There seems to be many ways to determine the fractional pool volume for use in the Steele equation when considering glucose kinetics. I am interested to know how people have justified their use of a particular volume in relation to the circumstances they have used it in. Currently I have used anything from 40ml to 200ml in humans during exercise but find it difficult to justify my usage of this volume. Any advice or guidance from people's experiences would be appreciated.
It is shown that this model introduces an error with two components, one dependent on the volume of the compartment, the other related to the complex configuration of the system.
This number is of course arbitrary and really only a 'fix' for dealing with non-steady state. It reflects the non-equilibrium of tacer and tracee throughout the body. Experimental design can have a major effect. If you have steady state before your intervention this make interpretation of the data easier. You can try different values and ensure you don't end up with negative Ra but interpretation of the data is easy without needing to calculate numbers - see attached paper as example
There is a very useful section on this topic in the book "Isotope tracers in Metabolic Research" by Robert Wolfe and David Chinkes. Basically they show that the best solution over a range of sampling times will be given by using a low value - they show 40mL. This is because the fractional pool volume will be low at the start of the infusion and become larger over time, but that the contribution of the non-steady state part of the equation will decrease with increasing sampling time (i.e. the magnitude of error introduced by an incorrect fractional pool volume will decrease over time).
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