The contribution of naturally labelled 13C fructose to glucose appearance in humans.

Among monosaccharides, fructose has a small hyperglycaemic effect. In order to better explain the mechanisms which cause this metabolic property, we used tracers labelled with stable isotopes (deuterated glucose and naturally 13C labelled fructose) to quantify the overall glucose appearance, the rate of appearance in plasma of the 13C glucose synthesized from fructose, and the fructose oxidation in vivo in man during a 6-h period following ingestion of 0.5 and 1 g.kg-1 fructose. Fructose had a very small effect on overall glucose appearance (NS). During the 6 h of the study, it was found that the overall glucose appearance was 0.87 +/- 0.06 and 0.89 +/- 0.06 g.kg-1 (NS). The amount of glucose synthesized from fructose was 0.27 +/- 0.04 and 0.51 +/- 0.03 g.kg-1 (p < 0.01) representing 31% and 57% of overall glucose appearance (p < 0.01); the non-fructose glucose production was 0.60 +/- 0.02 and 0.38 +/- 0.03 g.kg-1 (p < 0.05) after the 0.5 and 1 g.kg-1 load, respectively. Fructose oxidation was 0.28 +/- 0.03 and 0.59 +/- 0.07 g.kg-1 after the 0.5 and 1 g.kg-1 load respectively (p < 0.01) representing 56% and 59% of the fructose loads (NS). These data show that the low hyperglycaemic effect of fructose is explained by its very small effect on overall glucose appearance and that fructose has a sparing effect on glucose metabolism.