carbohydrate

(kär'bō-hī'drāt')
n.
Any of a group of organic compounds that includes sugars, starches, celluloses, and gums and serves as a major energy source in the diet of animals. These compounds are produced by photosynthetic plants and contain only carbon, hydrogen, and oxygen, usually in the ratio 1:2:1.

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Concept

Carbohydrates are nutrients, along with proteins and other types of chemical compounds, but they are much more than that. In addition to sugars, of which there are many more varieties than ordinary sucrose, or table sugar, carbohydrates appear in the form of starches and cellulose. As such, they are the structural materials of which plants are made. Carbohydrates are produced by one of the most complex, vital, and amazing processes in the physical world: photosynthesis. Because they are an integral part of plant life, it is no wonder that carbohydrates are in most fruits and vegetables. And though they are not a dietary requirement in the way that vitamins or essential amino acids are, it is difficult to eat without ingesting some carbohydrates, which are excellent sources of quick-burning energy. Not all carbohydrates are of equal nutritional value, however: in general, the ones created by nature are good for the body, whereas those produced by human intervention—some forms of pasta and most varieties of bread, white rice, crackers, cookies, and so forth—are much less beneficial.

How It Works

What Carbohydrates Are

Carbohydrates are naturally occurring compounds that consist of carbon, hydrogen, and oxygen, and are produced by green plants in the process of undergoing photosynthesis. In simple terms, photosynthesis is the biological conversion of light energy (that is, electromagnetic energy) from the Sun to chemical energy in plants. It is an extremely complex process, and a thorough treatment of it involves a great deal of technical terminology. Although we discuss the fundamentals of photosynthesis later in this essay, we do so only in the most cursory fashion.

Photosynthesis involves the conversion of carbon dioxide and water to sugars, which, along with starches and cellulose, are some of the more well known varieties of carbohydrate. Sugars can be defined as any of a number of water-soluble compounds, of varying sweetness. (What we think of as sugar—that is, table sugar—is actually sucrose, discussed later.) Starches are complex carbohydrates without taste or odor, which are granular or powdery in physical form. Cellulose is a polysaccharide, made from units of glucose, that constitutes the principal part of the cell walls of plants and is found naturally in fibrous materials, such as cotton. Commercially, it is a raw material for such manufactured goods as paper, cellophane, and rayon.

Monosaccharides

The preceding definitions contain several words that also must be defined. Carbohydrates are made up of building blocks called monosaccharides, the simplest type of carbohydrate. Found in grapes and other fruits and also in honey, they can be broken down chemically into their constituent elements, but there is no carbohydrate more chemically simple than a monosaccharide. Hence, they are also known as simple sugars or simple carbohydrates.

Examples of simple sugars include glucose, which is sweet, colorless, and water-soluble and appears widely in nature. Glucose, also known as dextrose, grape sugar, and corn sugar, is the principal form in which carbohydrates are assimilated, or taken in, by animals. Other monosaccharides include fructose, or fruit sugar, and galactose, which is less soluble and sweet than glucose and usually appears in combination with other simple sugars rather than by itself. Glucose, fructose, and galactose are isomers, meaning that they have the same chemical formula (C₆H₁₂O₆), but different chemical structures and therefore different chemical properties.

Disaccharides

When two monosaccharide molecules chemically bond with each other, the result is one of three general types of complex sugar: a disaccharide, oligosaccharide, or polysaccharide. Disaccharides, or double sugars, are composed of two monosaccharides. By far the most well known example of a disaccharide is sucrose, or table sugar, which is formed from the bonding of a glucose molecule with a molecule of fructose. Sugar beets and cane sugar provide the principal natural sources of sucrose, which the average American is most likely to encounter in refined form as white, brown, or powdered sugar.

Another disaccharide is lactose, or milk sugar, the only type of sugar that is produced from animal (i.e., mammal) rather than vegetable sources. Maltose, a fermentable sugar typically formed from starch by the action of the enzyme amylase, is also a disaccharide. Sucrose, lactose, and maltose are all isomers, with the formula C₁₂H₂₂O₁₁.

Oligosaccharides and Polysaccharides

The definitions of oligosaccharide and polysaccharide are so close as to be confusing. An oligosaccharide is sometimes defined as a carbohydrate containing a known, small number of monosaccharide units, while a polysaccharide is a carbohydrate composed of two or more monosaccharides. In theory, this means practically the same thing, but in practice, an oligosaccharide contains 3-6 monosaccharide units, whereas a polysaccharide is composed of more than six.

Oligosaccharides are found rarely in nature, though a few plant forms have been discovered. Far more common are polysaccharides ("many sugars"), which account for the vast majority of carbohydrate types found in nature. (See Where to Learn More for the Nomenclature of Carbohydrates Web site, operated by the Department of Chemistry at Queen Mary College, University of London. A glance at the site will suggest something about the many, many varieties of carbohydrates.)

Polysaccharides may be very large, consisting of as many as 10,000 monosaccharide units strung together. Given this vast range of sizes, it should not be surprising that there are hundreds of polysaccharide types, which differ from one another in terms of size, complexity, and chemical makeup. Cellulose itself is a polysaccharide, the most common variety known, composed of numerous glucose units joined to one another. Starch and glycogen are also glucose polysaccharides. The first of these polysaccharides is found primarily in the stems, roots, and seeds of plants. As for glycogen, this is the most common form in which carbohydrates are stored in animal tissues, particularly muscle and liver tissues.

Photosynthesis

Photosynthesis, as we noted earlier, is the biological conversion of light or electromagnetic energy from the Sun into chemical energy. It occurs in green plants, algae, and some types of bacteria and requires a series of biochemical reactions. Higher plants have structures called chloroplasts, which contain a dark green or blue-black chemical known as chlorophyll. Light absorption by chlorophyll catalyzes, or speeds up, the process of photosynthesis. (A catalyst is a substance that accelerates a chemical reaction without participating in it.)

In photosynthesis, carbon dioxide and water react with each other in the presence of light and chlorophyll to produce a simple carbohydrate and oxygen. This is one of those statements in the realm of science that at first glance sounds a bit dry and boring but which, in fact, encompasses one of life's great mysteries—a concept far more captivating than any number of imaginary, fantastic, or pseudoscientific ideas one could concoct. Photosynthesis is one of the most essential life-sustaining processes, making possible the nutrition of all things and the respiration of animals and other oxygen-breathing organisms.

In photosynthesis, plants take a waste product of human and animal respiration and, through a series of chemical reactions, produce both food and oxygen. The food gives nourishment to the plant, which, unlike an animal, is capable of producing its own nutrition from its own body with the aid only of sunlight and a few chemical compounds. Later, when the plant is eaten by an animal or when it dies and is consumed by bacteria and other decomposers, it will pass on its carbohydrate content to other creatures. (See Food Webs for more about plants as autotrophs and the relationships among primary producers, consumers, and decomposers.)

A carbohydrate is not the only useful product of the photosynthetic reaction. The reaction produces an extremely important waste by-product—waste, that is, from the viewpoint of the plant, which has no need of oxygen. Yet the oxygen it generates in photosynthesis makes life possible for animals and many single-cell life-forms, which depend on oxygen for respiration.

The Photosynthesis Equation

The photosynthesis reaction can be represented thus as a chemical equation:

Note that the arrow indicates that a chemical reaction has taken place with the assistance of light and chlorophyll. In the same way, heat from a Bunsen burner may be required to initiate some other chemical reaction, without actually being part of the reactants to the left of the arrow. In the present equation, neither the added energy nor the catalyst appears on the left side, because they are not actual physical participants consumed in the reaction, as the carbon dioxide and water are. The catalyst does not participate in the reaction, whereas the energy, while it is consumed in the reaction, is not a material or physical participant—that is, it is energy, not matter.

One might also wonder why the equation shows six molecules of carbon dioxide and six of water. Why not one of each, for the sake of simplicity? To produce a balanced chemical equation, in which the same number of atoms appears on either side of the arrow, it is necessary to show six carbon dioxide molecules reacting with six water molecules to produce six oxygen molecules and a single glucose molecule. Thus, both sides contain six atoms of carbon, 12 of hydrogen, and 18 of oxygen.

