can make between the spontaneous chemical reactions and the work Synthesis. ion pumping, muscular contraction, etc.). The efficiency of the process is then given by the ratio of the actual work done to the maximum possible work.

Although the energy input to the body can always be measured, the total amount of work performed by the body cannot. Therefore, the efficiency of energy utilization cannot be measured. However, it is possible to imagine an experiment in which changes in efficiency can be measured. If measurements of energy expenditure are made on the same individual, during two situations identical in total work (i.e., a given "regime" or "programme"), then any change in energy expenditure directly reflects a change in efficiency of energy utilization.