Human energy expenditure.

HYSIOLOGICAL STUDY of man's food needs and capacity for physical work began on a systematic basis about seventy years ago. Following upon Atwater's classic experiments with a human calorimeter at the end of the last century, the validity of using rates of oxygen consumption as the basis for measuring energy expenditure (indirect calorimetry) was firmly established. Throughout this early period rates of energy expenditure during a variety of human activities were recorded. Recently there has been a renewal of interest in such measurements. This is in part due to the development of apparatus which can be conveniently applied under many industrial conditions, where it had not hitherto been practicable to use the more cumbersome apparatus designed for laboratory studies. Also, methods of work in many industries and in agriculture have changed markedly. Machines increasingly do the work formerly done by manual labor. This change is more noticeable in the highly industrialized coun.tries, but it is spreading throughout the world. Much of 'heavy' industry now consists in controlling large sources of mechanical power by means of levers and switches and the work for the human operator is often 'light.' Further, with increased productiveness there has been a shortening of the working week. There is now time and opportunity for recreations on a scale altogether different from fifty years ago. Many recreations demand much expenditure of physical energy and a wage earner may use his muscles more in his leisure time than during his earning hours. These factors call for new assessments of human energy expenditure , Indirect calorimetry enables the energy expended to be determined while a de& nite activity is undertaken for a limited period of time, usually measured in minutes. More information is necessa.ry to assess the energy expenditure in daily life. This energy has, of course, to be provided by the food, if health and activity are to be maintained. In the past most assessments of total energy needs have been made from dietary studies. This was the method used by Voit, Atwater and others, who first stated the energy expenditure of a variety of occupations, after analyzing records of the diets consumed by persons of known employment. The method is open to the theoretical objection that it assumes that the diets consumed provide exactly enough energy with no surplus or deficiency. It provides no direct yardstick of requirement. Indeed in the East and elsewhere large numbers of …