The equation gives the impression that photosynthesis is a simple, one-step process, but nothing could be further from the truth. In fact, the process occurs one small step at a time. It also involves many, many intricacies and aspects that require the introduction of scores of new terms and ideas. Such a discussion is beyond the scope of the present essay, and therefore the reader is encouraged to consult a reliable textbook for further information on the details of photosynthesis.

Real-Life Applications

Fruits and Vegetables

One of the principal ways in which people obtain carbohydrates from their diets is through fruits and vegetables. The distinctions between these two are based not on science but on custom. Traditionally, vegetables are plant tissues (which may be sweet, but usually are not), that are eaten as a substantial part of a meal's main course. By contrast, fruits are almost always sweet and are eaten as desserts or snacks. It so happens, too, that people are much more likely to cook vegetables than they are fruits, though vegetables are nutritionally best when eaten raw.

Fruits and vegetables are heavy in carbohydrate content, in the form of edible sugars and starches but also inedible cellulose, whose role in the diet will be examined later. In a fresh vegetable, for instance, water may account for about 70% of the volume, and proteins, fat, vitamins, and minerals may make up a little more than 5%, with nearly 25% taken up either by edible sugars and starches or by inedible cellulose fiber.

The Example of the Artichoke

Every fruit or vegetable one could conceivably eat—and there are hundreds—contains both edible carbohydrates, which are a good source of energy, and inedible ones, which provide fiber. An excellent example of this edible-inedible mixture is the globe, or French, artichoke—Cynara scolymus, a member of the family Asteraceae, which includes the sunflower. The globe artichoke (not to be confused with the Jerusalem artichoke, or Helianthus tuberosus) appears in the form of an inflorescence, or a cluster of flowers. This vegetable usually is steamed, and the bracts, or leaves, are dipped in butter or another sauce.

Not nearly all of the bract is edible, however; to consume the starchy "meat" of the artichoke, which has a distinctive, nutty flavor, one must draw the leaves between the teeth. Most of the artichoke's best parts are thus hidden away, and the best part of all—the tender and fully edible "heart"—is enclosed beneath an intimidating shield of slender thistles. Whoever first discovered that an artichoke could be eaten must have been a brave person indeed, and whoever ascertained how to eat it was a wise one. Thanks to these adventurous souls, the world's cuisine has an unforgettable delicacy.

The Carbohydrate Content of Vegetables

In terms of edible carbohydrate content, the artichoke has a low percentage. A few vegetables have a smaller percentage of carbohydrates, whereas others have vastly higher percentages, as the list shown here illustrates. In general, it seems that the carbohydrate content of vegetables (and in each of these cases we are talking about edible carbohydrates, not cellulose) is in the range of about 5-10%, somewhere around 20%, or a very high 60-80%. There does not seem to be a great deal of variation in these ranges.

Water, Protein, and Carbohydrate Content of Selected Vegetables:

• Artichoke: 85% water, 2.9% protein, 10.6% carbohydrate
• Beets, red: 87.3% water, 1.6% protein, 9.9% carbohydrate
• Celery: 94.1% water, 0.9% protein, 3.9% carbohydrate
• Corn: 13.8% water, 8.9% protein, 72.2% carbohydrate
• Lima bean: 10.3% water, 20.4% protein, 64% carbohydrate
• Potato: 79.8% water, 2.1% protein, 17.1% carbohydrate
• Red pepper: 74.3% water, 3.7% protein, 18.8% carbohydrate
• Summer squash: 94% water, 1.1% protein, 4.2% carbohydrate

Starches

Not all the carbohydrates in these vegetables are the same. Some carbohydrates appear in the form of sugar and others in the form of inedible cellulose, discussed in the next section. In addition, some vegetables are high in starch content. As we noted earlier, starch is white and granular, and, unlike sugars, starches cannot be dissolved in cold water, alcohol, or other liquids that normally act as solvents.

Manufactured in plants' leaves, starch is the product of excess glucose produced during photosynthesis, and it provides the plant with an emergency food supply stored in the chloroplasts. Vegetables high in starch content are products of plants whose starchy portions happen to be the portions we eat. For example, there is the tuber, or underground bulb, of the potato as well as the seeds of corn, wheat, and rice. Thus, all of these vegetables, and foods derived from them, are heavy in the starch form of carbohydrate.

In addition to their role in the human diet, starches from corn, wheat, tapioca, and potatoes are put to numerous commercial uses. Because of its ability to thicken liquids and harden solids, starch is applied in products (e.g., cornstarch) that act as thickening agents, both for foods and nonfood items. Starch also is utilized heavily in various phases of the garment and garment-care industries to impart stiffness to fabrics. In the manufacture of paper, starch is used to increase the paper's strength. It also is employed in the production of cardboard and paper bags.

Cellulose

One of the aspects of fruits and vegetables to which we have alluded several times is the high content of inedible material, or cellulose. (Actually, it is edible—just not digestible.) A substance found in the cell walls of plants, cellulose is chemically like starch but even more rigid, and this property makes it an excellent substance for imparting strength to plant bodies. Animals do not have rigid, walled cells, but plants do. The heavy cellulose content in plants' cell walls gives them their erect, rigid form; in other words, without cellulose, plants might be limp and partly formless. Like human bone, plant cell walls are composed of fibrils (small filaments or fibers) that include numerous polysaccharides and proteins. One of these polysaccharides in cell walls is pectin, a substance that, when heated, forms a gel and is used by cooks in making jellies and jams. Some trees have a secondary cell wall over the primary one, containing yet another polysaccharide called lignin. Lignin makes the tree even more rigid, penetrable only with sharp axes.

Cellulose in Digestion

As we have noted, cellulose is abundant in fruits and vegetables, yet humans lack the enzyme necessary to digest it. Termites, cows, koalas, and horses all digest cellulose, but even these animals and insect do not have an enzyme that digests this material. Instead, they harbor microbes in their guts that can do the digesting for them. (This is an example of symbiotic mutualism, a mutually beneficial relationship between organisms, discussed in Symbiosis.)

Cows are ruminants, or animals that chew their cud—that is, food regurgitated to be chewed again. Ruminants have several stomachs, or several stomach compartments, that break down plant material with the help of enzymes and bacteria. The partially digested material then is regurgitated into the mouth, where it is chewed to break the material down even further. (If you have ever watched cows in a pasture, you have probably observed them calmly chewing their cud.) The digestion of cellulose by bacteria in the stomachs of ruminants is anaerobic, meaning that the process does not require oxygen. One of the by-products of this anaerobic process is methane gas, which is foul smelling, flammable, and toxic. Ruminants give off large amounts of methane daily, which has some environmentalists alarmed, since cow-borne methane may contribute to the destruction of the ozone high in Earth's stratosphere.

Alhough cellulose is indigestible by humans, it is an important dietary component in that it aids in digestion. Sometimes called fiber or roughage, cellulose helps give food bulk as it moves through the digestive system and aids the body in pushing out foods and wastes. This is particularly important inasmuch as it helps make possible regular bowel movements, thus ridding the body of wastes and lowering the risk of colon cancer. (See Digestion for more about the digestive and excretory processes.)

Overall Carbohydrate Nutrition

A diet high in cellulose content can be beneficial for the reasons we have noted. Likewise, a healthy diet includes carbohydrate nutrients, but only under certain conditions. First of all, it should be understood that the human body does not have an essential need for carbohydrates in and of themselves—in other words, there are no "essential" carbohydrates, as there are essential amino acids or fatty acids.

On the other hand, it is very important to eat fresh fruits and vegetables, which, as we have seen, are heavy in carbohydrate content. Their importance has little do with their nutritional carbohydrate content, but rather with the vitamins, minerals, proteins, and dietary fiber that they contain. For these healthy carbohydrates, it is best to eat them in as natural a form as possible: for example, eat the whole orange, rather than just squeezing out the juice and throwing away the pulp. Also, raw spinach and other vegetables contain far more vitamins and minerals than the cooked versions.

Sugar Highs and Fat Storage

Carbohydrates can give people a short burst of energy, and this is why athletes may "bulk up on carbs" right before competition. But if the carbohydrates are not quickly burned off, they eventually will be stored as fat. This is the case even with healthy carbohydrates, but the situation is much worse with junk-food carbohydrates, which offer only empty calories stripped of vitamin and mineral content. One example is a particular brand of candy bar that, over the years, has been promoted in commercials as a means of obtaining a quick burst of energy. In fact, this and all other white-sugar-based candies give only a quick "sugar high," followed almost immediately by a much lower energy "low"—and in the long run by the accumulation of fat.

