title: Liver glycogen turnover in fed and fasted humans.
abstract: Whether liver glycogen synthesis and breakdown occur simultaneously during net glycogen synthesis was assessed in fed and fasted healthy humans. The peak intensity of the carbon-1 (C1) resonance of the glycosyl units of glycogen was monitored with 13C nuclear magnetic resonance spectroscopy during [1-13C]glucose infusion followed by unlabeled glucose infusion. The C1 peak intensity increased almost linearly during the [1-13C]glucose infusion, reflecting a near linear rate of glycogen synthesis. When switched to unlabeled glucose, the C1 peak intensity reached a plateau in the fasted subjects and declined in the fed subjects, reflecting active glycogenolysis during a time of net glycogen synthesis. We conclude that liver glycogen synthesis and degradation occur simultaneously in humans under conditions of net glycogen synthesis. The relative turnover rate was significantly higher in the fed (57 +/- 3%) than in the fasted state (31 +/- 8%; P < 0.01). The results indicate that glycogen may regulate its rate of breakdown and that liver glycogen turnover may be an important factor in limiting the accumulation of liver glycogen in humans.
authors.affiliation: Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut 06510.
authors.forename: I
authors.initials: I
authors.lastname: Magnusson
authors.forename: D L
authors.initials: DL
authors.lastname: Rothman
authors.forename: B
authors.initials: B
authors.lastname: Jucker
authors.forename: G W
authors.initials: GW
authors.lastname: Cline
authors.forename: R G
authors.initials: RG
authors.lastname: Shulman
authors.forename: G I
authors.initials: GI
authors.lastname: Shulman
mesh_terms: D000328:Adult; D001786:Blood Glucose / Q000378:metabolism; D002247:Carbon Isotopes; D004435:Eating / Q000502:physiology*; D005215:Fasting / Q000502:physiology*; D005947:Glucose / Q000378:metabolism*; D015309:Glucose Clamp Technique; D006801:Humans; D006943:Hyperglycemia / Q000378:metabolism; D007328:Insulin / Q000097:blood*; D007700:Kinetics; D008112:Liver Glycogen / Q000096:biosynthesis / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D008433:Mathematics; D008954:Models, Biological; D014532:Uridine Diphosphate Glucose / Q000378:metabolism
chemical_list: D001786:Blood Glucose; D002247:Carbon Isotopes; D007328:Insulin; D008112:Liver Glycogen; D005947:Glucose; D014532:Uridine Diphosphate Glucose