Energy Metabolism: Errors in Gas-Exchange Conversion Factors

Abstract

When an animal fasts throughout the period of measurement of CO2 production but is presumed to be eating, the error of estimating energy metabolism can be as high as 23% for herbivores; the error is small (≤3.2%) for uricotelic carnivores, and it increases steadily with the protein content of the diet for ureotelic carnivores. Errors in converting CO2 production measurements to energy metabolism are associated with small errors (for animals that are eating daily) when a respiratory quotient of 0.83 for all herbivores, 0.80–0.83 for mammalian carnivores, and 0.72 for uricotelic carnivores is used. Similar conversions of oxygen consumption measurements into energy metabolism are small (<±3%) when urinary nitrogen excretion is not measured and RQ is assumed to be 0.8, regardless of the actual protein, fat, and carbohydrate mixture being catabolized by fed carnivores, omnivores, or herbivores. The error is even smaller in nearly all cases if the respiratory quotient is measured and is used to derive energy conversion factors (<±0.6% for uricotelic herbivores and carnivores).