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Fructolysis
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 metabolized widely in the body, fructose is metabolized almost completely in the liver in humans, where it is directed toward replenishment of liver glycogen and triglyceride synthesis.〔 Under one percent of ingested fructose is directly converted to plasma triglyceride.〔http://www.nutritionandmetabolism.com/content/9/1/89〕 29% - 54% of fructose is converted in liver to glucose, and about quarter of fructose is converted to lactate. 15% - 18% is converted to glycogen.〔http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3649104/〕 Glucose and lactate are then used normally as energy to fuel cells all over the body.〔
Fructose is a dietary monosaccharide present naturally in fruits and vegetables, either as free fructose or as part of the disaccharide sucrose, and as free monosaccharides in honey. 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 and as high fructose corn syrups. About 10% of the calories contained in the Western diet are supplied by fructose (approximately 55 g/day).〔https://www.inkling.com/read/illustrated-reviews-biochemistry-harvey-5th/chapter-12/fructose-metabolism〕
Unlike glucose, fructose is not an insulin secretagogue, and can in fact lower circulating insulin.〔http://press.endocrine.org/doi/abs/10.1210/jc.2003-031855〕 In addition to liver, fructose is metabolized in intestine, testis, kidney, skeletal muscle, fat tissue and brain, but it is not transported into cells via insulin-sensitive pathways (insulin regulated transporters GLUT1 and GLUT4). Instead fructose is taken in by GLUT5.
==Fructolysis and glycolysis are independent pathways==
Although the metabolism of fructose and glucose share many of the same intermediate structures, they have very different metabolic fates in human metabolism. Fructose is metabolized almost completely in the liver in humans, and is directed toward replenishment of liver glycogen and triglyceride synthesis, while much of dietary glucose passes through the liver and goes to skeletal muscle, where it is metabolized to CO2, H2O and ATP, and to fat cells where it is metabolized primarily to glycerol phosphate for triglyceride synthesis as well as energy production. The products of fructose metabolism are liver glycogen and ''de novo'' lipogenesis of fatty acids and eventual synthesis of endogenous triglyceride can be divided into two main phases: The first phase is the synthesis of the trioses, dihydroxyacetone(DHAP) and glyceraldehyde; the second phase is the subsequent metabolism of these trioses either in the gluconeogenic pathway for glycogen replenishment and/or the complete metabolism in the fructolytic pathway to pyruvate, which enters the Krebs cycle, is converted to citrate and subsequently directed toward ''de novo'' synthesis of the free fatty acid palmitate.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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