Isotopic estimation of the hepatic glucose balance in vivo.

Conventional determination of "hepatic glucose production" in the refed state is based on the 30-year-old Steele model, which omits glucose<-->glucose-6P cycling. Using the more complete model, conventional hepatic glucose production is shown to be a complex ratio of fluxes, and not a simple physiological flux. Our new model allows some prospective isotopic approaches to the estimation of glucose-6-phosphatase and glucokinase fluxes, and thus real net hepatic glucose production or uptake.