Fat is the only form in which the body can store carbohydrates for the long haul, meaning that the "fat-free" stickers on many a package of cookies or cakes in the supermarket are as meaningless as the calories themselves are empty. Carbohydrate consumption is one of the main reasons why the average American is so overweight. With an in active lifestyle, as is typical of most adults in modern life, all those French fries, cookies, dinner rolls, and so on have no place to go but to the fat-storage centers in the abdomen, buttocks, and thighs. Of all carbohydrate-containing foods, the least fattening, of course, are natural nonstarches, such as fruits and vegetables (assuming they are not cooked in fat). Next on the least-fattening list are starchy natural foods, such as potatoes, and most fattening of all are processed starches, whether they come in the form of rice, wheat, or potato products.

Why You Can Eat More Carbohydrates Than Proteins

One of the biggest problems with starches is that the body can consume so many of them compared with proteins and fats. How many times have you eaten a huge plate of mashed potatoes or rice, mountains of fries, or piece after piece of bread? All of us have done it: with carbohydrates, and particularly starches, it seems we can never get enough. But how many times have you eaten a huge plate of nothing but chicken, steak, or eggs? Probably not very often, and if you have tried to eat too much of these protein-heavy foods at one time, you most likely started to get sick.

The reason is that when you eat protein or fat, it triggers the release of a hormone called cholecystokinin (CCK) in the small intestine. CCK tells the brain, in effect, that the body is getting fed, and if enough CCK is released, it signals the brain that the body has received enough food. If one continues to consume proteins or fats beyond that point, nausea is likely to follow. Carbohydrates, on the other hand, do not cause a release of CCK; only when they enter the bloodstream do they finally send a signal to the brain that the body is satisfied. By then, most of us have piled on more mashed potatoes, which are destined to take their place in the body as fat stores.

Where to Learn More

Carbohydrates. Hardy Research Group, Department of Chemistry, University of Akron (Web site). <http://ull.chemistry.uakron.edu/genobc/Chapter_17/>.

Dey, P. M., and R. A. Dixon. Biochemistry of Storage Carbohydrates in Green Plants. Orlando, FL: Academic Press, 1985.

Carpi, Anthony. "Food Chemistry: Carbohydrates." Visionlearning.com (Web site). <http://www.vision learning.com/library/science/chemistry-2/CHE2.5-carbohydrates.htm>.

Food Resource, Oregon State University (Web site). <http://food.orst.edu/>.

Kennedy, Ron. "Carbohydrates in Nutrition." The Doctors' Medical Library (Web site). <http://www.medicallibrary.net/sites/carbohydrates_in_nutrition.html>.

"Nomenclature of Carbohydrates." Queen Mary College, University of London, Department of Chemistry (Web site). <http://www.chem.qmw.ac.uk/iupac/2carb/>.

Snyder, Carl H. The Extraordinary Chemistry of Ordinary Things. New York: John Wiley and Sons, 1998.

Spallholz, Julian E. Nutrition, Chemistry, and Biology. Englewood Cliffs, NJ: Prentice-Hall, 1989.

Wiley, T. S., and Bent Formby. Lights Out: Sleep, Sugar, and Survival. New York: Pocket Books, 2000.

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A term applied to a group of substances which include the sugars, starches, and cellulose, along with many other related substances. This group of compounds plays a vitally important part in the lives of plants and animals, both as structural elements and in the maintenance of functional activity. Plants are unique in that they alone in nature have the power to synthesize carbohydrates from carbon dioxide and water in the presence of the green plant chlorophyll through the energy derived from sunlight, by the process of photosynthesis. This process is responsible not only for the existence of plants but for the maintenance of animal life as well, since animals obtain their entire food supply directly or indirectly from the carbohydrates of plants.

The term carbohydrate originated in the belief that naturally occurring compounds of this class, for example, D-glucose (C₆H₁₂O₆), sucrose (C₁₂H₂₂O₁₁), and cellulose (C₆H₁₀O₅)_n, could be represented formally as hydrates of carbon, that is, C_x(H₂O)_y. Later it became evident that this definition for carbohydrates was not a satisfactory one. New substances were discovered whose properties clearly indicated that they had the characteristics of sugars and belonged in the carbohydrate class, but which nevertheless showed a deviation from the required hydrogen-to-oxygen ratio. Examples of these are the important deoxy sugars, D-deoxyribose, L-fucose, and L-rhamnose, the uronic acids, and such compounds as ascorbic acid (vitamin C). The retention of the term carbohydrate is therefore a matter of convenience rather than of exact definition. A carbohydrate is usually defined as either a polyhydroxy aldehyde (aldose) or ketone (ketose), or as a substance which yields one of these compounds on hydrolysis. However, included within this class of compounds are substances also containing nitrogen and sulfur.

The properties of many carbohydrates differ enormously from one substance to another. The sugars, such as D-glucose or sucrose, are easily soluble, sweet-tasting, and crystalline; the starches are colloidal and paste-forming; and cellulose is completely insoluble. Yet chemical analysis shows that they have a common basis; the starches and cellulose may be degraded by different methods to the same crystalline sugar, D-glucose.

The carbohydrates usually are classified into three main groups according to complexity: monosaccharides, oligosaccharides, and polysaccharides. Monosaccharides are simple sugars that consist of a single carbohydrate unit which cannot be hydrolyzed into simpler substances. These are characterized, according to their length of carbon chain, as trioses (C₃H₆O₃), tetroses (C₄H₈O₄), pentoses (C₅H₁₀O₅), hexoses (C₆H₁₂O₆), heptoses (C₇H₁₄O₇), and so on. Oligosaccharides are compound sugars that are condensation products of two to five molecules of simple sugars and are subclassified into disaccharides, trisaccharides, tetrasaccharides, and pentasaccharides, according to the number of monosaccharide molecules yielded upon hydrolysis. Polysaccharides comprise a heterogeneous group of compounds which represent large aggregates of monosaccharide units, joined through glycosidic bonds. They are tasteless, nonreducing, amorphous substances that yield a large and indefinite number of monosaccharide units on hydrolysis. Their molecular weight is usually very high, and many of them, like starch or glycogen, have molecular weights of several million. They form colloidal solutions, but some polysaccharides, of which cellulose is an example, are completely insoluble in water. On account of their heterogeneity they are difficult to classify. See also Polysaccharide.

The sugars are also classified into two general groups, the reducing and nonreducing. The reducing sugars are distinguished by the fact that because of their free, or potentially free, aldehyde or ketone groups they possess the property of readily reducing alkaline solutions of many metallic salts, such as those of copper, silver, bismuth, mercury, and iron. The most widely used reagent for this purpose is Fehling's solution. The reducing sugars constitute by far the larger group. The monosaccharides and many of their derivatives reduce Fehling's solution. Most of the disaccharides, including maltose, lactose, and the rarer sugars cellobiose, gentiobiose, melibiose, and turanose, are also reducing sugars. The best-known nonreducing sugar is the disaccharide sucrose. Among other nonreducing sugars are the disaccharide trehalose, the trisaccharides raffinose and melezitose, the tetrasaccharide stachyose, and the pentasaccharide verbascose.

The sugars consist of chains of carbon atoms which are united to one another at a tetrahedral angle of 109°28′. A carbon atom to which are attached four different groups is called asymmetric. A sugar, or any other compound containing one or more asymmetric carbon atoms possesses optical activity; that is, it rotates the plane of polarized light to the right or left. See also Optical activity.

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Carbohydrates are the major source of metabolic energy, the sugars and starches. Chemically they are composed of carbon, hydrogen, and oxygen in the ratio C_n : H_2n : O_n. The basic carbohydrates are the monosaccharide sugars, of which glucose, fructose, and galactose are the most important nutritionally.

Disaccharides are composed of two monosaccharides: nutritionally the important disaccharides are sucrose (a dimer of glucose + fructose), lactose (a dimer of glucose + galactose) and maltose (a dimer of two glucose units).

A number of oligosaccharides occur in foods, consisting of 3-5 monosaccharide units; in general these are not digested, and should be considered among the unavailable carbohydrates.

