Submitted to: American Journal of Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 21, 1997
Publication Date: N/A
Interpretive Summary: A room calorimeter chamber at BHNRC, Beltsville, Maryland can determine the number of calories a person is burning by either directly measuring the heat they give off (heat emission) or by measuring the oxygen they consume and the carbon dioxide they produce (energy expenditure). Because these measurements are very different physically the signals from the calorimeter rfor heat emission and energy expenditure do not match up in time. The signals are what is called smeared. A mathematical relationship between the calories the person in the calorimeter is actually burning and the signals for heat emission and energy expenditure was developed. These equations were tested by simulating a human being with a burning alcohol lamp. These test show that the equations correct the smearing of the heat emission and energy expenditure signals. Experiments in humans show that both heat emission and energy expenditure are the same when averaged for the entire day. However, during sleep the heat emission was actually greater than energy expenditure. The test in humans demonstrated how people lose some of the heat in their body at night but make it up in the morning when they "warm up". This research will enhance the useful information gathered from calorimeter experiments which will help us understand thermoregulation and energy metabolism and eventually benefit the general population.
Technical Abstract: A dual direct/indirect room sized calorimeter is used at the Beltsville Human Nutrition Research Center to measure heat emission and energy expenditure in humans. Because the response time of a gradient layer direct calorimeter and an indirect calorimeter are not equivalent, the respective rate of heat emission and energy expenditure have not been directly compared. A system of equations has been developed and tested which can correct the respective outputs of the direct gradient layer calorimeter and indirect calorimeter for delays due to the response times of the measurement systems. Performance tests using alcohol combustion to simulate a human subject indicate comparative accuracy of measurements of heat production from indirect (100.6% +/- 1.1%), corrected indirect (100.6% +/- 1.1%), direct (100.2% +/- 1.4%) and corrected direct (100.5% +/- 1.5%) calorimetry systems. Results from 24-hour measurements in ten human subjects indicate that corrected heat emission is equivalent to (99.8% +/- 2.5% of) corrected energy expenditure. However, heat emission was significantly greater (17%) than energy expenditure during sleep suggesting a change in the energy stored as heat in the body. These results illustrate how the simultaneous measurement of heat emission and energy expenditure provide insights into thermoregulation.