carbohydrate

For more information on carbohydrate, visit Britannica.com.

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.

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.

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.

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.

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.

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

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

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

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.

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.

Lactose is a disaccharide found in milk. It consists of a molecule of D-galactose and a molecule of D-glucose bonded by a-1-4 glycosidic linkage.

A carbohydrate is an organic compound with general formula C_m(H₂O)_n, that is, consisting only of carbon, hydrogen and oxygen, the last two in the 2:1 atom ratio. Carbohydrates can be viewed as hydrates of carbon, hence their name.

The term is most commonly used in biochemistry, where it is essentially a synonym of saccharide, a large family of natural carbohydrates that fill numerous roles in living things, such as the storage and transport of energy (e.g., starch, glycogen) and structural components (e.g., cellulose in plants and chitin in arthropods). This word comes from the Greek σάκχαρον (sákcharon), meaning "sugar". Saccharides and their derivatives include many other important biomolecules that play key roles in the immune system, fertilization, pathogenesis, blood clotting, and development.^[1]

In food science and in many informal contexts, the term carbohydrate often means any food that is particularly rich in starch (such as cereals, bread and pasta) or sugar (such as candy, jams and desserts).

While the scientific nomenclature of carbohydrates is complex, the names of carbohydrates very often end in the suffix -ose.

Contents 1 Structure 2 Monosaccharides 2.1 Classification of monosaccharides 2.2 Ring-straight chain isomerism 2.3 Use in living organisms 3 Disaccharides 4 Oligosaccharides and polysaccharides 5 Nutrition 5.1 Classification 6 Metabolism 6.1 Catabolism 7 Carbohydrate chemistry 8 See also 9 Notes 10 References 11 External links

Structure

Formerly the name "carbohydrate" was used in chemistry for any compound with the formula C_m(H₂O)_n. Following this definition, some chemists considered formaldehyde CH₂O to be the simplest carbohydrate,^[2] while others claimed that title for glycolaldehyde ^[3] Today the term is generally understood in the biochemistry sense, which excludes compounds with only one or two carbons.

Natural saccharides are generally built of simple carbohydrates called monosaccharides with general formula (CH₂O)_n where n is three or more. A typical monosaccharide has the structure H-(CHOH)_x(C=O)-(CHOH)_y-H, that is, an aldehyde or ketone with many hydroxyl groups added, usually one on each carbon atom that is not part of the aldehyde or ketone functional group. Examples of monosaccharides are glucose, fructose, and glyceraldehyde. However, some biological substances commonly called "monosaccharides" do not conform to this formula (e.g., uronic acids and deoxy-sugars such as fucose), and there are many chemicals that do conform to this formula but are not considered to be monosaccharides (e.g., formaldehyde CH₂O and inositol (CH₂O)₆).^[4]

The open-chain form of a monosaccharide often coexists with a closed ring form where the oxygen of the carbonyl group C=O is replaced by an internal -O- bridge.

Monosaccharides can be linked together into what are called polysaccharides (or oligosaccharides) in a large variety of ways. Many carbohydrates contain one or more modified monosaccharide units that have had one or more groups replaced or removed. For example, deoxyribose, a component of DNA, is a modified version of ribose; chitin is composed of repeating units of N-acetylglucosamine, a nitrogen-containing form of glucose.

Monosaccharides

Main article: Monosaccharide

D-glucose is an aldohexose with the formula (C·H₂O)₆. The red atoms highlight the aldehyde group, and the blue atoms highlight the asymmetric center furthest from the aldehyde; because this -OH is on the right of the Fischer projection, this is a D sugar.

Monosaccharides are the simplest carbohydrates in that they cannot be hydrolyzed to smaller carbohydrates. They are aldehydes or ketones with two or more hydroxyl groups. The general chemical formula of an unmodified monosaccharide is (C•H₂O)_n, literally a "carbon hydrate." Monosaccharides are important fuel molecules as well as building blocks for nucleic acids. The smallest monosaccharides, for which n = 3, are dihydroxyacetone and D- and L-glyceraldehyde.

