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where FCo2 is mol. RQ is calculated from dietary information or assumed to have a particular population value, such as 0.85.

Insertion of typical Western adult values (NO = 2000, Nd = 2066, KO = 0.12, and KD = 0.10) into the relevant equations and 'what if' experimentation will allow the reader to test the effect of making changes to the assumptions and values. Table 2 provides examples that show that serious errors or bias, for groups or individuals, are unlikely unless the applied population means for assumed values are grossly incorrect or the coefficient of variation (CV) is large.

Experimentation with the data, however, will also show that the magnitude of the difference between KONO and KDND is crucial. The method depends on precisely determining a relatively small difference between these two experimentally measured, larger values. This difference is approximately 20% in the example but can be much less when water turnover is high relative to carbon dioxide production (e.g., very young infants or subjects living in the tropics).

For the slopes (KO and KD) a minimum of two time points are required sufficiently far apart in time (two or three biological half-lives) to allow good precision on the slope determination with doses of sufficient magnitude to avoid detrimental effects of natural abundance variations and the limitations of analytical precision, especially at the end of the measurement period. In some protocols, more than two samples are measured, and this permits error calculations based on the goodness of fit of the data. Isotope distribution

Table 2 'What if' calculations for a typical subject (NO = 2000, Nd = 2066, KO = 0.12, Kd = 0.10)

Fractionated water losses defined in CO2 production relative terms of Fco2 (Ff = 2.1 Fco2) for mean and assumed CV = 10% -2 SD = 1.68 FCO2 Mean = 2.1 Fco2 +2 SD = 2.58 Fco2 Assumed RQ (typical mean ±2 SD)

to value for mean

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