Influence of delayed isotopic equilibration in urine on the accuracy of the 2H2 18O method in the elderly

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Abstract
Isotopic determination of total energy expenditure (TEE) by the doubly labeled water (DLW) method may be affected by urine retention in the elderly. The isotopic enrichments in urine and plasma sampled simultaneously 4 h post-DLW dose were compared in a subset of 281 subjects [139 women, 142 men, 75 ± 3 (SD) yr] of the 3,075 participants in the Health, Aging, and Body Composition study. Based on analytic precisions, a ±2% urine-plasma difference was set as the cut-off value. Ten percent of the population presented a difference lower than −2%, suggesting a delay in urine isotopic equilibration. This −13 ± 10% urine-plasma difference was not linked to analytic errors, illnesses, the sampling time, or the time and quantity of water intake, suggesting that urine retention may be the main factor. The consequences are an 18 ± 13 and 21 ± 16% overestimation of the total body water and the TEE, respectively. Unexpectedly, 21% of the population presented a urine-plasma difference higher than ±2% that resulted, however, in a nonsignificant TEE underestimation of −3 ± 5%. In conclusion, the delayed isotopic equilibration observed in urine reduces the accuracy of the DLW method in the elderly. It is recommended, when blood sampling is impossible, to adopt the intercept method with urine sampling 24 h postdose.