Larger polymers of carbohydrates are known as polysaccharides or complex carbohydrates. Nutritionally two classes of polysaccharides can be distinguished: (1)starches, which are polymers of glucose units, either as a straight chain (amylose) or with a branched structure (amylopectin) and are digested; and(2)a variety of other polysaccharides which are collectively known as non-starch polysaccharides (NSP), and are not digested by human digestive enzymes.

The reserve of carbohydrate in the liver and muscles is glycogen, a glucose polymer with a similar structure to the amylopectin form of starch, but more branched.

Carbohydrates form the major part of the diet, providing between 50 and 70% of the energy intake, largely from starch and sucrose. The metabolic energy yield of carbohydrates is 4 kcal (17 kJ)/g. More precisely, monosaccharides yield 3.74 kcal (15.7 kJ), disaccharides 3.95 kcal (16.6 kJ), and starch 4.18 kcal (17.6 kJ)/g. Glycerol is a 3-carbon sugar alcohol, and classified as a carbohydrate; it yields 4.32 kcal (18.1 kJ)/g.

See also sugar; sugar alcohols.

Carbohydrates are the most accessible and most used form of fuel in the body. One gram yields approximately 4 Calories of energy (about 16.7 joules).

Simple carbohydrates include the sugars (monosaccharides and disaccharides) found in jams, confectionery, cakes, biscuits, pastries, and table sugar. Complex carbohydrates include starches and fibres found in unrefined foods (e.g. bread, potatoes, rice, pasta, cereals, and nuts). Most carbohydrates (but not fibre) are broken down during digestion to glucose and other monosaccharides.

Although the precise figures differ, most nutritionists and dietitians recommend that carbohydrates (mainly complex carbohydrates because they contain other nutrients and fibre) should be the prime source of energy for everyone (see also glycaemic index). A typical recommendation is that 55-60 per cent of daily calories should come from carbohydrates, of which 85 per cent should be complex carbohydrates. The figure should be even higher (about 70 per cent of daily calories) for those who are regularly engaged in strenuous endurance activities. This means that an adult male exerciser in hard training and requiring about 4000 Calories a day, would need to eat about 500 grams of carbohydrate; a large plateful of pasta supplies about 110 grams of carbohydrate.

The ability of the body to store carbohydrate is limited, but relatively small amounts of glycogen can be stored in the liver and muscle (about 2000 kcal of energy, or the equivalent of the energy needed for about 32 km of running). Diets rich in carbohydrate help to replace muscle glycogen which is the main fuel used during exercise. There is also some evidence that carbohydrates are less easily converted to body fat than is dietary fat. See also carbohydrate loading and glucose.

A broad category of sugars, starches, fibers and starchy vegetables that the body eventually converts to glucose, the body's primary source of energy. There are two classes of carbohydrates-simple and complex. Simple carbohydrates are the sugars, which include glucose and fructose from fruits and vegetables, sucrose from beet or cane sugar and lactose from milk. Simple carbohydrates are absorbed by the body very quickly. Complex carbohydrates include starches and fiber and are most commonly found in whole grains and legumes. Complex carbohydrates, which are generally large chains of glucose molecules, take longer to digest and provide more nutrients than simple carbohydrates.

Carbohydrates are one of the major classes of biological molecules, along with proteins, lipids (fats), and nucleic acids. Carbohydrates range from small molecules — mono, - di-, or tri-saccharides — to large molecules called polysaccharides. As these names imply, carbohydrates are made up of sugars.

The most abundant organic compound in the biosphere is a carbohydrate — cellulose, a substance which gives strength and integrity to plant cell walls. It consists of linked linear chains of glucose molecules. This configuration allows arrays of long, parallel straight fibrils to form, giving cellulose its characteristic properties. Cellulose is difficult to break down, and only some bacteria, fungi, and protozoa secrete the enzymes (cellulases) that can do so. So mammals are unable to digest cellulose, except some ruminants that have cellulase-secreting bacteria in their rumens. (If it were not for these bacterial cellulases, the disposal of 10¹⁵ kg of plant waste per year world-wide would present an enormous pollution problem.) Nevertheless, in man, cellulose is an important component of the diet as ‘roughage’.

glycogen, another polysaccharide made of linked chains of glucose molecules, is used by mammals as a way of storing energy in the cells of most tissues, but notably in liver cells as a store for the whole body, and in muscles for their own use. Here the type of linkage results in the formation of an open helix, readily broken down by the relevant enzymes (glycogenases) when the sudden need for energy arises.

Carbohydrate-rich foods are starchy ones, such as the staples bread, potatoes, and pasta. Large quantities of carbohydrates are ingested as common sugar and in confectionary. Common dietary carbohydrates are sucrose, lactose, and mannose, all disaccharides (formed from two simple sugars), which are broken down by digestive processes to monosaccharides and used to derive energy.

The conversion of monosaccharides to energy in the form of adenosine triphosphate (ATP) follows a common pathway — used also for the conversion of fats and, to a lesser extent, proteins into energy — which involves the utilization of oxygen. The end result of this metabolism is that one molecule of glucose generates 38 molecules of ATP. The respiratory quotient (RQ) for carbohydrates is 1, where the RQ is the ratio of the number of CO₂ molecules formed to the number of molecules of oxygen consumed in oxidizing (burning) one sugar molecule. As carbohydrates have the formula (CH₂O) _n, hydrogen and oxygen are in the correct ratio to form water. (When fats are oxidized, extra oxygen is needed to form water, such that the RQ for fats is around 0.7.)

In an ideally balanced diet, about two-thirds of the energy supply should be from carbohydrates. But unlike proteins and fats, which must provide certain essential components, no particular dietary carbohydrates are necessary for health. This may seem paradoxical, in that the brain crucially needs a constant supply of glucose — but this can, if necessary, be made internally from proteins. Excess of dietary carbohydrate, when the glycogen stores are filled, is converted and stored as fat, and this can be released as fatty acids when needed for energy production. In starvation, the mobilization of fatty acids from body fat results in their uptake by the liver and production from them of ketone bodies, an alternative energy source. At this stage acetone can be smelt (like pear drops) on the breath. The same happens when carbohydrate starvation occurs at the cellular level in diabetes because glucose entry into cells is impaired.

— Alan W. Cuthbert

See also blood sugar; energy balance; metabolism; starvation; sugars.

An organic compound composed of carbon, hydrogen, and oxygen, with the general chemical formula of C_x (H₂O)_y. Carbohydrates are major sources of energy, each gram yielding approximately 4 kcal of energy. During a short-duration exercise of maximal effort, energy is supplied almost exclusively by carbohydrates. Carbohydrates include simple sugars, glycogen, and starches. Sugars are simple carbohydrates. Starches are complex carbohydrates found in legumes, potatoes, and other vegetables. Carbohydrates are classified according to the number of sugar-units they contain. Monosaccharides have one sugar-unit, disaccharides two, oligosaccharides a few, and polysaccharides many. The body stores of carbohydrate are mainly in the form of glycogen in the liver and skeletal muscle. The stores are limited to less than 2000 kcal, only enough energy for about a 32-km (20-mile) run. Without an adequate intake of dietary carbohydrate, muscle and brain cells can be deprived of their main energy source.

Low-Carb Diets Low-carbohydrate diets, such as the Atkins and South Beach diets, are based on the proposition that it's not fat that makes you fat. Allowing dieters to eat steak, butter, eggs, bacon, and other high-fat foods, these diets instead outlaw starches and refined carbohydrates on the theory that they are metabolized so quickly that they lead to hunger and overeating. This theory, which was first popularized in the nineteenth century, came under scathing criticism from the medical establishment during the early 1970s when Dr. Robert Atkins published the phenomenally popular low-carb diet bearing his name. According to the American Medical Association (AMA), the Atkins diet was a "bizarre regimen" that advocated "an unlimited intake of saturated fats and cholesterol-rich foods" and therefore presented a considerable risk of heart disease. Most doctors recommended instead a diet low in fat and high in carbohydrates, with plenty of grains, fruits, and vegetables and limited red meat or dairy products. This became the received wisdom during the 1980s, at the same time that the U.S. waistline began to expand precipitously. As dieters found that weight loss was difficult to maintain on a low-fat diet, low-carb diets regained popularity—with as many as 30 million people trying a low-carb diet in 2003. Several small-scale studies began to suggest that a low-carb diet may indeed be effective and may not have the deleterious effects its detractors have claimed; other research found that any benefits of a low-carb diet are short-lived, and that the negative effects will take decades to become evident. The National Institutes of Health has pledged $2.5 million for a five-year study of the Atkins diet with 360 subjects. While the results of this and other large-scale studies are awaited, many researchers stress that the key issue in maintaining a healthy weight is the number of calories consumed, not the type of calories. The National Academy of Sciences recommends that adults obtain 45 to 65 percent of their calories from carbohydrates, 20 to 35 percent from fat, and 10 to 35 percent from protein.

