Fructolysis refers to the
metabolism of
fructose from
dietary sources. Though the
metabolism of
glucose through
glycolysis uses many of the same
enzymes and intermediate structures as those in fructolysis, the two sugars have very different metabolic fates in human metabolism. Unlike glucose, which is directly metabolized widely in the body, fructose is almost entirely metabolized in the liver in humans, where it is directed toward replenishment of liver
glycogen and
triglyceride synthesis.
[1] Under one percent of ingested fructose is directly converted to plasma triglyceride.
[2] 29% - 54% of fructose is converted in liver to glucose, and about a quarter of fructose is converted to
lactate. 15% - 18% is converted to
glycogen.
[3] Glucose and lactate are then used normally as energy to fuel cells all over the body.
[2]
Fructose is a dietary
monosaccharide present naturally in
fruits and
vegetables, either as free fructose or as part of the
disaccharidesucrose, and as its polymer
inulin. It is also present in the form of refined sugars including
granulated sugars (white crystalline table sugar,
brown sugar,
confectioner's sugar, and
turbinado sugar), refined
crystalline fructose , as
high fructose corn syrups as well as in honey. About 10% of the calories contained in the Western diet are supplied by fructose (approximately 55 g/day).
[4]
Unlike glucose, fructose is not an
insulinsecretagogue, and can in fact lower circulating insulin.
[5] In addition to liver, fructose is metabolized in intestine, testis, kidney, skeletal muscle, fat tissue and brain,
[6][7] but it is not transported into cells via insulin-sensitive pathways (insulin regulated transporters
GLUT1 and
GLUT4). Instead fructose is taken in by
GLUT5. Fructose in muscles and adipose tissue is phosphorylated by hexokinase.
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