Fructose is a monosaccharide, meaning the molecule consists of a single sugar ring. Like glucose, commonly called blood sugar, fructose is taken up by cells and used to provide for cellular energy needs. The specific function of fructose in the body is to provide cells with energy, and the mechanism by which this is accomplished differs somewhat from that of glucose.
Significance
Fructose is a nutritive molecule, meaning that when consumed, it provides cells with energy. Unlike glucose, which can be taken up by any body cells, fructose uptake and metabolism tends to be limited to the liver, which has enzymes necessary to process the molecule. Enzymes, explain Drs. Reginald Garrett and Charles Grisham, are protein-based biochemical catalysts, meaning molecules that help chemical reactions take place faster than they otherwise would. Cells without fructose-related enzymes can't process fructose.
Source
Fructose in the diet comes from several major sources. Naturally, fructose occurs in high concentration in fruit. In fact, the name "fructose" comes from a derivation of "fruit sugar." Table sugar also contains fructose--the table sugar molecule consists of a unit of fructose bound to a unit of glucose, note Drs. Mary Campbell and Shawn Farrell in their book "Biochemistry." High-fructose corn syrup also contains fructose, though in quantities very similar to those found in table sugar.
Effects
For fructose to fulfill its primary role in that body--that is, for fructose to provide cells with energy--cells must chemically burn it to release the stored energy in its bonds. This takes place through the action of several enzymes. First, note Drs. Garrett and Grisham, an enzyme called fructokinase adds a phosphate group to fructose. A second enzyme, called aldolase, then splits the fructose molecule into two parts. With some modification, these two molecular pieces can then enter normal fructose metabolic pathways.
Misconceptions
Some individuals wrongly blame fructose for the negative health effects of high-fructose corn syrup. In fact, the concentration of fructose in high-fructose corn syrup is nearly identical to that of fructose in table sugar, note Drs. Campbell and Farrell. The difference, however, is that in table sugar, fructose and glucose are linked chemically, whereas in high fructose corn syrup, they're not. This represents a significant chemical difference to cells in terms of metabolism.
Considerations
One important consideration with regard to the function of fructose in the body is that while it provides cells with energy and adds sweetness to food, it's much sweeter-tasting than table sugar, note Drs. Garrett and Grisham. As such, far less fructose--and therefore far fewer calories--satisfy the desire for sweetness as compared with table sugar. For this reason, fructose is sometimes considered a reduced-calorie sweetener. Fructose also doesn't require insulin release by the pancreas to be taken up by cells.
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