Intestinal metabolism of orally administered glucose and fructose in Yucatan miniature swine.

Absorption rates and metabolism of orally administered glucose and fructose were quantitated in conscious, overnight-fasted, Yucatan miniature swine. The pigs were surgically prepared, with catheters in the portal vein and a carotid artery. A continuous wave Doppler flow cuff was placed around the portal vein. Four to 7 days after surgery, arterial and portal venous blood samples were obtained in the basal state and at 15-minute intervals for 4 hours following oral administration of glucose or fructose (1.5 g/kg body weight). Absorption rates and intestinal metabolism were calculated from portal blood flow and arterio-venous concentration differences for glucose, fructose and lactate. Oral fructose administration was accompanied by a rise in intestinal lactate production, which correlated significantly with the fructose absorption rate. Intestinal lactate output during fructose absorption could account for 12% of the fructose absorbed or metabolized. A small rise in glucose output from the gut was also observed. In contrast, after oral glucose administration, only a modest rise in intestinal lactate output was noted. It could account for only 2% of the glucose absorbed or metabolized. No correlation between glucose absorption rate and intestinal lactate output was observed. These studies indicate that a significant portion of oral fructose is converted to lactate during absorption but that the major part enters the portal vein as fructose. Only minimal intestinal metabolism of ingested glucose occurs.