Carbohydrates are one of three macronutrients that provide the body with energy (protein and fats being the other two). The chemical compounds in carbohydrates are found in both simple and complex forms, and in order for the body to use carbohydrates for energy, food must undergo digestion, absorption, and glycolysis. It is recommended that 55 to 60 percent of caloric intake come from carbohydrates.

Chemical Structure
Carbohydrates are a main source of energy for the body and are made of carbon, hydrogen, and oxygen. Chlorophyll in plants absorbs light energy from the sun. This energy is used in the process of photosynthesis, which allows green plants to take in carbon dioxide and release oxygen and allows for the production of carbohydrates. This process converts the sun's light energy into a form of chemical energy useful to humans. Plants transform carbon dioxide (CO) from the air, water (HO) from the ground, and energy from the sun into oxygen (O) and carbohydrates (CHO) (6 CO+ 6 HO + energy = CHO + 6 O). Most carbohydrates have a ratio of 1:2:1 of carbon, hydrogen, and oxygen, respectively.

Humans and other animals obtain carbohydrates by eating foods that contain them. In order to use the energy contained in the carbohydrates, humans must metabolize, or break down, the structure of the molecule in a process that is opposite that of photosynthesis. It starts with the carbohydrate and oxygen and produces carbon dioxide, water, and energy. The body utilizes the energy and water and rids itself of the carbon dioxide.

Simple Carbohydrates
Simple carbohydrates, or simple sugars, are composed of monosaccharide or disaccharide units. Common monosaccharides (carbohydrates composed of single sugar units) include glucose, fructose, and galactose. Glucose is the most common type of sugar and the primary form of sugar that is stored in the body for energy. It sometimes is referred to as blood sugar or dextrose and is of particular importance to individuals who have diabetes or hypoglycemia. Fructose, the primary sugar found in fruits, also is found in honey and high-fructose corn syrup (in soft drinks) and is a major source of sugar in the diet of Americans. Galactose is less likely than glucose or fructose to be found in nature. Instead, it often combines with glucose to form the disaccharide lactose, often referred to as milk sugar. Both fructose and galactose are metabolized to glucose for use by the body.

Oligosaccharides are carbohydrates made of two to ten monosaccharides. Those composed of two sugars are specifically referred to as disaccharides, or double sugars. They contain two monosaccharides bound by either an alpha bond or a beta bond. Alpha bonds are digestible by the human body, whereas beta bonds are more difficult for the body to break down.

There are three particularly important disaccharides: sucrose, maltose, and lactose. Sucrose is formed when glucose and fructose are held together by an alpha bond. It is found in sugar cane or sugar beets and is refined to make granulated table sugar. Varying the degree of purification alters the final product, but white, brown, and powdered sugars all are forms of sucrose. Maltose, or malt sugar, is composed of two glucose units linked by an alpha bond. It is produced from the chemical decomposition of starch, which occurs during the germination of seeds and the production of alcohol. Lactose is a combination of glucose and galactose. Because it contains a beta bond, it is hard for some individuals to digest in large quantities. Effective digestion requires sufficient amounts of the enzyme lactase.

Sugar	Carbohydrate	Monosaccharide or disaccharide	Additional information
Beet sugar (cane sugar)	Sucrose	Disaccharide (fructose and glucose)	Similar to white and powdered sugar, but varied degree of purification
Brown sugar	Sucrose	Disaccharide (fructose and glucose)	Similar to white and powdered sugar, but varied degree of purification
Corn syrup	Glucose	Monosaccharide
Fruit sugar	Fructose	Monosaccharide	Very sweet
High-fructose corn syrup	Fructose	Monosaccharide	Very sweet and inexpensive Added to soft drinks and canned or frozen fruits
Honey	Fructose and glucose	Monosaccharides
Malt sugar	Maltose	Disaccharide (glucose and glucose)	Formed by the hydrolysis of starch, but sweeter than starch
Maple syrup	Sucrose	Disaccharide (fructose and glucose)
Milk sugar	Lactose	Disaccharide (glucose and galactose)	Made in mammary glands of most lactating animals
Powdered sugar	Sucrose	Disaccharide (fructose and glucose)	Similar to white and brown sugar, but varied degree of purification
White sugar	Sucrose	Disaccharide (fructose and glucose)	Similar to brown and powdered sugar, but varied degree of purification
SOURCE: Mahan and Escott-Stump, 2000; Northwestern University; Sizer and Whitney, 1997; and Wardlaw and Kessel, 2002.

Complex Carbohydrates
Complex carbohydrates, or polysaccharides, are composed of simple sugar units in long chains called polymers. Three polysaccharides are of particular importance in human nutrition: starch, glycogen, and dietary fiber.

Starch and glycogen are digestible forms of complex carbohydrates made of strands of glucose units linked by alpha bonds. Starch, often contained in seeds, is the form in which plants store energy, and there are two types: amylose and amylopectin. Starch represents the main type of digestible complex carbohydrate. Humans use an enzyme to break down the bonds linking glucose units, thereby releasing the sugar to be absorbed into the bloodstream. At that point, the body can distribute glucose to areas that need energy, or it can store the glucose in the form of glycogen.

Glycogen is the polysaccharide used to store energy in animals, including humans. Like starch, glycogen is made up of chains of glucose linked by alpha bonds; but glycogen chains are more highly branched than starch. It is this highly branched structure that allows the bonds to be more quickly broken down by enzymes in the body. The primary storage sites for glycogen in the human body are the liver and the muscles.

Another type of complex carbohydrate is dietary fiber. In general, dietary fiber is considered to be polysaccharides that have not been digested at the point of entry into the large intestine. Fiber contains sugars linked by bonds that cannot be broken down by human enzymes, and are therefore labeled as indigestible. Because of this, most fibers do not provide energy for the body. Fiber is derived from plant sources and contains polysaccharides such as cellulose, hemicellulose, pectin, gums, mucilages, and lignins.

The indigestible fibers cellulose, hemicellulose, and lignin make up the structural part of plants and are classified as insoluble fiber because they usually do not dissolve in water. Cellulose is a nonstarch carbohydrate polymer made of a straight chain of glucose molecules linked by beta bonds and can be found in whole-wheat flour, bran, and vegetables. Hemicellulose is a nonstarch carbohydrate polymer made of glucose, galactose, xylose, and other monosaccharides; it can be found in bran and whole grains. Lignin, a noncarbohydrate polymer containing alcohols and acids, is a woody fiber found in wheat bran and the seeds of fruits and vegetables.

In contrast, pectins, mucilages, and gums are classified as soluble fibers because they dissolve or swell in water. They are not broken down by human enzymes, but instead can be metabolized (or fermented) by bacteria present in the large intestine. Pectin is a fiber made of galacturonic acid and other monosaccharides. Because it absorbs water and forms a gel, it is often used in jams and jellies. Sources of pectin include citrus fruits, apples, strawberries, and carrots. Mucilages and gums are similar in structure. Mucilages are dietary fibers that contain galactose, manose, and other monosaccharides; and gums are dietary fibers that contain galactose, glucuronic acid, and other monosaccharides. Sources of gums include oats, legumes, guar, and barley.

Digestion and Absorption
Carbohydrates must be digested and absorbed in order to transform them into energy that can be used by the body. Food preparation often aids in the digestion process. When starches are heated, they swell and become easier for the body to break down. In the mouth, the enzyme amylase, which is contained in saliva, mixes with food products and breaks some starches into smaller units. However, once the carbohydrates reach the acidic environment of the stomach, the amylase is inactivated. After the carbohydrates have passed through the stomach and into the small intestine, key digestive enzymes are secreted from the pancreas and the small intestine where most digestion and absorption occurs. Pancreatic amylase breaks starch into disaccharides and small polysaccharides, and enzymes from the cells of the small-intestinal wall break any remaining disaccharides into their monosaccharide components. Dietary fiber is not digested by the small intestine; instead, it passes to the colon unchanged.

