Sugars represent a subgroup of a broad class of compounds known as carbohydrates. Carbohydrates consist of carbon, hydrogen and oxygen with a 2:1 hydrogen to oxygen ratio. Plants produce carbohydrates during photosynthesis and the cells of living organisms consume them to produce energy. Food manufacturers add sugars to food, candy and beverages to improve taste, and fructose has become the manufacturers' sugar of choice because of its intense sweetness and low cost. Researchers at Princeton University ignited a controversy, however, when they published findings that fructose consumption in rats contributed to obesity.
Sugars
The simplest sugars -- known as monosaccharides -- contain at least four and as many as nine or more carbon atoms, with six carbon atoms being most common. Sugars with six carbon atoms -- called hexoses -- contain six oxygen atoms and 12 hydrogen atoms and include glucose and fructose. More complex sugars -- disaccharides -- consist of two monosaccharides bound together. Even more complex sugars -- polysaccharides -- also exist and may contain thousands of saccharide units. Cellulose and starch, for example, contains anywhere from 1,000 to 26,000 linked glucose molecules.
Fructose
Fructose represents one of three primary monosaccharides of dietary importance, along with glucose and galactose. It, like most sugars, exhibits high solubility in water; 375 grams of fructose will dissolve in 100 grams of water, which is a solubility more than 10 times greater than table salt. It melts near 105 degrees Celsius and caramelizes near 100 Celsius.
Fructose comprises one-half of the disaccharide sucrose -- common table sugar. Fructose exhibits a perceived sweetness greater than both sucrose and glucose. This represents the fundamental underpinning of its popularity with food, candy and soft drink manufacturers: achieving a desired sweetness level requires less fructose than any other sugar.
High Fructose Corn Syrup
Corn starch is a polysaccharide derived from corn kernels. Under hydrolysis, which involves the addition of water, and in the presence of an enzyme known as glucoamylase, it breaks apart into predominantly glucose molecules. A second treatment with a different enzyme, glucose isomerase, results in a syrup that contains approximately 42 percent fructose. This technically constitutes "high-fructose" corn syrup and many food manufacturers use it as such. However, further purification by a separation technique known as chromatography concentrates the fructose to 55 percent to yield the high-fructose corn syrups preferred by soft drink manufacturers.
Biochemistry
After ingestion, fructose travels to the small intestine intact. In the small intestine, a protein known as GLUT5 binds the fructose molecules to the intestine's mucosal membrane. From there, another protein, GLUT2, transports the fructose across the cellular membrane and into the blood stream, where the liver quickly metabolizes it and converts it to fatty acids. This metabolic pathway differs from that of glucose, which stimulates production of insulin. This difference may have consequences for the way in which the human body produces energy and stores fat.
Health Controversy
In 2010, a research group at Princeton University led by Dr. Bart Hoebel published the results of a study in the peer-reviewed journal "Pharmacology, Biochemistry and Behavior" that found rats, when fed a diet of high-fructose corn syrup, gained significantly more weight than rats fed table sugar. Scientists observed the weight gain even when the rats eating the different sugars consumed the same number of total calories.
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