VALIDATION IN SHEEP OF THE DOUBLY LABELED WATER METHOD FOR ESTIMATING CO2 PRODUCTION

Carbon dioxide production (r(co2)) was estimated in four sheep over a period of 10 days using doubly labeled water (H-2 and O-18) and was compared with simultaneous respiration chamber measurements of CO2. The excess H-2 and O-18 measurements were corrected for the empirically determined effects of isotope rebreathing within the confines of the chambers. A weighted monoexponential curve was then fitted to the data from which isotope flux rates and ultimately r(co2) and water turnover (r(H2O)) estimates were made. The curve fits were weighted assuming a Poisson model. Selection of this weighting policy did not bias the results, and curvature in the data also appeared to have little effect on the r(co2) estimates. Fractionated evaporative water loss expressed as a fraction of r(H2O) (X) was estimated from water balance and breath water production estimates; the mean X was 0.145 and ranged from 0.108 to 0.183. Corrections for H-2 loss in fecal solids reduced the mean r(H2O) (4,746 g/day) by 35.5 g/day and increased the mean r(co2) (3332.3 l/day) by 21.2 l/day. Further corrections to account for H-2 loss in methane (mean production rate 27.2 l/day) reduced r(H2O) by 33.8 g/day and increased r(co2) by 20.3 l/day. The final Isotopic estimates of r(H2O) were 14.6 +/- 6.59% (n = 4) lower than direct measurements and the mean r(CO2) was 3.5 +/- 14.48% (n = 4) lower than the chamber measured r(co2). However, in one of the animals studied the r(co2) deviated markedly from the chamber-derived value, and this discrepancy has yet to be explained. When this animal was excluded from the comparisons, the standard deviation was greatly reduced (+/-3.6, n = 3) and the mean overall error on r(co2) was +3.6%.