ROBERT WOLFE: I presented values derived almost entirely in steady state conditions. The kinetic modeling in nonsteady state with a stable isotope is more complex, but I would say it can be done. But I think that in terms of nutritional aspects and total substrate oxidation I would definitely recommend staying away from that. The problems, however, are more with modeling than isotope discrimination.

DENNIS BIER: With regard to the isotope effects, there are rather sizable isotope effects if you are talking about deuterium and tritium. If you look at the mass differences that are smaller, I mean, people have investigated ¹³C and ¹²C isotope effects on the molecular level, on the biochemical level, on the cellular level, and even with totally ¹³C-labeled enzymes, for example, compared to their substrates, the differences are about 1 to 2 percent. I mean, they are small enough that we cannot measure them.

Now, we can tell those differences by isotope ratio mass spectrometry because that can measures parts per 100,000. That is exactly the basis of isotope fractionation in breath, the kind of thing Bob talked about with substrate oxidation, but it is below the limits of any detection that you can do.

DONALD McCORMICK: At steady state.

DENNIS BIER: Right. There is at least one set of experiments that I know of where people tried to look at transport in an organ with different isotope-labeled materials, specifically glucose, and that was done by Riccardo Bonnadonna, Ralph DeFronzo, Clyde Epidelli, myself, and others, where we gave ¹³C with different labels and modeled the differences between those isotopes as they appeared inside the cell and they appeared in metabolites, et cetera, to discriminate transport, and in fact, we think we were successful.