Isotopic interactions between carbohydrate and protein turnover

A reasonably plausible model representing the interactions between carbohydrate, protein and lipid metabolism in the fasted state is presented. Using computer solutions, we fit the model to literature data. The data and model are insufficient for totally rigorous conclusions, but they suggest tentatively that in 60 hr fasted man, a considerable fraction of the gluconeogenic flux may proceed (in the liver or kidney) directly from Krebs cycle intermediates, not through pyruvate. Using the [U-C-14]glucose incorporation into protein found by Shipley et al. [Am. J. Physiol. 213: 1149-1158, 1967], we illustrate the effect that the isotopic interactions betwen protein and carbohydrate turnover have in making difficult the use of simple models, e.g. for the estimation of gluconeogenesis. Ne also outline a possible approach for the estimation of the kidney contribution to gluconeogenesis.