LIVER-GLYCOGEN TURNOVER IN FED AND FASTED HUMANS

Whether liver glycogen synthesis and breakdown occur simultaneously du ring net glycogen synthesis was assessed in fed and fasted healthy hum ans. The peak intensity of the carbon-1 (C1) resonance of the glycosyl units of glycogen was monitored with C-13 nuclear magnetic resonance spectroscopy during [1-C-13]glucose infusion followed by unlabeled glu cose infusion. The C1 peak intensity increased almost linearly during the [1-C-13]glucose infusion, reflecting a near linear rate of glycoge n synthesis. When switched to unlabeled glucose, the C1 beak intensity reached a plateau in the fasted subjects and declined in the fed subj ects, reflecting active glycogenolysis during a time of net glycogen s ynthesis. We conclude that liver glycogen synthesis and degradation oc cur simultaneously in humans under conditions of net glycogen synthesi st The relative turnover rate was significantly higher in the fed (57 +/- 3%) than in the fasted state (31 +/- 8%; P < 0.01). The results in dicate that glycogen may regulate its rate of breakdown and that liver glycogen turnover may be an important factor in limiting the accumula tion of liver glycogen in humans.