On problems of calculating energy expenditure and substrate utilization from respiratory exchange data

Abstract

Indirect calorimetry based on respiratory exchange measurement has been successfully used from the beginning of the century to obtain an estimate of heat production (energy expenditure) in human subjects and animals. The errors inherent to this classical technique can stem from various sources: 1) model of calculation and assumptions, 2) calorimetric factors used, 3) technical factors and 4) human factors. The physiological and biochemical factors influencing the interpretation of calorimetric data include a change in the size of the bicarbonate and urea pools and the accumulation or loss (via breath, urine or sweat) of intermediary metabolites (gluconeogenesis, ketogenesis). More recently, respiratory gas exchange data have been used to estimate substrate utilization rates in various physiological and metabolic situations (fasting, post-prandial state, etc.). It should be recalled that indirect calorimetry provides an index of overall substrate disappearance rates. This is incorrectly assumed to be equivalent to substrate "oxidation" rates. Unfortunately, there is no adequate golden standard to validate whole body substrate "oxidation" rates, and this contrasts to the "validation" of heat production by indirect calorimetry, through use of direct calorimetry under strict thermal equilibrium conditions. Tracer techniques using stable (or radioactive) isotopes, represent an independent way of assessing substrate utilization rates. When carbohydrate metabolism is measured with both techniques, indirect calorimetry generally provides consistent glucose "oxidation" rates as compared to isotopic tracers, but only when certain metabolic processes (such as gluconeogenesis and lipogenesis) are minimal or / and when the respiratory quotients are not at the extreme of the physiological range. However, it is believed that the tracer techniques underestimate true glucose "oxidation" rates due to the failure to account for glycogenolysis in the tissue storing glucose, since this escapes the systemic circulation. A major advantage of isotopic techniques is that they are able to estimate (given certain assumptions) various metabolic processes (such as gluconeogenesis) in a noninvasive way. Furthermore when, in addition to the 3 macronutrients, a fourth substrate is administered (such as ethanol), isotopic quantification of substrate "oxidation" allows one to eliminate the inherent assumptions made by indirect calorimetry. In conclusion, isotopic tracers techniques and indirect calorimetry should be considered as complementary techniques, in particular since the tracer techniques require the measurement of carbon dioxide production obtained by indirect calorimetry. However, it should be kept in mind that the assessment of substrate oxidation by indirect calorimetry may involve large errors in particular over a short period of time. By indirect calorimetry, energy expenditure (heat production) is calculated with substantially less error than substrate oxidation rates.

The Acute Effects of L-arginine on Hormonal and Metabolic Responses During Submaximal Exercise in Trained Cyclists

“…McConell et al (2006) and Linden et al (2011) combined glucose tracers and respiratory analysis to assess substrate oxidation. A limitation of the current study is substrate oxidation was measured through indirect calorimetry (Schutz, 1997). In addition, the current study found no effect on markers of NO production.…”

Leptin Deficiency Causes Insulin Resistance Induced by Uncontrolled Diabetes

“…However, the increased metabolic rate in STZ-induced diabetic animals could not be attributed to changes in ambulatory activity (number of beam breaks: 2,953 ± 243 for veh-veh vs. 1,562 ± 87 for STZ-veh vs. 2,215 ± 163 for STZ-lep vs. 2,927 ± 207 for STZ-veh-PF). Respiratory quotient and metabolic heat production are not reported because the assumptions involved in using the respiratory quotient to estimate heat production are not met in abnormal metabolic states, including those characterized by a shift to ketone utilization as a metabolic substrate or during rapid depletion of protein and fat stores (19,20). Nonetheless, these data collectively suggest that increased energy expenditure in STZ-veh-PF relative to STZ-lep-treated animals probably contributes to their excessive weight loss and depletion of lean mass, despite consuming equal amounts of food.…”

Sick of sitting

“…For example, if a person rakes leaves all day, they become hungry. It is unlikely that the integration of NEAT and food intake is limited to humans, because when wolves, horses and dogs run a lot, they eat a lot, too [92]. …”