Midwood A J, Haggarty P, McGaw B A
Rowett Research Institute, Aberdeen, United Kingdom.
Am J Physiol. 1993 Mar;264(3 Pt 2):R561-7. doi: 10.1152/ajpregu.1993.264.3.R561.
The doubly labeled water (DLW) technique allows the CO2 production (rCO2) of free living animals to be estimated from the difference between the turnover of 2H2O and H218O in the body water. A fundamental assumption of this technique is that neither of the isotopes used are lost in products other than CO2 and H2O. We found, however, that 2H was lost in both exchangeable and nonexchangeable positions in the feces of sheep. Negligible amounts of 18O were lost in exchangeable positions. 2H losses led to a 0.75% (SE 0.06, n = 4) overestimation of the measured 2H2O flux, leading to an average error in rCO2 estimates of 20.3 l/day. For a typical rCO2 rate of 370 l/day, this would amount to an error of approximately 5% (range -7.0 to -4.3%, n = 4). Correction factors to account for this loss were presented. The error in rCO2 due to 2H sequestration into fat was calculated to be at most 2.1 l/day or about -0.66% in lambs with a rCO2 of 320 l/day. In a triply labeled water (TLW) study the maximum error in the estimation of fractionated evaporative water loss (X) would lead to a 0.81% underestimation of rCO2. We recommend that during a DLW study involving ruminant animals the correction factors presented here be used to compensate for 2H loss in feces. This may be particularly important where the diet has a high roughage content leading to a significant fecal dry matter production.
双标记水（DLW）技术可根据体内水分中2H2O和H218O的周转率差异来估算自由活动动物的二氧化碳产生量（rCO2）。该技术的一个基本假设是，所使用的两种同位素都不会在二氧化碳和水以外的产物中损失。然而，我们发现，绵羊粪便中可交换和不可交换位置的氢都有损失。可交换位置的氧-18损失量可忽略不计。氢的损失导致测得的2H2O通量被高估了0.75%（标准误0.06，n = 4），导致rCO2估计值的平均误差为每天20.3升。对于典型的每天370升的rCO2速率，这相当于约5%的误差（范围-7.0至-4.3%，n = 4）。我们给出了考虑这种损失的校正因子。在rCO2为每天320升的羔羊中，由于氢被隔离在脂肪中导致的rCO2误差计算最多为每天2.1升，或约-0.66%。在一项三标记水（TLW）研究中，分馏蒸发水损失（X）估计值的最大误差将导致rCO2被低估0.81%。我们建议，在涉及反刍动物的DLW研究中，使用此处给出的校正因子来补偿粪便中的氢损失。在日粮粗饲料含量高导致粪便干物质产量显著的情况下，这可能尤为重要。
