Splanchnic metabolism was studied in the fed state during prolonged intravenous administration (30 g/h) of either fructose or glucose to hypertriglyceridemic men who had been maintained on a high-carbohydrate diet for 2 wk. Splanchnic exchange of amino acids and carbohydrates was quantified by measurement of splanchnic flow and of blood or plasma arteriohepatic venous concentration gradients. Results obtained in subjects receiving fructose were compared with those obtained in (a) similar subjects receiving glucose and (b) postabsorptive controls maintained on isocaloric, balanced diets. Mean arterial plasma levels of alanine, glycine, serine, threonine, methionine, proline, valine, leucine, histidine, lysine, and ornithine were significantly higher in subjects given fructose than in those give glucose (P less than 0.05). The mean arterial concentration and splanchnic uptake of alanine were significantly higher in subjects given fructose than in postabsorptive controls, despite a significantly lower fractional extraction of alanine in the former (P less than 0.05). The mean arterial plasma levels of serine and ornithine were significantly lower in subjects receiving fructose than in postabsorptive controls (P less than 0.05). About half of the administered fructose or glucose was taken up in the splanchnic region, where approximately 15% was converted to CO2 and 10% to lactate. Half of the fructose taken up in the splanchnic region was converted to glucose released from the liver. The amount of hexose carbon remaining for hepatic synthesis of liquids in subjects given fructose was less than half of that of subjects given glucose. These studies demonstrate that fructose and glucose have divergent effects on amino acid metabolism and that during hypercaloric infusion of glucose (as with fructose), the human liver is a major site of lactate production.
B M Wolfe, S P Ahuja, E B Marliss