AJP - Regulatory, Integrative and Comparative Physiology, Vol 264, Issue 3 561-R567, Copyright © 1993 by American Physiological Society
The doubly labeled water method: errors due to deuterium exchange and sequestration in ruminants
A. J. Midwood, P. Haggarty and B. A. McGaw
Rowett Research Institute, Aberdeen, United Kingdom.
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.
