Galactose is one of the monosaccharides of importance in human nutrition. It is converted to glucose-1-phosphate in the liver and subsequently stored as glycogen, or is converted to glucose and released into the circulation. The increase in plasma glucose is known to be modest following galactose ingestion. Whether this is due to a small increase in hepatic glucose output, or to a relatively large increase in hepatic glucose output but a concomitant increase in glucose disposal, is not known in humans. Therefore, the rates of glucose appearance (R a ) and disappearance (R d ) were determined over an 8-hour period in normal subjects using an isotope dilution technique. The subjects ingested 50 g galactose or water alone in random order at 8 AM on separate occasions. Plasma glucose, glucagon, lactate, urea nitrogen, total amino acids, and uric acid and serum insulin and triglycerides also were determined. Following galactose ingestion, there was a modest transient increase in peripheral glucose and insulin concentrations. This was associated with a modest increase in the glucose R a . The calculated amount of glucose appearing in the circulation as a result of galactose ingestion was 9.8 g, while the amount of glucose disappearing over the 8 hours was 9.9 g. Thus, following ingestion of 50 g galactose by overnight-fasted men, approximately 20% appears as additional glucose in the circulation. Data obtained in animals suggest that a large amount of the galactose is stored as glucose in glycogen. Nevertheless, the conversion of galactose to glucose in the liver may have been greater than suggested by the increase in glucose appearance in the circulation due to substitution for other gluconeogenic substrates. Copyright © 2001 by W.B. Saunders Company G LUCOSE, GALACTOSE, AND FRUCTOSE are the major monosaccharides absorbed following the ingestion of a mixed meal. Galactose (milk sugar) is quantitatively the least important of the three, except in infancy. It is almost always ingested in the form of lactose, a disaccharide composed of glucose and galactose. The hydrolysis of lactose in the intestine is so rapid that in the circulation the rate of increase of the products of its hydrolysis (ie, glucose and galactose) is largely dependent on the rate at which they can be absorbed, 1 except in people with lactose deficiency. The latter is uncommon in Caucasians. 2 The absorbed galactose also is readily removed by the liver. 3 Indeed, in the past, it has been used to assess the general functional capacity of this organ. 4 In the liver, galactose is readily converted to glucose-1-phosphate and subsequently stored as glucose in glycogen. [5][6][7] It also may be converted to glucose and released into the circulation. 5-7 However, galactose increases the plasma glucose concentration only modestly. Ingestion of relatively large amounts of galactose (25 to 50 g) results in a mean increase of the peripheral circulating glucose of only 1 mmol/L (18 mg/dL) or less in normal adults and in people with type 2 diabetes. 1,[8][9][10][1...