Fructose

Fructose is a six-carbon monosaccharide, or simple sugar, with the molecular formula C6H12O6. It is a ketohexose, meaning that its open-chain form contains a ketone group. Fructose occurs naturally as a free sugar and is also one of the two monosaccharides that make up sucrose, in which fructose is chemically linked to glucose .^1,2

Dietary fructose is absorbed mainly in the small intestine. At the brush-border membrane of enterocytes, uptake occurs primarily through the facilitative transporter GLUT5 and does not require sodium cotransport. Fructose can then leave the intestinal cell toward the circulation through facilitative sugar transporters, including GLUT2 .^2,3

After absorption, fructose is metabolized substantially by the small intestine and liver. A major fructose-metabolizing pathway begins when ketohexokinase, also called fructokinase, converts fructose to fructose-1-phosphate. Subsequent reactions produce intermediates whose carbon can enter pathways involved in glucose, lactate, glycogen, and lipid metabolism. The relative contribution of intestinal and hepatic metabolism depends partly on the amount of fructose entering the digestive system .²

Fructose is chemically distinct from glucose even though both are six-carbon monosaccharides with the same molecular formula. It is also distinct from sucrose and high-fructose corn syrup. Sucrose contains glucose and fructose joined together as a disaccharide, whereas high-fructose corn syrup contains these sugars mainly as separate monosaccharides in solution .^1,2