Classification of monosaccharides

The α and β anomers of glucose. Note the position of the anomeric carbon (red or green) relative to the CH₂OH group bound to carbon 5: they are either on the opposite sides (α), or the same side (β).

Monosaccharides are classified according to three different characteristics: the placement of its carbonyl group, the number of carbon atoms it contains, and its chiral handedness. If the carbonyl group is an aldehyde, the monosaccharide is an aldose; if the carbonyl group is a ketone, the monosaccharide is a ketose. Monosaccharides with three carbon atoms are called trioses, those with four are called tetroses, five are called pentoses, six are hexoses, and so on. ^[5] These two systems of classification are often combined. For example, glucose is an aldohexose (a six-carbon aldehyde), ribose is an aldopentose (a five-carbon aldehyde), and fructose is a ketohexose (a six-carbon ketone).

Each carbon atom bearing a hydroxyl group (-OH), with the exception of the first and last carbons, are asymmetric, making them stereocenters with two possible configurations each (R or S). Because of this asymmetry, a number of isomers may exist for any given monosaccharide formula. The aldohexose D-glucose, for example, has the formula (C·H₂O)₆, of which all but two of its six carbons atoms are stereogenic, making D-glucose one of 2⁴ = 16 possible stereoisomers. In the case of glyceraldehyde, an aldotriose, there is one pair of possible stereoisomers, which are enantiomers and epimers. 1,3-dihydroxyacetone, the ketose corresponding to the aldose glyceraldehyde, is a symmetric molecule with no stereocenters). The assignment of D or L is made according to the orientation of the asymmetric carbon furthest from the carbonyl group: in a standard Fischer projection if the hydroxyl group is on the right the molecule is a D sugar, otherwise it is an L sugar. The "D-" and "L-" prefixes should not be confused with "d-" or "l-", which indicate the direction that the sugar rotates plane polarized light. This usage of "d-" and "l-" is no longer followed in carbohydrate chemistry.^[6]

Ring-straight chain isomerism

Glucose can exist in both a straight-chain and ring form.

The aldehyde or ketone group of a straight-chain monosaccharide will react reversibly with a hydroxyl group on a different carbon atom to form a hemiacetal or hemiketal, forming a heterocyclic ring with an oxygen bridge between two carbon atoms. Rings with five and six atoms are called furanose and pyranose forms, respectively, and exist in equilibrium with the straight-chain form.^[7]

During the conversion from straight-chain form to cyclic form, the carbon atom containing the carbonyl oxygen, called the anomeric carbon, becomes a stereogenic center with two possible configurations: The oxygen atom may take a position either above or below the plane of the ring. The resulting possible pair of stereoisomers are called anomers. In the α anomer, the -OH substituent on the anomeric carbon rests on the opposite side (trans) of the ring from the CH₂OH side branch. The alternative form, in which the CH₂OH substituent and the anomeric hydroxyl are on the same side (cis) of the plane of the ring, is called the β anomer. You can remember that the β anomer is cis by the mnemonic, "It's always better to βe up". Because the ring and straight-chain forms readily interconvert, both anomers exist in equilibrium.^[7]

Use in living organisms

Monosaccharides are the major source of fuel for metabolism, being used both as an energy source (glucose being the most important in nature) and in biosynthesis. When monosaccharides are not immediately needed by many cells they are often converted to more space efficient forms, often polysaccharides. In many animals, including humans, this storage form is glycogen, especially in liver and muscle cells. In plants, starch is used for the same purpose.

Disaccharides

Sucrose, also known as table sugar, is a common disaccharide. It is composed of two monosaccharides: D-glucose (left) and D-fructose (right).

Main article: Disaccharide

Two joined monosaccharides are called a disaccharide and these are the simplest polysaccharides. Examples include sucrose and lactose. They are composed of two monosaccharide units bound together by a covalent bond known as a glycosidic linkage formed via a dehydration reaction, resulting in the loss of a hydrogen atom from one monosaccharide and a hydroxyl group from the other. The formula of unmodified disaccharides is C₁₂H₂₂O₁₁. Although there are numerous kinds of disaccharides, a handful of disaccharides are particularly notable.

