Chemically fructose is a hexose monosaccharide. This means is a six carbon sugar (hexose) and is not bonded to any other sugar (monosaccharide). Fructose is absorbed in the small intestine like most other sugars, and from here it passes to the liver through the hepatic portal vein. In the liver the fructose enters hepatocytes and here it is phosphorylated (has a phosphate added to it) to form fructose 1-phosphate. Fructose 1-phosphate can then enters a pathway called glycolysis, and this pathway determines the fate of the compound. If the hepatocytes are low in energy, the fructose can proceed down glycolysis to form a molecule of pyruvate. This pyruvate then enters the citric acid cycle. The passing of fructose down the glycolytic pathway and round the citric acid cycle produces energy that can be used to drive chemical reactions in the liver. This pathway for the fructose has no negative metabolic consequences and is in fact of great use to a liver cell that requires energy to fuel its extensive metabolic activity.
Another possible fate for fructose, if the energy state of the hepatocyte is low, is to be converted to glucose 6-phosphate and then joined end on end with other glucose molecules to form glycogen. Glycogen is identical in structure to starch that we find in many carbohydrate foods. Storage of fructose as liver glycogen is only possible if depletion of glycogen has occurred as the liver has a finite capacity to store glucose as glycogen. The liver can store roughly 100 grams of glucose as glycogen (depending on body, and therefore liver size), and once this store is full no further synthesis of glycogen can proceed. This 100 grams is roughly enough to fuel the metabolic needs of the body, to maintain adequate blood sugar, during sleep. The fructose can also be converted to glucose and release to the circulation to increase blood sugar levels without the need to be converted to glycogen first. This results from activity of the enzyme hexokinase that resides in the endoplasmic reticulum of liver cells.
However, if the energy content of the liver is adequate, and the blood glucose levels are adequate, dietary fructose has a quite different fate. In this case the fructose will pass down the glycolytic pathway to be converted to pyruvate and then to acetyl CoA. Acetyl CoA is a two carbon molecule that under the influence of high insulin levels with undergo synthesis to fatty acid, a process that involves sequentially adding acetyl CoA to a growing chain of carbons. Also under the influence of insulin, this fatty acid will then be added along with 2 others to a molecule of glycerol (that has also been formed in the glycolytic pathway) to form a triglyceride. The resulting triglycerides are then packaged into very low density lipoproteins (VLDL) and exported to the blood for transport to peripheral tissues such as adipocytes and muscle tissue. As the VLDL particle loses triglycerides to the peripheral tissues, it becomes relatively higher in cholesterol, and is thus renamed a low density lipoprotein (LDL) particle.
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