Sugars such as galactose, glucose, and fructose that are found naturally in foods or are produced by the breakdown of polysaccharides enter into absorptive intestinal cells. After absorption, they are transported to the liver where galactose and fructose are converted to glucose and released into the bloodstream. The glucose may be sent directly to organs that need energy, it may be transformed into glycogen (in a process called glycogenesis) for storage in the liver or muscles, or it may be converted to and stored as fat.

Glycolysis
The molecular bonds in food products do not yield high amounts of energy when broken down. Therefore, the energy contained in food is released within cells and stored in the form of adenosine triphosphate (ATP), a high-energy compound created by cellular energy-production systems. Carbohydrates are metabolized and used to produce ATP molecules through a process called glycolysis.

Glycolysis breaks down glucose or glycogen into pyruvic acid through enzymatic reactions within the cytoplasm of the cells. The process results in the formation of three molecules of ATP (two, if the starting product was glucose). Without the presence of oxygen, pyruvic acid is changed to lactic acid, and the energy-production process ends. However, in the presence of oxygen, larger amounts of ATP can be produced. In that situation, pyruvic acid is transformed into a chemical compound called acetyle coenzyme A, a compound that begins a complex series of reactions in the Krebs Cycle and the electron transport system. The end result is a net gain of up to thirty-nine molecules of ATP from one molecule of glycogen (thirty-eight molecules of ATP if glucose was used). Thus, through certain systems, glucose can be used very efficiently in the production of energy for the body.

Recommended Intake
At times, carbohydrates have been incorrectly labeled as "fattening." Evidence actually supports the consumption of more, rather than less, starchy foods. Carbohydrates have four calories per gram, while dietary fats contribute nine per gram, so diets high in complex carbohydrates are likely to provide fewer calories than diets high in fat. Recommendations are for 55 to 60 percent of total calories to come from carbohydrates (approximately 275 to 300 grams for a 2,000-calorie diet). The majority of carbohydrate calories should come from complex rather than simple carbohydrates. Of total caloric intake, approximately 45 to 50 percent of calories should be from complex carbohydrates, and 10 percent or less from simple carbohydrates.

It is important to consume a minimum amount of carbohydrates to prevent ketosis, a condition resulting from the breakdown of fat for energy in the absence of carbohydrates. In this situation, products of fat breakdown, called ketone bodies, build up in the blood and alter normal pH balance. This can be particularly harmful to a fetus. To avoid ketosis, daily carbohydrate intake should include a minimum of 50 to 100 grams. In terms of dietary fiber, a minimum intake of 20 to 35 grams per day is recommended.

Exchange System
The exchange system is composed of lists that describe carbohydrate, fat, and protein content, as well as caloric content, for designated portions of specific foods. This system takes into account the presence of more than one type of nutrient in any given food. Exchange lists are especially useful for individuals who require careful diet planning, such as those who monitor intake of calories or certain nutrients. It is particularly useful for diabetics, for whom carbohydrate intake must be carefully controlled, and was originally developed for planning diabetic diets.

Diabetes, Carbohydrate-Modified Diets, and Carbohydrate Counting
Diabetes is a condition that alters the way the body handles carbohydrates. In terms of diet modifications, diabetics can control blood sugar levels by appropriately managing the carbohydrates, proteins, and fats in their meals. The amount of carbohydrates, not necessarily the source, is the primary issue. Blood glucose levels after a meal can be related to the process of food preparation, the amount of food eaten, fat intake, sugar absorption, and the combination of foods in the meal or snack.

One method of monitoring carbohydrate levels—carbohydrate counting—assigns a certain number of carbohydrate grams or exchanges to specific foods. Calculations are used to determine insulin need, resulting in better control of blood glucose levels with a larger variety of foods. Overall, diabetic diets can include moderate amounts of sugar, as long as they are carefully monitored.

See also Diabetes mellitus; Fats; Nutrients; Protein; Weight loss diets.

Bibliography
Bounds, Laura E.; Agnor, Dottiedee; Darnell, Gayden S.; and Shea, Kirstin Brekken (2003). Health and Fitness: A Guide to a Healthy Lifestyle, 2nd edition. Dubuque, IA: Kendall/Hunt.
Duyff, Roberta Larson (2002). American Dietetic Association: Complete Food and Nutrition Guide, 2nd edition. Hoboken, NJ: John Wiley.
Mahan, L. Kathleen, and Escott-Stump, Sylvia (2000). Krause's Food, Nutrition, and Diet Therapy, 10th edition. Philadelphia: W. B. Saunders.
Robbins, Gwen; Powers, Debbie; and Burgess, Sharon (2002). A Wellness Way of Life, 5th edition. New York: McGraw-Hill.
Sizer, Frances, and Whitney, Eleanor (1997). Nutrition: Concepts and Controversies, 7th edition. Belmont, CA: Wadsworth Publishing.
Wardlaw, Gordon M., and Kessel, Margaret (2002). Perspectives in Nutrition, 5th edition. New York: McGraw-Hill.
Wilmore, Jack H., and Costill, David L. (1999). Physiology of Sport and Exercise, 2nd edition. Champaign, IL: Human Kinetics.

Internet Resources
American Diabetes Association. http://www.diabetes.org
American Dietetic Association. http://www.eatright.org
Carpi, Anthony. "Carbohydrates." Visionlearning. Available from http://www.visionlearning.com
Kennedy, Ron. "Carbohydrates in Nutrition." Doctor's Medical Library. Available from http://www.medical-library.net/sites

Northwestern University, Department of Preventive Medicine. "Nutrition Fact Sheets: Carbohydrates." Available from http://www.feinberg.northwestern.edu/nutrition

KEY TERMS Diabetes—A condition characterized by inadequate use of insulin preventing a person from controlling blood sugar levels. Fructose—A monosaccharide known as fruit sugar. Galactose—A monosaccharide known as milk sugar. Glucose—A monosaccharide used for energy; also known as blood sugar. Lactose—A disaccharide known as milk sugar. Low-density lipoprotein (LDL)—The so-called bad cholesterol that contains a large amount of cholesterol and transports lipids (fats) to other tissues in the body. Maltose—A disaccharide known as malt sugar. Sucrose—A disaccharide known as table sugar. Polysaccharides—Long chains of glucose units linked together.

    Description
    Precautions
    Interactions
    Aftercare
    Complications
    Parental concerns
    Resources

What are Carbohydrates?

Carbohydrates are compounds that consist of carbon, hydrogen, and oxygen, linked together by energy-containing bonds. There are two types of carbohydrates: complex and simple. The complex carbohydrates, such as starch and fiber, are classified as

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polysaccharides. Simple carbohydrates are known as sugars and they are classified as either monosacchar-ides (one sugar molecule) or disaccharides (two sugar molecules).

What is the Purpose of Carbohydrates?

In the digestive tract, carbohydrates are broken down into glucose, which provides energy for the body’s cells and tissues. Glucose is the body’s primary source of fuel.

Carbohydrates

Refined and processed carbohydrates	Whole grain and high-fiber carbohydrates
White bread	100% whole wheat bread
White rice	Oatmeal
White potatoes	Brown rice
Pasta	Whole wheat pasta
Sugary cereals	Whole grain crackers
Cinnamon toast	Popcorn
Sweets	Cornmeal
Jellies	Hulled barley
Candy	Whole wheat bulgur
Soft drinks	Bran cereals
Sugars	Rye wafer crackers
Fruit drinks (fruitades and fruit punch)	English muffins Dry beans and peas
Cakes, cookies and pies	Navy beans
Dairy desserts	Kidney beans
Ice cream	Split peas
Sweetened yogurt	Lentils
Sweetened milk	White beans
	Pinto beans
	Green peas
	Soybeans
	Whole fruits, fresh, frozen or canned
	Vegetables
	Low-fat milk

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Plants manufacture and store carbohydrates as their main source of energy through photosynthesis. Once consumed, these organic compounds can be digested, absorbed, and metabolized, supplying humans or animals with energy. Carbohydrates provide roughly half of the total caloric intake of the average human diet. These calories may be used immediately for energy metabolism or may be transformed and stored as glycogen or fat to be used as an energy source as demanded. Dietary carbohydrates are comprised of a wide array of compounds ranging from the simple oneor two-unit sugars to the long chain starches, glycogen and cellulose. Carbohydrates can be classified as monosaccharides, di-and oligosaccharides, and polysaccharides.

