REVISION OF CALCULATIONS IN THE DOUBLY LABELED WATER METHOD FOR MEASUREMENT OF ENERGY-EXPENDITURE IN HUMANS

In the doubly labeled water (DLW) method for the measurement of energy expenditure in humans; the basis of the calculation for CO2 productio n is the difference between the products of the rate constants for the disappearance of O-18 and H-2 from body water (K-O, and K-D, respecti vely) and the matching isotope dilution spaces (N-O, and N-D, respecti vely). Thus, omitting corrections for isotope fractionation, CO2 produ ction = 0.5 (KONO - KDND). In this calculation, it is also customary t o normalize observed N-O and N-D values to a fixed value for N-D/N-O. The increasing use of the method has resulted in the generation of sub stantially more information on the normal value for N-D/N-O than exist ed at the time the method was first developed, and recent work has sug gested that revisions of the originally used value of 1.03 may now be deemed appropriate. Values of 1.034 or 1.0427 have recently been sugge sted, but when applied in energy expenditure studies these estimates w ould lead to significantly different expenditure measurements. It can, however, be shown from published work and direct experimental study t hat N-D/N-O values are method dependent, and for these reasons the low er revised value of 1.034 appears to be more acceptable. The possibili ty that particular populations may ultimately be shown to be different from 1.034 should not, however, be dismissed entirely, and for this r eason we suggest that information derived in individual experiments co uld be used in a Bayesian fashion to generate new N-D/N-O estimates. T he appropriate techniques are described.