AJP - Regulatory, Integrative and Comparative Physiology, Vol 251, Issue 6 1137-R1143, Copyright © 1986 by American Physiological Society
Doubly labeled water method: in vivo oxygen and hydrogen isotope fractionation
D. A. Schoeller, C. A. Leitch and C. Brown
The accuracy and precision of the doubly labeled water method for measuring
energy expenditure are influenced by isotope fractionation during
evaporative water loss and CO2 excretion. To characterize in vivo isotope
fractionation, we collected and isotopically analyzed physiological fluids
and gases. Breath and transcutaneous water vapor were isotopically
fractionated. The degree of fractionation indicated that the former was
fractionated under equilibrium control at 37 degrees C, and the latter was
kinetically fractionated. Sweat and urine were unfractionated. By use of
isotopic balance models, the fraction of water lost via fractionating
routes was estimated from the isotopic abundances of body water, local
drinking water, and dietary solids. Fractionated water loss averaged 23%
(SD = 10%) of water turnover, which agreed with our previous estimates
based on metabolic rate, but there was a systematic difference between the
results based on O2 and hydrogen. Corrections for isotopic fractionation of
water lost in breath and (nonsweat) transcutaneous loss should be made when
using labeled water to measure water turnover or CO2 production.
