Validation of the Doubly-Labeled Water (DLW) Method for Estimating CO2Production and Water Flux in Growing Poultry Chicks

This study is the first validation of the doubly-labeled water (DLW) method on birds (1) to evaluate the accuracy of 2 points versus multiple points for computing fractional isotopic washout rates (k) and CO2production$({\rm rCO}_{2})$, (2) to measure CO2production and water flux each day over a 4-day period, (3) to compare measured fractional evaporative water loss$({\rm r}_{{\rm G}})$with assumed values that provide DLW estimates of${\rm rCO}_{2}$with zero error, and (4) to measure the effect of assumed${\rm r}_{{\rm G}}$on the error of estimating water influx and efflux. Percent error of CO2production of six growing poultry chicks estimated by the DLW method was not correlated with mean daily relative growth rates of up to 5% nor with daily rates of energy retained in growth of up to 320 kJ/day/kg, nor was it significantly reduced by using multiple points (5 points) rather than 2 points to compute fractional isotopic washout rates (k) and isotope pool sizes. Its seems clear from our study and the previous 5 validations on growing birds that average relative daily growth rates of up to about 20% do not increase the error of estimating${\rm rCO}_{2}$by the DLW method. Arithmetic error was significantly less when using one isotopic pool, rather than two pools, to compute${\rm rCO}_{2}$and was less when using an assumed fractional evaporative water loss$({\rm r}_{{\rm G}})$of 0.45 rather than an assumed${\rm r}_{{\rm G}}$of 0.25 or 0.5 (the two values used predominantly in previous DLW studies). Our study supports Speakman's (1997) suggestion that the one-pool model is more appropriate than the two-pool model for birds weighing < 1 kg. We recommend using an assumed${\rm r}_{{\rm G}}$of 0.45 to compute${\rm rCO}_{2}$of poultry, which is a compromise between the two schools of${\rm r}_{{\rm G}}$useage, i.e.,${\rm r}_{{\rm G}}$= 0.25 or 0.5, however we hesitate to recommend 0.45 for all birds in all settings. Close agreement between measured${\rm r}_{{\rm G}}$and an assumed${\rm r}_{{\rm G}}$that produced zero${\rm rCO}_{2}$error supports the validity of using the pooled fractionation correction factors$({\rm f}_{\text{pool}})$of 0.0339 for tritiated water and 0.0249 for deuterated water. Absolute error decreased with the percent washout of${\rm H}_{2}{}^{18}{\rm O}$during measurement periods of 1 to 4 days. Accuracy of estimating${\rm rCO}_{2}$was not significantly different for durations of 2, 3, and 4 days using either tritiated or deuterated water. The ari