The doubly labeled water method: errors due to deuterium exchange and sequestration in ruminants

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.