Hepatic glycogen in humans. I. Direct formation after oral and intravenous glucose or after a 24-h fast.

The formation of hepatic glycogen by the direct pathway is assessed in humans 1) after a 12-h fast and oral loading (100 g) or 2) intravenous infusion (90 g) and 3) after a 24-h fast and the same oral glucose load. The methodology used is based on the double tracer method. [3-3H]glucose is infused at a constant rate for the determination of the metabolic clearance of glucose. [1-14C]glucose is administered with the glucose load. One hour after absorption or the intravenous glucose infusion is terminated, a glucagon infusion is initiated to mobilize the glycogen labeled with [1-14C]glucose and formed during the absorptive period. At this time a third tracer, [6-3H]glucose, is administered to measure glucose clearance. It was found that after the 12-h fast and oral glucose loading 7.2 +/- 1.1 g of hepatic glycogen appears to be formed directly from glucose compared with 8.4 +/- 1.0 g after the same load and a 24-h fast and 8.5 +/- 0.4 g after a 12-h fast and an equivalent intravenous glucose infusion. When the amount of label ([14C]glucose) mobilized that was not corrected for metabolic recycling was calculated, the data suggested that the amount of glycogen formed by gluconeogenic pathways was probably at least equal to that formed by direct uptake. It was also approximately 60% greater after a 24-h fast. It can be concluded that the amount of hepatic glycogen formed directly from glucose during glucose loading is not significantly altered by the route of entry or the extension of the fasting period to 24 h. The data suggest, however, that gluconeogenetic formation of glycogen increases with fasting.