Energy expenditure from doubly labeled water: some fundamental considerations in humans.

The isotopic loading dose and metabolic period for the measurement of energy expenditure in humans by the doubly labeled water method were predicted by a propagation of error analysis. Factors considered for sources of error were analytical errors in the mass spectrometric determination of isotopic enrichments, biological variation in the isotopic enrichments, uncertainties in total body water (TBW), evaporative water loss, and metabolic fuel. The predicted optimal isotopic loading doses were 0.3 g H2 18O/kg TBW and 0.12 g 2H2O/kg TBW for all subjects except neonates in which they were 0.4 g H2 18O/kg TBW and 0.16 g 2H2O/kg TBW. The optimal metabolic periods were 3 to 14 days in neonates and children, and 5 to 28 days in adults. The theoretical coefficient of variation of the doubly labeled water method for the measurement of energy expenditure is between 4 and 8%.