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 ([3-3H]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 mumol.kg-1.min-1, respectively, both P < 0.001) and glucose oxidation declined approximately 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 mumol.kg-1.min-1 and 1.5 +/- 0.4 to 4.2 +/- 0.8 mumol.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 mumol.kg-1.min-1, P < 0.01), which contributed approximately 75% of resting energy requirements after the 60-h fast and increased the supply of glycerol for gluconeogenesis. Proteolysis and protein oxidation increased approximately 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.
Department of Medicine, Vanderbilt University School of Medicine, Nashville, TN 37232-2230.
M G
MG
Carlson
W L
WL
Snead
P J
PJ
Campbell
D000273:Adipose Tissue / Q000378:metabolism*; D000328:Adult; D001786:Blood Glucose / Q000378:metabolism; D050260:Carbohydrate Metabolism; D002851:Chromatography, High Pressure Liquid; D004734:Energy Metabolism; D005215:Fasting / Q000378:metabolism*; D005230:Fatty Acids, Nonesterified / Q000097:blood; D005943:Gluconeogenesis; D006728:Hormones / Q000097:blood; D006801:Humans; D007328:Insulin / Q000097:blood; D050356:Lipid Metabolism; D008066:Lipolysis; D008297:Male; D011506:Proteins / Q000378:metabolism
D001786:Blood Glucose; D005230:Fatty Acids, Nonesterified; D006728:Hormones; D007328:Insulin; D011506:Proteins