The measurement of energy exchange in man: an analysis.

This report analyzes two kinds of studies of human energy balance; direct and indirect calorimetry for 24-hr periods, and complete measurements of food intake, waste, and tissue storage for 3 weeks and longer. Equations of energy balance are written to show that the daily quantity of metabolic energy, QM, is coupled with an unidentified quantity of unmeasured energy, QX, in order to make the equation balance. The equations challenge the assumed equivalence of direct and indirect calorimetry. The analysis takes the form of employing experimental data to calculate values for the arguable quantity, QX. Studies employing 24-hr direct calorimetry, 202 complete days, show that when food intake nearly matches QM, values for QX are small and probably insignificant, but when there is a large food deficit, large positive values for QX appear. Calculations are also made from studies of nutrient balance during prolonged overeating and undereating, and in nearly all cases there were large negative values for QX. In 52 sets of data from studies lasting 3 weeks or longer, where all the terms in the balance equation except QX were either directly measured or could be readily estimated, the average value for QX amounts to 705 kcal/day, or 27% of QM. A discussion of the nature of QX considers error and the noninclusion of small quantities like the energy of combustible gases, which are not thought to be sufficient to explain QX. It might represent the cost of mobilizing stored fuel, or of storing excess fuel, or it might represent a change in internal energy other than fuel stores, but none of these is thought to be likely. Finally, it is emphasized that entropy exchange in man as an open thermodynamic system is not presently included in the equations of energy balance, and perhaps it must be, even though it is not directly measurable. The significance of unmeasured energy is considered in light of the poor control of obesity, of the inability to predict weight change during prolonged diet restriction or intentional overeating, and of the energetics of tissue gain in growth and loss in cachexia. It is not even well established how much food man requires to maintain constant weight. New studies as they are undertaken should try to account completely for all the possible terms of energy exchange.