Fructose

of fructose to glucose, with minor differences. HFCS has simply replaced sucrose as a sweetener.
Therefore, despite the changes in the sweetener consumption, the ratio of glucose to fructose
intake has remained relatively constant.
Figure 4: Hydrolysis of sucrose to glucose and fructose by sucrase.
The mechanism of fructose absorption in the small intestine is not completely understood. Some
evidence suggests active transport, because fructose uptake has been shown to occur against
a concentration gradient. However, the majority of research supports the claim that fructose
absorption occurs on the mucosal membrane via facilitated transport involving GLUT5 transport
proteins. An electrical factor stimulated by the brush border effect of fiber in the diet. Since the
concentration of fructose is higher in the lumen, fructose is able to flow down a concentration
gradient into the enterocytes, assisted by transport proteins. Fructose may be transported out of
the enterocyte across the basolateral membrane by either GLUT2 or GLUT5, although the GLUT2
transporter has a greater capacity for transporting fructose and therefore the majority of fructose
is transported out of the enterocyte through GLUT2.
Figure 3: Adjusted consumption of refined sugar per capita in the U.S.
Fructose digestion and absorption in humans
Fructose exists in foods as either a monosaccharide (free fructose) or as a unit of a disaccharide
(sucrose). Free fructose is absorbed directly by the intestine; however, when fructose is consumed
in the form of sucrose, digestion occurs entirely in the upper small intestine. As sucrose comes
into contact with the membrane of the small intestine, the enzyme sucrase catalyzes the cleavage
of sucrose to yield one glucose unit and one fructose unit. Fructose is absorbed in the small
intestine, then enters the portal vein and is directed toward the liver.
Figure 5: Intestinal sugar transport proteins.
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