Table 1

Carbohydrate classification
Classification	Number of sugar units**	Examples
Monosaccharides	1	Glucose, galactose, fructose
Disaccharides	2	Sucrose, lactose, maltose
Oligosaccharides	2–10	Includes the disaccharides
Polysaccharides	> 10	Glycogen, starch, cellulose
**A "sugar unit" is one monosaccharide—each unit is not necessarily the same monosaccharide. For example, sucrose consists of one glucose uni and one fructose unit.

Monosaccharides, often referred to as simple sugars, are the simplest form of carbohydrates and are seldom found free in nature. The three that can be absorbed by the human body include glucose, galactose, and fructose. Glucose is the most abundant of the monosaccharides and the most important nutritionally. It is the repeating monosaccharide unit in starch, glycogen, and cellulose, and is found in all edible disaccharides.

Oligosaccharides are short chains of monosaccharide units that are joined by glycosidic bonds. They generally have between two to ten units, with the disaccharides, those chains containing two units, being the most abundant. The most common disaccharides include:

 Sucrose (from table, cane, and beet sugars), consisting of glucose and fructose
 Lactose (from milk sugar), consisting of glucose and galactose
 Maltose (from malt sugar), consisting of two glucose units 

Polysaccharides are long chains of monosaccharide units. The major polysaccharides include the digestible forms (glycogen and starch) and nondigestible forms (cellulose, hemicellulose, lignin, pectin, and gums).

Starch is the most common digestible polysaccharide found in plants. It can be found in two forms—amylose and amylopectin. Amylose is a linear, unbranched molecule that is bound solely by a-1,4 glycosidic bonds. Amylopectin, which makes up the greatest percent of the total starch content, is branched with a-1,6 bonds at the branch points.

Glycogen is the major storage form of carbohydrates in animals, found primarily in the liver and skeletal muscle. When energy intake exceeds energy expenditure, excess calories from fat, protein, and carbohydrate can be used to form glycogen. It is made up of repeating glucose units and is highly branched. During times of fasting or in between meals, these chains can be broken down to single glucose units and used as an energy source for the body. Although found in animal tissue, animal products do not contain large amounts of glycogen because it is depleted at the time of slaughter due to stress hormones.

Cellulose is the major component of cell walls in plants. Just as starch and glycogen, it too is made up of repeating glucose molecules. However, the glycosidic bonds connecting the units are b-1,4. These bonds are resistant to mammalian digestive enzymes rendering cellulose, and other substances containing these bonds, indigestible. Thus, cellulose is not considered to be a significant source of energy for the body. However, as a fiber, it is important for intestinal bacteria.

Since cellulose is a major part of the plant cell wall, it also encases some of the starch, preventing the digestive enzymes from reaching it and decreasing the digestibility of some raw foods such as potatoes and grains. Cooking causes the granules to swell and also softens and ruptures the cellulose wall, allowing the starch to be digested.

Dietary Fiber

Fiber can be classified as soluble and insoluble. Soluble fiber, which includes pectin and gums, dissolves in water to form a gel in the digestive tract. This increases the time the food is in the small intestine, thus increasing the chance of nutrients being absorbed. It is believed that soluble fiber plays a role in lowering blood LDL cholesterol. This could be due to the binding and increased excretion of fat and bile acid (a derivative of cholesterol) or other mechanisms not yet understood. Bacteria in the bowel can use fiber as a food source. These bacteria can degrade the fiber and release some components that can then be absorbed and used by the body. The increased nutrition for the bacteria can increase microbial growth, which can then lead to increased stool bulk, with little of the fiber actually found in the stool.

Insoluble fiber, including cellulose, hemicellulose, and lignin (a noncarbohydrate component of the cell wall that is often included as dietary fiber), absorbs water, thereby increasing the bulk and volume of the stool. It helps to speed the movement through the intestinal tract, preventing constipation, and is prescribed in the treatment of irritable bowel syndrome. It has also been shown that insoluble fibers bind fat-soluble carcinogens and remove them from the gastrointestinal tract, helping to decrease cancer risk.

Refined and processed foods have not only most of the fiber removed, but along with it many of the vitamins, minerals, and phytochemicals (chemicals found in plants believed to contain protective properties) that contribute to the health benefits of whole grain foods. The federal government's Dietary Guidelines for Americans encourage individuals to include whole grain foods in their diet to ensure adequate fiber to promote proper bowel function, as well as to receive other added health benefits.

Digestion, Absorption, and Transportation

In order for carbohydrates to be absorbed by the intestinal mucosal cells, they must first be converted into monosaccharides. The digestive process begins in the mouth with salivary a-amylase that partially breaks down starch by hydrolyzing some of the a-1,4 bonds. However, the digestion that takes place here is of little significance since food remains in the mouth for only a brief period, although this may differ depending on chewing time. The enzyme continues to work for a short time in the stomach until the pH is lowered due to hydrochloric acid that inhibits the enzyme.

Table 2

Examples of carbohydrate food sources
Monosaccharides
Glucose	Fructose	Galactose
Fruit	High-fructose corn syrup	Milk
Vegetables	Honey	Milk products
Honey	Fruit
Disaccharides
Sucrose	Lactose	Maltose
Table sugar	Milk	Beer
Maple sugar	Milk products	Malt liquor
Fruit
Vegetables
Honey
Polysaccharides
Starch (rye, oats, wheat, rice, potatoes, legumes, cereals, bread)
Dietary Fiber
Soluble
Pectin	Gums
Fruits (apples, berries)	Oats, barley
Jams and jellies (additive)	Ice cream (additive) Legumes
Insoluble
Cellulose	Hemicellulose	Lignin
Whole wheat foods	Whole grains	Fruit
Bran		Seeds
Leafy vegetables		Bran, wheat Vegetables

The bulk of carbohydrate digestion occurs in the small intestine by pancreatic a-amylase. The pH of the small intestines is increased due to the addition of bicarbonate and bile, allowing the enzyme activity to occur. Specific disaccharidases located on the intestinal mucosal cells help to further break down the carbohydrates into the monosaccharides: glucose, fructose, and galactose.

Once the carbohydrates have been broken down, the monosaccharides can be absorbed by the mucosal cells. Glucose and galactose enter by active transport, which requires energy as well as specific receptors and carriers. Fructose is absorbed by facilitated diffusion. Like active transport, facilitated diffusion requires a specific carrier, but instead of needing energy, it relies on the low levels of fructose inside the cell to "pull" the fructose inside. Once transported through the intestinal wall, the monosaccharides enter the blood through the capillaries and are carried to the portal circulation and then to the liver.

Metabolism of Carbohydrates

The liver is the major site of galactose and fructose metabolism, where they are taken up, converted to glucose derivatives, and either stored as liver glycogen or used for energy immediately when needed. Although glucose is metabolized extensively in the liver, unlike galactose and fructose, it is also passed into the blood supply to be used by other tissues. Tissues like skeletal muscle and adipose tissue depend on insulin for glucose uptake, whereas the brain and liver do not. This dependence on insulin becomes a problem for diabetics who either cannot make insulin (IDDM) or are resistent to insulin (NIDDM). For individuals left untreated, dietary carbohydrates cause glucose levels to rise, resulting in hyperglycemia, which will lead to serious consequences if steps are not taken to correct it.

Once in the tissues, the fate of glucose depends on the energy demands of the body. Glucose can be metabolized through the glycolysis pathway to pyruvate where it is either converted to lactate or completely oxidized to CO₂, H₂O, and energy. Liver and skeletal muscle can convert excess glucose to glycogen through a pathway known as glycogenesis. The glycogen is stored after meals to be used as an energy source when energy demands are higher than intake. At this time the glycogen is broken down into individual glucose units, a process known as glycogenolysis, and the glucose can be metabolized further. Excess carbohydrates also can be used as a substrate for fat synthesis.

