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Carbohydrates are naturally occurring compounds composed of carbon, hydrogen, and oxygen. The carbohydrate group includes sugars, starches, cellulose, and a number of other chemically related substances. For the most part, these carbohydrates are produced by green plants through the process known as photosynthesis. Countless varieties of plants use this process to synthesize a simple sugar (glucose, mostly) from the light energy absorbed by the chlorophyll in their leaves, water from the soil, and carbon dioxide from the air. Typically, plants use some of this simple sugar to form the more complex carbohydrate cellulose (which makes up the plant's supporting framework) and some to provide energy for its own metabolic needs; the rest is stored away for later use in the form of seeds, roots, or fruits.

Interestingly, the digestive and metabolic processes in animals and humans work almost in reverse fashion.

When a fruit is eaten, for instance, the complex carbohydrates are broken down in the digestive tract to simpler glucose units. The glucose is then used primarily to produce energy in a process which involves oxidation and the excretion of carbon dioxide and water as waste products. In the mid-1800s, German chemist Justus von Liebig was one of the first to recognize that the body derived energy from the oxidation of foods recently eaten, and also declared that it was carbohydrates and fats that served to fuel the oxidation-not carbon and hydrogen as Antoine-Laurent Lavoisier had thought.

Carbohydrates are usually divided into three main categories. The first category, the monosaccharides, are simple sugars that consist of a single carbohydrate unit that cannot be broken down into any simpler substances. The three most common sugars in this group are glucose (or dextrose), the most frequently seen sugar in fruits and vegetables (and, in digestion, the form of carbohydrate to which all others are eventually converted); fructose, associated with glucose in honey and in many fruits and vegetables; and galactose, derived from the more complex milk sugar, lactose. Each of these simple but nutritionally important sugars is a hexose, which means it contains six carbon atoms, 12 hydrogen atoms, and six oxygen atoms. All three require virtually no digestion but are readily absorbed into the bloodstream from the intestine.

Slightly more complex sugars are the disaccharides which contain two hexose units. The three most nutritionally important of these are sucrose (ordinary table sugar), maltose (derived from starch), and lactose, which is formed in the mammary glands and is the only sugar not found in plants. In the digestive tract, specific enzymes split all of these sugars into the more easily absorbed monosaccharides. If needed for future energy use, glucose units are typically squeezed together into larger, more slowly absorbed units and stored as polysaccharides, whose molecules often contain a hundred times the number of glucose units as do the simple sugars. These highly complex carbohydrates include dextrin, starch, cellulose, and glycogen. More efficient and more stable than the simple sugars, they are much easier to store. On the other hand, most of them need to be broken down by the digestive tract's enzymes before they can be absorbed. Some of them—cellulose, for instance—are almost impossible for humans to digest, but this indigestibility is useful since the colon needs a certain amount of bulk, or roughage, to perform at its best.

Glycogen is the form in which most of the body's excess glucose is stored. Both the liver and muscle are able to store glycogen, with muscle glycogen used primarily to fuel muscle contractions and liver glycogen used (when necessary) to replenish the bloodstream's dwindling supply of glucose.

Glycogen was named by French physiologist Claude Bernard, who in 1856 discovered a starchlike substance in the liver of mammals. This substance, he later showed, was not only built out of glucose taken from the blood, but could be broken down again into sugar whenever it was needed. In 1891, German physiologist Karl von Voit demonstrated that mammals could make glycogen even when fed sugars more complex than glucose. In 1919, Otto Meyerhof was able to show that glycogen is converted into lactic acid in working muscles. It was not until the 1930s, however, that the complicated process by which glycogen, stored in the liver and muscle, is broken down in the body and resynthesized was discovered by Czech-American biochemists Carl Cori and Gerty Cori. Building on their work, Fritz Lipmann was able a few years later to further clarify the way carbohydrates can be converted into the forms of chemical energy most usable by the body.

The chemical structure of the various sugars was worked out in great detail by German biochemist Emil Fischer, who began his Nobel Prize-winning work in 1884. Fischer not only was able to synthesize glucose and 30 other sugars, he also showed that the shape of their molecules was even more important than their chemical composition.

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