Validation of the doubly labeled water method in rats during isolation andsimulated weightlessness

Total energy expenditure (TEE) of rats during simulated microgravity is unk nown. The doubly labeled water method (DLW) reliably measures TEE, but the results depend on the methods of calculation. These methods were validated and appraised by indirect calorimetry in eight rats during isolation (7 day s) and simulated microgravity (10 days). There were no effects on CO2 produ ction in the method used to derive constant flux rates as in the regression models. r(CO2) estimates were dependent on the assumed fractionation proce sses, the derivation of constant flux rate methods, and the selected pool m odels. Use of respiratory or food quotients did not influence TEE estimatio ns, which were similar during isolation and simulation. During either isola tion with growth or simulation with a stabilized mass, the one-pool model o f Speakman (Speakman JR. Doubly Labelled Water. Theory and Practice. London : Chapman and Hall, 1997) resulted in the more reliable validation (0.8 +/- 2.2 and 2.2 +/- 3.4% vs. calorimetry, respectively). However, during simul ation, agreement was also observed with the single pool model of Lifson (Li fson N, Gordon GB, and McClintock R. J Appl Physiol 7: 704-710, 1955) (-2.5 +/- 2.5%), and two two-pool models [Schoeller (Schoeller DA. J Nutr 118: 1 278-1289, 1988) (0.5 +/- 3.1%) and Speakman (Speakman, JR. Doubly Labelled Water. Theory and Practice. London: Chapman and Hall, 1997) (-1.9 +/- 2.7%) ]. This latter finding seems linked to the stable body mass and to fraction ation consideration close to the single-pool model of Speakman. During isol ation or simulated microgravity, the other equations underestimated TEE by 10-20%.