Meeting Abstract
The doubly labeled water (DLW) method is an important tool for measuring energy expenditure in free-living animals, including humans. Here we present the results of a validation study where we measured isotope ratios in real-time from the water vapor of exhaled breath with a cavity ring-down (laser) spectrometer (CRDS). We administered DLW via intramuscular injection to five zebra finches (Taeniopygia guttata, mean mass = 16 g), five brown-headed cowbirds (Molothrus ater, mean mass = 52 g), and five European starlings (Sturnus vulgaris, mean mass = 79 g). Blood and breath samples were taken prior to enrichment (background), at equilibration, and 24 h following administration of the isotopes. Both breath and blood samples were measured with the CRDS. Isotope values from the blood were validated by comparing estimates of dilution space (g) with total body water (g) measured with quantitative magnetic resonance. We found a very strong linear relationship between dilution space and total body water for both isotopes (R2 = 0.99), indicating the accuracy of isotope ratios measured from the blood. When comparing isotope concentrations from the breath and blood, we also found very strong linear relationships for both 18H (p < 0.001, R2 = 0.98) and 18O (p < 0.001, R2 = 0.99). Comparing estimates of whole body CO2 production (mol/h) between the breath and blood, we again found a strong positive linear relationship (p < 0.001, R2 = 0.90). Together our results suggest isotope ratios measured real-time from the water vapor in breath with a CRDS represents a promising new, less invasive, efficient, and simple method for quantifying energy expenditure in free-living animals.
