Inborn errors of fructose metabolism

Most people in developed countries ingest 50 to 150 g fructose equivalents daily in their diet and the use of this sugar in food and drinks is increasing globally. Fructose is absorbed rapidly by a carrier mechanism that facilitates transport across the intestinal epithelium, metabolized (mainly in the liver) by the enzymes ketohexokinase (fructokinase), aldolase B, and triokinase, and eventually converted into glucose or glycogen. Dietary sugar has particular effects on those whose capacity to metabolise fructose is limited. Fructose occurs either as a free monosaccharide, as a component of sucrose, a disaccharide from which it is released by digestion; fructose may also be derived from the metabolism of the sugar alcohol, sorbitol....

Absorption and metabolism of fructose by rat jejunum

The absorption and metabolism of fructose was investigated in the vascularly perfused jejunum of fructose-fed rats. With 10 mM-glutamate and 10 mM-fructose in the lumen, the viability of the tissue is maintained and fructose is absorbed and utilized at high rates. With 28 mM-fructose in the lumen, glucose appears in the vascular bed. With 10 mM- or 28 mM-fructose in the presence of 10 mM- or mM-glucose in the lumen, the fructose absorption is decreased. From 10 mM- or 28 mM-sucrose in the lumen, fructose uptake is also less than from the equivalent concentration of free fructose. The rate of appearance of fructose in the vascular bed is independent of the source of fructose from which it is derived. In the presence of glucose, either free or as sucrose, there is a marked decrease in the utilization of fructose, defined as the difference between that absorbed by the jejunum and that transported unchanged into the vascular bed. In all cases about half of the carbohydrate absorbed from the lumen is converted into lactate, most of which is secreted into the blood. The absorption of glucose and the rate of vascular appearance of glucose from glucose in the lumen are about 1.5 times greater than those of fructose from fructose in the lumen. It is concluded: firstly, that fructose uptake from the lumen of rat jejunum is determined by its concentration and by the demand for it as a fuel for the intestine, a demand that is severely decreased in the presence of glucose; secondly, that in the vascularly perfused jejunum there is no evident kinetic advantage for uptake of fructose or glucose from sucrose rather than from free monosaccharide in the lumen; thirdly, that some fructose can be converted into glucose.