Impact of 2H and 18O pool size determinations on the calculation of total energy expenditure

Measurement of total energy expenditure using [2H,18O] water requires both accurate and precise determination of the rates of disappearance of 2H and 18O from body water over time and determination of the 2H and 18O pool sizes. However, the impact of the isotopic determination of body water upon the determination of energy expenditure is often overlooked. For measurement of total body water per se, the delay after administration before sampling body fluids becomes important, and saliva sampling can be used to resolve the timing of early samples for body water determination. For energy expenditure measurement per se, linear regression can be used to define the initial dilution. Because the hydrogen tracer dilutes into a pool significantly larger than body water pool per se due to the presence of labile hydrogens, a correction to the isotope pool size must be applied. The theoretical calculations of the exchangeable hydrogen pool presented here suggest that the hydrogen pool size is < 3% greater than the body water pool and data are provided to support this idea. Finally, the two approaches used to define the body water pool space contribution to the calculation of energy expenditure using 2H2(18)O are reviewed. Using a pool size based upon the average of the two pool spaces limits the effect of pool size error in the calculation of energy expenditure.