Sucrose, pictured to the right, is the most abundant disaccharide, and the main form in which carbohydrates are transported in plants. It is composed of one D-glucose molecule and one D-fructose molecule. The systematic name for sucrose, O-α-D-glucopyranosyl-(1→2)-D-fructofuranoside, indicates four things:

• Its monosaccharides: glucose and fructose
• Their ring types: glucose is a pyranose, and fructose is a furanose
• How they are linked together: the oxygen on carbon number 1 (C1) of α-D-glucose is linked to the C2 of D-fructose.
• The -oside suffix indicates that the anomeric carbon of both monosaccharides participates in the glycosidic bond.

Lactose, a disaccharide composed of one D-galactose molecule and one D-glucose molecule, occurs naturally in mammalian milk. The systematic name for lactose is O-β-D-galactopyranosyl-(1→4)-D-glucopyranose. Other notable disaccharides include maltose (two D-glucoses linked α-1,4) and cellulobiose (two D-glucoses linked β-1,4).

Oligosaccharides and polysaccharides

Amylose is a linear polymer of glucose mainly linked with α(1→4) bonds. It can be made of several thousands of glucose units. It is one of the two components of starch, the other being amylopectin.

Main articles: Oligosaccharide and Polysaccharide

Oligosaccharides and polysaccharides are composed of longer chains of monosaccharide units bound together by glycosidic bonds. The distinction between the two is based upon the number of monosaccharide units present in the chain. Oligosaccharides typically contain between two and nine monosaccharide units, and polysaccharides contain greater than ten monosaccharide units. Definitions of how large a carbohydrate must be to fall into each category vary according to personal opinion. Examples of oligosaccharides include the disaccharides mentioned above, the trisaccharide raffinose and the tetrasaccharide stachyose.

Oligosaccharides are found as a common form of protein posttranslational modification. Such posttranslational modifications include the Lewis and ABO oligosaccharides responsible for blood group classifications and so of tissue incompatibilities, the alpha-Gal epitope responsible for hyperacute rejection in xenotransplanation, and O-GlcNAc modifications.

Polysaccharides represent an important class of biological polymers. Their function in living organisms is usually either structure- or storage-related. Starch (a polymer of glucose) is used as a storage polysaccharide in plants, being found in the form of both amylose and the branched amylopectin. In animals, the structurally-similar glucose polymer is the more densely-branched glycogen, sometimes called 'animal starch'. Glycogen's properties allow it to be metabolized more quickly, which suits the active lives of moving animals.

Cellulose and chitin are examples of structural polysaccharides. Cellulose is used in the cell walls of plants and other organisms, and is claimed to be the most abundant organic molecule on earth.^[8] It has many uses such as a significant role in the paper and textile industries, and is used as a feedstock for the production of rayon (via the viscose process), cellulose acetate, celluloid, and nitrocellulose. Chitin has a similar structure, but has nitrogen-containing side branches, increasing its strength. It is found in arthropod exoskeletons and in the cell walls of some fungi. It also has multiple uses, including surgical threads.

Other polysaccharides include callose or laminarin, chrysolaminarin, xylan, mannan, fucoidan, and galactomannan.

Nutrition

Grain products: rich sources of complex and simple carbohydrates

Foods high in carbohydrates include breads, pastas, beans, potatoes, bran, rice, and cereals. Most such foods are high in starch. Carbohydrates require less water to digest than proteins or fats and are the most common source of energy in living things. Proteins and fat are necessary building components for body tissue and cells, and are also a source of energy for most organisms.

Carbohydrates are not essential nutrients in humans: the body can obtain all its energy from protein and fats^[9][10]. However, the brain and neurons generally cannot burn fat and need glucose for energy; the body can make some glucose from a few of the amino acids in protein and also from the glycerol backbone in triglycerides. Carbohydrate contains 15.8 kilojoules (3.75 kilocalories) and proteins 16.8 kilojoules (4 kilocalories) per gram, while fats contain 37.8 kilojoules (9 kilocalories) per gram. In the case of protein, this is somewhat misleading as only some amino acids are usable for fuel. Likewise, in humans, only some carbohydrates are usable for fuel, as in many monosaccharides and some disaccharides. Other carbohydrate types can be used, but only with the assistance of gut bacteria. Ruminants and termites can even process cellulose, which is indigestible to other organisms.

