To determine the contributions of galactose and fructose to glucose formation, 6 subjects (26 +/- 2 years old; body mass index, 22.4 +/- 0.2 kg/m(2)) (mean +/- SE) were studied during fasting conditions. Three subjects received a primed constant intravenous infusion of [6,6-(2)H(2)]glucose for 3 hours followed by oral bolus ingestion of galactose labeled to 2% with [U-(13)C]galactose (0.72 g/kg); the other 3 subjects received a primed constant intravenous infusion of [6,6-(2)H(2)]glucose followed by either a bolus ingestion of fructose alone (0.72 g/kg) (labeled to 2% with [U-(13)C]fructose) or coingestion of fructose (labeled with [U-(13)C]fructose) (0.72 g/kg) and unlabeled glucose (0.72 g/kg). Four hours after ingestion, subjects received 1 mg of glucagon intravenously to stimulate glycogenolysis. When galactose was ingested alone, the area under the curve (AUC) of [(13)C(6)]glucose and [(13)C(3)]glucose was 7.28 +/- 0.39 and 3.52 +/- 0.05 mmol/L per 4 hours, respectively. When [U-(13)C]fructose was ingested with unlabeled fructose or unlabeled fructose plus glucose, no [(13)C(6)]glucose was detected in plasma. The AUC of [(13)C(3)]glucose after fructose and fructose plus glucose ingestion was 20.21 +/- 2.41 and 6.25 +/- 0.34 mmol/L per 4 hours, respectively. Comparing the AUC for the (13)C(3) vs (13)C(6) enrichments, 67% of oral galactose enters the systemic circulation via a direct route and 33% via an indirect route. In contrast, fructose only enters the systemic circulation via the indirect route. Finally, when ingested alone, fructose and galactose contribute little to glycogen synthesis. After the coingestion of fructose and glucose with the resultant insulin response from the glucose, fructose is a significant contributor to glycogen synthesis.