F

KoNo - KdNd

Assumption 6 relates to the requirement that a predose sample should represent the effect of normal natural abundance isotope input. In most cases, background isotopic enrichment is likely to vary only randomly during a measurement period and so the issues are about the relationship between the background sample measured, the mean background and its random variation during the experimental period, the extent to which background variations in 2H and 18O are covariant, and the size of isotope doses and postdose enrichments in relation to these variations. In most experimental situations investigated with affordable isotopic doses, background variation contributes to the internal errors of the method and limits the extent to which better analytical precision improves results. In some circumstances (e.g., subjects moving from one place to another and use of large amounts of rehydration fluids in hospitalised patients), it is possible that a predose sample taken to represent isotopic background is not at all meaningful and the best advice may be to avoid these circumstances rather than try to correct for them.

Finally, FCo2 values have to be converted into values for energy expenditure based on a fixed relationship between these quantities that depends on metabolic fuels used, expressed as a respiratory quotient (RQ). We can write

Energy expenditure (kJ) = FCOl

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