FUEL AND ENERGY-METABOLISM IN FASTING HUMANS

Fuel and energy homeostasis was examined in six male volunteers during a 60-h fast by using a combination of isotopic tracer techniques ([3-H-3]glucose, [H-2(5)]glycerol, [1-C-14]palmitate, and L-[1-C-13]leuccne) and indirect calorimetry. Plasma glucose concentration and hepatic glucose production decreased by 30% with fasting (5.2 +/- 0.1 to 3.8 +/- 0.2 mmol/L and 11.8 +/- 0.5 to 8.2 +/- 0.6 mu mol.kg(-1).min(-1), respectively, both P < 0.001) and glucose oxidation declined approximate to 85% (P < 0.01). Lipolysis and primary (intraadipocyte) free fatty acid (FFA) reesterification increased 2.5-fold (1.7 +/- 0.2 to 4.2 +/- 0.2 mu mol kg(-1).min(-1) and 1.5 +/- 0.4 to 4.2 +/- 0.8 mu mol.kg(-1).min(-1), respectively, both P < 0.05). This provided substrate for the increase in fat oxidation (from 2.7 +/- 0.3 to 4.3 +/- 0.1 mu mol.kg(-1).min(-1), P < 0.01), which contributed approximate to 75% of resting energy requirements after the 60-h fast and increased the supply of glycerol for gluconeogenesis. Proteolysis and protein oxidation increased approximate to 50% during fasting (P < 0.01 and P < 0.05, respectively). We conclude that the increase in FFA reesterification with fasting modulates FFA availability for oxidation and maximizes release of glycerol from triglyceride for gluconeogenesis.