Validation in sheep of the doubly labeled water method for estimating CO2 production

Abstract

Carbon dioxide production (rCO2) was estimated in four sheep over a period of 10 days using doubly labeled water (2H and 18O) and was compared with simultaneous respiration chamber measurements of CO2. The excess 2H and 18O 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 rCO2 and water turnover (rH2O) 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 rCO2 estimates. Fractionated evaporative water loss expressed as a fraction of rH2O (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 2H loss in fecal solids reduced the mean rH2O (4,746 g/day) by 35.5 g/day and increased the mean rCO2 (332.3 l/day) by 21.2 l/day. Further corrections to account for 2H loss in methane (mean production rate 27.2 l/day) reduced rH2O by 33.8 g/day and increased rCO2 by 20.3 l/day. The final isotopic estimates of rH2O were 14.6 +/- 6.59% (n = 4) lower than direct measurements and the mean rCO2 was 3.5 +/- 14.48% (n = 4) lower than the chamber measured rCO2. However, in one of the animals studied the rCO2 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 rCO2 was +3.6%.

Feeding biology of two functionally different foregut‐fermenting mammals, the marsupial red kangaroo and the ruminant sheep: how physiological ecology can inform land management

“…Sheep lost body mass on average (± sem ) (% initial body mass change was −0.73 ± 0.16% day −1 , which was significantly different from zero, Z =−3.75, P <0.001). However, this level of mass loss was not considered as biologically significant because sheep can drink 8–10% of their body mass in a single bout (Squires, 1981), and this level of body mass change was consistent with that generally observed for sheep over comparable periods (Midwood et al , 1994). Moreover, initial body masses for sheep were measured in the afternoon, after animals had visited water, but final measurements were made in the morning, before sheep were able to drink (pers.…”

Assessment of body fat in the pony: Part II. Validation of the deuterium oxide dilution technique for the measurement of body fat

“…Thawed plasma samples were filtered (Costar Spin‐X centrifuge tube with 0.22 µm nylon filter) 8 and analysed in triplicate by gas isotope ratio mass spectrometry (Sira 10) 9 following zinc reduction of the water containing hydrogen isotopes present in the samples (modifications by Fuller et al . 2004 and Midwood 1990 of the methods described by Wong et al . 1987).…”

“…Plasma was immediately harvested from each blood sample (10 min, 2000 g at 4°C, Hermle Z 300 K) 6 and stored in 1-2 ml aliquots in air-tight tubes (Safe-T-Seal tubes and Screw Cap with Gasket) 7 at -80°C pending analysis. Thawed plasma samples were filtered (Costar Spin-X centrifuge tube with 0.22 mm nylon filter) 8 and analysed in triplicate by gas isotope ratio mass spectrometry (Sira 10) 9 following zinc reduction of the water containing hydrogen isotopes present in the samples (modifications by Fuller et al 2004 andMidwood 1990 of the methods described by Wong et al 1987). Deuterium abundances (expressed as parts per 1000 different from reference, d‰) in plasma samples were determined relative to a reference water (Aberdeen tap water) that had previously been normalised against the international standards V-SMOW and SLAP according to Gonfiantini (1978).…”

Myocardial substrate metabolism influences left ventricular energetics in vivo

“…and analyzed later for determination of 3 H 2 O and substrate levels. The content of 3 H 2 O in plasma was determined by vacuum sublimation as described by Midwood (21). The 14 CO 2 content of the blood was assessed by a diffusion method as described by Wisneski et al (39).…”