Carbohydrate
Carbohydrates are compounds consisting of carbon, hydrogen, and oxygen and are important sources of dietary energy. For most of the people in the world, carbohydrates--chiefly in the form of cereal grains and potatoes (or other root vegetables)--serve as the major source of energy. Even in the wealthier countries, such as the United States, carbohydrates provide 45 to 50 percent of the daily calorie count. In poorer countries, these easily digested and relatively inexpensive nutrients often provide well over 80 percent of every day's calories.
Chemically, carbohydrates are naturally occurring compounds composed of carbon, hydrogen and oxygen (CHO). In many cases the hydrogen-to-oxygen ratio of carbohydrates is the same 2 to 1 ratio as that of water. (Originally, chemists believed that all members of the class could be described as "carbon hydrates," which is how the name originated. Although this assumption was soon disproved, the name remained.)
The carbohydrate group includes sugars, starches, cellulose and a number of other chemically-related substances. For the most part, these various carbohydrates are produced by green plants. In the process known as photosynthesis, countless varieties of plants are able to synthesize a simple sugar (glucose, mostly) using the light energy absorbed by the chlorophyll in their leaves. They also utilize water from the soil and carbon dioxide from the air. Typically, the simple sugar they produce is used partly to form the more complex carbohydrate cellulose, which makes up the plant' s supporting framework, and partly to provide energy for its own metabolic needs. The rest, however, 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, Justus von Liebig, the famed German chemist, was one of the first to maintain 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 and 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, twelve 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 the only sugar not found in plants. All these sugars are split into the more easily absorbed monosaccharides in the digestive tract by specific enzymes. 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 Claude Bernard, the French physiologist who, in 1856, discovered a starch-like substance in the liver of mammals. The 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, another physiologist, the German Karl von Voit, demonstrated that mammals could make glycogen even when fed more complex sugars than glucose. In 1919, Otto Meyerhof was able to show that, in working muscles, glycogen was converted into lactic acid. It wasn't until the 1930s, however, that two Czech-American biochemists, Carl Cori and Gerty Cori, were able to detail the complicated process by which glycogen, stored in the liver and muscle, was broken down in the body and resynthesized. Building on the work of the Coris, Fritz Lipmann, a few years later, was able to clarify even further the way carbohydrates could 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 Emil Fischer, the noted German biochemist, who began his Nobel-prize winning work in 1884. Fischer not only was able to synthesize glucose and 30 more sugars, he also showed that the shape of these molecules was even more important than their chemical composition.
In the last 100 years much has been learned about carbohydrates' role in diet and health. We now know that not all carbohydrates are created equal. A 1988 University of Toronto study of 65,000 women showed that those with diets high in carbohydrates from white bread, potatoes, white rice, and pasta had more than twice the risk for Type 2 diabetes compared to those who ate a diet rich in high-fiber carbohydrates like whole wheat bread and whole grain pasta. The overall conclusion was that a high-carbohydrate diet that is low in fiber is not as healthy. According to the author of the study, the type of carbohydrate is more important than the total amount consumed. Low-fiber carbohydrates behave like sugar because they are digested fast and quickly drive up blood sugar levels and such rapid rises can contribute to diabetes. On the other hand, high-fiber carbohydrates like whole grains are digested more slowly and blood sugar levels rise more gradually and safely.
In addition to research on the dietary aspects of carbohydrates, there are efforts underway to create improved carbohydrate molecules and to identify new applications. For example, advances have been made with genetically altered carbohydrate-rich crops. A new type of potato has been cultivated which is designed to absorb less fat when fried. Other improved carbohydrate technologies include a new corn fiber byproduct; a natural sweetener produced by fermenting glucose; a modified cellulose polymer which may have utility in low-fat foods; and an edible film that could be used as a coating for fruits and vegetables to prevent spoilage.
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