Effect of bolus fluid intake on energy expenditure values as determined by the doubly labeled water method

The doubly labeled water (DLW, ²H₂¹⁸O) method is a highly accurate method for measuring energy expenditure (EE). A possible source of error is bolus fluid intake before body water sampling. If there is bolus fluid intake immediately before body water sampling, the saliva may reflect the ingested water disproportionately, because the ingested water may not have had time to mix fully with the body water pool. To ascertain the magnitude of this problem, EE was measured over a 5-day period by the DLW method. Six subjects were dosed with ²H₂¹⁸O. After the reference salivas for the two-point determination were obtained, subjects drank water (700-1,000 ml), and serial saliva samples were collected for the next 3 h. Expressing the postbolus saliva enrichments as a percentage of the prebolus value, we found 1) a minimum in the saliva isotopic enrichments were reached at ~30 min with the minimum for ²H (95.48 ± 0.43%) being significantly lower than the minimum for ¹⁸O (97.55 ± 0.44, P < 0.05) and 2) EE values calculated using the postbolus isotopic enrichments are appreciably higher (19.9 ± 7.5%) than the prebolus reference values. In conclusion, it is not advisable to collect saliva samples for DLW measurements within ~1 h of bolus fluid intake.