Plasma glucose and free fatty acid metabolism in normal and long-fasted dogs

Abstract

Plasma glucose and plasma free fatty acid turnover and prompt conversion to CO2 were studied in the dog in the postabsorptive state and after 7-11 days of fasting. Glucose-U-Cl4 [uniformly labeled with Cl4] and palmitate-l-Cl4 were used in these studies. Total CO2 output, plasma glucose concentration, and plasma free fatty acid concentration were also measured. In the postabsorptive state, carbon derived promptly from plasma glucose forms 16% of the total CO2; carbon from plasma free fatty acids, 25%. The CO2 derived promptly from plasma glucose represents 47% of the total uptake by the tissues of plasma glucose; that from plasma free acids 35% of total uptake of plasma free fatty acids. In the long-fasted state total CO2 production is reduced by about 22%. to this state plasma glucose turnover and prompt conversion to CO2 decrease in proportion, with no change in plasma glucose concentration; plasma free fatty acid concentration and turnover increase markedly and the prompt conversion of free fatty acids to CO2 increases to 47% of total free fatty acid uptake. The result is that CO2 carbon derived promptly from plasma glucose then constitutes 14% of total CO2 expired; that from plasma free fatty acids, 61%. It is shown that the yields of C14O2 from the tagged circulating metabolites indicate minimum, rather than exact, values for the contributions of these metabolites to energy production.