Maintenance of a normal plasma glucose concentration requires precise matching of glucose utilization and endogenous glucose production or dietary glucose delivery. Glucose is derived from 3 sources: the intestinal absorption that follows the digestion of dietary carbohydrates, glycogenolysis, and gluconeogenesis. Glucose is transported into cells through multiple metabolic pathways: it may be stored as glycogen; it may undergo glycolysis to pyruvate; finally, it may be released into the circulation by the liver and kidneys, the sole organs containing glucose-6-phosphatase, the enzyme necessary for the release of glucose into the circulation.
Information about normal hepatic glucose metabolism may help to understand pathogenic mechanisms underlying obesity and diabetes mellitus. In addition, liver glucose metabolism is involved in glycosylation reactions and connected with fatty acid metabolism. The liver receives dietary carbohydrates directly from the intestine via the portal vein. Glucokinase phosphorylates glucose to glucose 6-phosphate inside the hepatocyte, ensuring that an adequate flow of glucose enters the cell to be metabolized. Glucose 6-phosphate may proceed to several metabolic pathways. During the post-prandial period, most glucose 6-phosphate is used to synthesize glycogen via the formation of glucose 1-phosphate and UDP–glucose. Minor amounts of UDP–glucose are used to form UDP–glucuronate and UDP–galactose, which are donors of monosaccharide units used in glycosylation.