Fuel and energy metabolism in fasting humans

• Published in: The American journal of clinical nutrition Vol. 60; no. 1; pp. 29 - 36
• Main Authors: Carlson, MG, Snead, WL, Campbell, PJ
• Format: Journal Article
• Language: English
• Published: Bethesda, MD Elsevier Inc 01-07-1994; American Society for Clinical Nutrition; American Society for Clinical Nutrition, Inc
• Subjects: Adipose Tissue - metabolism; Adult; Biological and medical sciences; Blood Glucose - metabolism; Carbohydrate Metabolism; Chromatography, High Pressure Liquid; Energy Metabolism; Fasting; Fasting - metabolism; Fatty Acids, Nonesterified - blood; free fatty acids; Fundamental and applied biological sciences. Psychology; Gluconeogenesis; glycerol; Hormones - blood; Humans; Insulin - blood; Intermediate and energetic metabolism; Lipid Metabolism; Lipolysis; Male; Metabolisms and neurohumoral controls; Proteins - metabolism; proteolysis; Vertebrates: anatomy and physiology, studies on body, several organs or systems; free fatty acids; Fasting; lipolysis; gluconeogenesis; glycerol; proteolysis; Human; Proteins; Proteolysis; Glycerol; Lipids; Carbohydrate; Free fatty acid; Metabolism; Lipolysis; Gluconeogenesis
• ISSN: 0002-9165; 1938-3207
• DOI: 10.1093/ajcn/60.1.29

Fuel and energy homeostasis was examined in six male volunteers during a 60-h fast by using a combination of isotopic tracer techniques ([3-3H]glucose, [2H5]glycerol, [1-14C]palmitate, and l-[1-13C]leucine) 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 µmol·kg−1·min−1, respectively, both P < 0.001) and glucose oxidation declined ≈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 µmol·kg−1·min−1 and 1.5 ± 0.4 to 4.2 ± 0.8 µ 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 µ mol·kg−1·min−1, P < 0.01), which contributed ≈75% of resting energy requirements after the 60-h fast and increased the supply of glycerol for gluconeogenesis. Proteolysis and protein oxidation increased ≈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.