Energy balance in man measured by direct and indirect calorimetry

Abstract

In six 24-hr measurements of energy balance, direct and indirect calorimetry agreed within ±3%, which is probably the range of experimental error. But in seven other 24-hr periods there was disagreement in the range of 8 to 23%, and these were usually days when the subjects ate much less than they spent metabolically. Our direct calorimeter is an insulated, water cooled suit. Continuous measurements of O₂ consumption and CO₂ production provided data on metabolic expenditure (M) by indirect calorimetry. The 24-hr values for M matched the energy losses within ±60 kcal (±3% of M) in four men who rested all day and lay down to sleep at night. Similar agreement was seen in one of the four who worked on a treadmill for 4 hr and stayed busy all day, but in another energy losses were 342 kcal greater than M (10% of M). When the experiments gave values for M minus the losses greater than ±60 kcal, this is called “unmeasured energy”. In further experiments, two subjects stayed awake for 24 hr, and their unmeasured energies were 279 and 393 kcal. The same two men, eating sparingly, also worked for 24 hr so as to double their resting metabolic expenditures; the unmeasured energies were even larger, 380 and 958 kcal. When they repeated the 24 hr of mild work, but ate nearly as much as they spent metabolically, one man was near energy balance, while the other showed an unmeasured energy of −363 kcal. Little heat storage was evident in these experiments; therefore, heat balance was present and energy balance should have been present. In the group of 13 experiments, it appeared that the greater the food deficit, the larger was the unmeasured energy (excess of metabolic expenditure over loss of energy).