Aj. Midwood et al., VALIDATION IN SHEEP OF THE DOUBLY LABELED WATER METHOD FOR ESTIMATINGCO2 PRODUCTION, The American journal of physiology, 266(1), 1994, pp. 180000169-180000179
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 co
mpared with simultaneous respiration chamber measurements of CO2. The
excess H-2 and O-18 measurements were corrected for the empirically de
termined effects of isotope rebreathing within the confines of the cha
mbers. A weighted monoexponential curve was then fitted to the data fr
om which isotope flux rates and ultimately r(co2) and water turnover (
r(H2O)) estimates were made. The curve fits were weighted assuming a P
oisson model. Selection of this weighting policy did not bias the resu
lts, 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/da
y 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 meas
ured r(co2). However, in one of the animals studied the r(co2) deviate
d markedly from the chamber-derived value, and this discrepancy has ye
t to be explained. When this animal was excluded from the comparisons,
the standard deviation was greatly reduced (+/-3.6, n = 3) and the me
an overall error on r(co2) was +3.6%.