VALIDATION OF THE DOUBLY LABELED WATER METHOD IN GROWING PIGS

The CO2 production (r(CO2)) of eight growing pigs was determined by co ntinuous collection of CO2 over 21 days and simultaneously estimated u sing the doubly labeled water (DLW) method. The aim was to assess the accuracy of the method before and after correction for known sources o f error and to test for any residual discrepancy arising from as yet u nidentified sources of error. Mass spectrometer accuracy was verified by analyzing serial dilutions of the dose material in the form of an a rtificial decay curve; no significant bias was detected. The physiolog ical errors were linearly dependent on weight gain. DLW-derived r(CO2) (corrected only for fractionated water loss) underestimated the true value by 0.270 l CO2/g wt gain or -8% in the restricted (group R) and -16% in the ad libitum-fed (group AL) groups. Known sources of error a ccounted for -0.006 (methane), -0.032 (fecal H-2 losses), -0.108 (fat synthesis), and -0.146 (changing pool size) l CO2/g wt gain. After cor rection for these sources of error the DLW-derived r(CO2) differed fro m the true value by -2 +/- 3% ingroup R and 0 +/- 3% in group AL. Thus there was no significant bias in the DLW method after correction for known sources of error, even during rapid weight gain or at weight sta bility with or without correction. The precision estimates include bot h dose and background errors and uncertainty in the correction factors used. Strategies for optimizing precision are presented.