Future directions for energy requirements and food energy values.

The energy values of foods have been expressed in units of metabolizable energy (ME) since Atwater's time. Atwater "specific factors" for protein, fat and carbohydrate of food groups are used to compute energy values for individual foods. Although remarkably successful for many years this system is changing in order to provide the means to predict the energy value of new foods and formulations containing food substitutes, bulking agents, sweeteners, etc. New methods of analysis will allow prediction of energy value of new and unusual foods from chemical analyses. Originally used to determine the needed food supply for whole populations, statements of energy requirements are now needed for diet formulation for individuals. The mean energy requirements of populations are very adequately defined by WHO and the NRC RDA's by establishing ratios of total energy expenditure to basal metabolic rate (BMR). These estimates have been validated by use of the doubly labelled water (DLW) method to measure energy expenditure of free-living subjects over periods of 10-14 days. Substantial improvement in the prediction of energy needs for individuals will require the means to quantify physical effort, resolve effects of body composition on basal metabolic rate, and define the extent of variation in the use of metabolizable energy (ME) in non-steady state situations such as are caused by systematic changes in diet composition, body weight, and patterns of exercise.