Carbohydrates are an essential part of a healthy diet. They provide an easily available energy source, are an important vehicle for micronutrients and phytochemicals, help to maintain adequate blood glucose, and are important in maintaining the integrity and function of the gastrointestinal tract. Table 2 contains examples of foods that contain the various types of carbohydrates.

Starch

Starch from plants makes up about half of our dietary carbohydrates. Starch molecules can aggregate to form granules that differ by size and shape depending on the source of the starch, for example, corn, potato, and manioc. Although there is no difference in the nutritional value between the starches since all cooked starches are broken down in the body into glucose molecules, they do differ by characteristics such as solubility, flavor, and thickening power. Because of these characteristics, starch is often removed from the source to use commercially. For example, the starch can be removed from tubers such as potatoes and manioc (also known as cassava) through a wet milling process, or in the case of manioc, through leaching and drying. The potato starch is often used as a thickener or instead of cornstarch in recipes, while manioc is best known as tapioca.

Bibliography

Ettinger, Susan. "Macronutrients: Carbohydrates, Proteins and Lipids." In Krause's Food, Nutrition, and Diet Therapy, edited Kathleen L. Mahan and Sylvia Escott-Stump. 10th ed. Philadelphia, Pa.: W. B. Saunders, 2000.

FAO/WHO. Carbohydrates in Human Nutrition: Report of a JointFAO/WHO Expert Consultation, Rome, 14–18 April 1997. Rome: World Health Organization, Food and Agriculture Organization of the United Nations, 1998.

Guthrie, Joanne, and Joan Morton. "Food Sources of Added Sweetners in the Diets of Americans." Journal of the American Dietetic Association 100 (2000): 43–48, 51.

Kiens, B., and E. A. Richter. "Types of Carbohydrates in an Ordinary Diet Affect Insulin Action and Muscle Substrates in Humans." American Journal of Clinical Nutrition. 63 (1996): 47–53.

Macdonald, I. A. "Carbohydrate as a Nutrient in Adults: Range of Acceptable Intakes." European Journal of Clinical Nutrition. 53 (1999): S101–S106.

—Debra Coward McKenzie Rachel K. Johnson

IN BRIEF: Plural of any of a group of neutral compounds composed of a series of elements including oxygen.

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Carbohydrates are organic compounds composed of carbon, hydrogen, and oxygen. The general chemical formula for carbohydrates is CH2O, indicating there is twice as much hydrogen as oxygen. Carbohydrates are the major source of energy for cells and cellular activities.

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Next question:	How are carbohydrates classified?

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Substances composed of long chains of oxygen, hydrogen, and carbon molecules. Sugar, starch, and cellulose are all carbohydrates. In the human body, carbohydrates play a major role in respiration; in plants, they are important in photosynthesis.

Carbohydrates in food provide energy for the body and, if present in excess, are stored as fat.

A group of substances that provide the body with one of the main sources of energy (the other is fat). Carbohydrates include sugar and starch. Glucose, fructose, and galactose are monosaccharides or mono sugars. Sucrose, lactose, and maltose are disugars or disaccharides. Multiple sugars are named according to their average molecular weight and to their type characteristics. From lower molecular weight ingredients to higher polysaccharides, they range from dextrins, maltodextrins, starches, and cellulose, and other soluble and insoluble fibers. Amylopectin and amylose are two polysaccha-rides that make up starches. In the body, polysaccharides are broken down into monosaccharides by the presence of enzymes. The enzyme that breaks down starch is called amylase. Cellulose and other complex polysaccharides such as gums pass through the body unabsorbed. Unlike some ruminant animals that can digest cellulose due to the presence of a fore stomach, allowing the animal to chew its cud, humans cannot digest cellulose due to the absence of cellulases and the necessary organ structure. Polysaccha-rides that dissolve in water but are not digestible are called soluble fibers. Soluble fibers are important in aiding the digestion process, and aid in good absorption of nutrients and bowel health. Monosaccharides are absorbed through the intestinal wall into the bloodstream and subsequently are distributed throughout the body. Brain cells and blood cells must have glucose, a common monosaccharide, to survive. Cells burn the glucose, immediately generating energy and heat. The unused glucose is distributed to the liver, fat cells, and muscle tissues and is converted into glycogen (animal starch) or into fat for storage. Insulin secreted by the pancreas takes high levels of glucose in the blood and converts it to glycogen and fat. When needed, the glycogen can then be reconverted to glucose at a later time to then be metabolized as described above. When blood sugar level is low, glucagon, also produced by the pancreas stimulates glycogen conversion again to glucose and is then introduced into the bloodstream. Other factors, such as stress, epinephrine, and corticosteroids have the same effect as glucagon. Galactose and fructose are converted into glucose in the liver. See Sugar(s) and Polyhydroxyl Compounds, Non-Nutritive Sweeteners, Dextrin, Maltodextrin, Cellulose, Fat.

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any of a group of organic compounds based on the general formula C_x(H₂O)_y. The group comprises the monosaccharides, oligosaccharides, and polysaccharides, and is usually extended to include their derivatives and, sometimes, the cyclitols. Some of the simplest members of the group may, notionally, be considered as hydrates of carbon. However, 2-deoxy-d-ribose (C₅H₁₀O₄) does not fit the empirical formulation. Carbohydrates are also components of nucleosides, nucleotides, RNA and DNA, glycoproteins, glycolipids, and glycosaminoglycans. Examples include amylose, amylopectin, chitin, and glycogen.

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A compound of carbon, hydrogen and oxygen, the latter two usually in the proportions of water (CH₂O)_n. They are classified into mono-, di-, tri-, poly- and heterosaccharides. Carbohydrates in food are an important and immediate source of energy for the body; 1 gram of carbohydrate yields 3.75 calories (16 kilojoules). They are present, at least in small quantities, in most foods, but the chief sources are the sugars and starches of plants. Herbivores are able to utilize the insoluble polysaccharides (crude fiber) because of bacterial conversion to volatile fatty acids by fermentation in the rumen and cecum.
Carbohydrates may be stored in the body as glycogen for future use. If they are eaten in excessive amounts they are converted to and stored as fat. Rapid ingestion of very large amounts in ruminants and horses causes carbohydrate engorgement.

• complex c. — polysaccharides containing either α- and β-type glycosidic bonds. Usually occurring in mixtures in food.
• dietary c. — the carbohydrate components of food.
• c. loading — depletion/repletion means of maximally loading glycogen into type II muscle for increased power of muscle contraction.
• c. loss — glucose loss in urine due to diabetes mellitus or chronic renal disease.
• c. metabolism — series of related enzymic reactions involved in the synthesis and catabolism of carbohydrates.
• c. tolerance test — see glucose tolerance test.

n.pl

A group of organic compounds with the class name saccharides, which are the aldehydic or ketonic derivatives of polyhydric alcohols. Carbohydrates such as sugar, starch, cellulose, and gum are generally synthesized by green plants. Carbohydrates constitute the main energy source in the diet and are classified as mono-, di-, tri-, and polysaccharides.

For a list of words related to carbohydrate, see:

• Nutrition For Fitness - carbohydrate: essential nutrient substance in sugars and starches
• Physiology - carbohydrate: any of various organic saccharide sugars, starches, and celluloses that supply energy to body when consumed

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See words rhyming with "carbohydrate."

  See crossword solutions for the clue Carbohydrate.

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Dansk (Danish)
n. - kulhydrat

Nederlands (Dutch)
koolhydraat

Français (French)
n. - hydrate de carbone

Deutsch (German)
n. - Kohlenhydrat

Ελληνική (Greek)
n. - (χημ.) υδατάνθρακας

Italiano (Italian)
carboidrato

Português (Portuguese)
n. - carboidrato (m) (Quím.)

Русский (Russian)
карбогидрат

Español (Spanish)
n. - hidrato de carbono, carbohidrato

Svenska (Swedish)
n. - kolhydrat

中文（简体）(Chinese (Simplified))
碳水化合物, 醣

中文（繁體）(Chinese (Traditional))
n. - 碳水化合物, 醣

한국어 (Korean)
n. - 탄수화물

日本語 (Japanese)
n. - 炭水化物

العربيه (Arabic)
‏(الاسم) الكربوهيدرات, مادة مكونه من كربون و هيدروجين و أوكسجين‏

עברית (Hebrew)
n. - ‮פחמימה‬

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