Based on the effects on risk of heart disease and obesity, the Institute of Medicine recommends that American and Canadian adults get between 40-65% of dietary energy from carbohydrates.^[11] The Food and Agriculture Organization and World Health Organization jointly recommend that national dietary guidelines set a goal of 55-75% of total energy from carbohydrates, but only 10% directly from sugars (their term for simple carbohydrates).^[12]

Classification

For dietary purposes, carbohydrates can be classified as simple (monosaccharides and disaccharides) or complex (oligosaccharides and polysaccharides). The term complex carbohydrate was first used in the U.S. Senate Select Committee on Nutrition and Human Needs publication Dietary Goals for the United States (1977), where it denoted "fruit, vegetables and whole-grains".^[13] Dietary guidelines generally recommend that complex carbohydrates, and such nutrient-rich simple carbohydrate sources such as fruit (glucose or fructose) and dairy products (lactose) make up the bulk of carbohydrate consumption. This excludes such sources of simple sugars as candy and sugary drinks.

The USDA's Dietary Guidelines for Americans 2005 dispensed with the simple/complex distinction, instead recommending fiber-rich foods and whole grains.^[14]

The glycemic index and glycemic load concepts have been developed to characterize food behavior during human digestion. They rank carbohydrate-rich foods based on the rapidity of their effect on blood glucose levels. The insulin index is a similar, more recent classification method that ranks foods based on their effects on blood insulin levels, which are caused by glucose (or starch) and some amino acids in food. Glycemic index is a measure of how quickly food glucose is absorbed, while glycemic load is a measure of the total absorbable glucose in foods.

Metabolism

Main article: Carbohydrate metabolism

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Catabolism

Catabolism is the metabolic reaction cells undergo in order to extract energy. There are two major metabolic pathways of monosaccharide catabolism: glycolysis and the citric acid cycle

In glycolysis, oligo/polysaccharides are cleaved first to smaller monosaccharides by enzymes called Glycoside hydrolases. The monosaccharide units can then enter into monosaccharide catabolism. In some cases, as with humans, not all carbohydrate types are usable as the digestive and metabolic enzymes necessary are not present. For instance, neither horses nor humans nor cats can digest and use cellulose, but ruminants and termites can.

Carbohydrate chemistry

Carbohydrate chemistry is a large and economically important branch of organic chemistry. Some of the main organic reactions that involve carbohydrates are:

• Carbohydrate acetalisation
• Cyanohydrin reaction
• Lobry-de Bruyn-van Ekenstein transformation
• Amadori rearrangement
• Nef reaction
• Wohl degradation
• Koenigs-Knorr reaction

See also

• Biochemistry
• Bioplastic
• Gluconeogenesis
• Glycolipid
• Glycoprotein
• Glycoinformatics
• Low-carbohydrate diet
• No-carbohydrate diet
• Macromolecules
• Nutrition
• Pentose phosphate pathway
• Photosynthesis
• Fermentation
• Sugar.

Notes

References

1. ^ Maton, Anthea; Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. pp. 52–59. ISBN 0-13-981176-1. 
2. ^ John Merle Coulter, Charler Reid Barnes, Henry Chandler Cowles (1930), A Textbook of Botany for Colleges and Universities"
3. ^ Carl A. Burtis, Edward R. Ashwood, Norbert W. Tietz (2000), Tietz fundamentals of clinical chemistry
4. ^ Matthews, C. E.; K. E. Van Holde; K. G. Ahern (1999) Biochemistry. 3rd edition. Benjamin Cummings. ISBN 0-8053-3066-6
5. ^ Campbell, Neil A.; Brad Williamson; Robin J. Heyden (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall. ISBN 0-13-250882-6. http://www.phschool.com/el_marketing.html. 
6. ^ Pigman, Ward; Horton, D. (1972). "Chapter 1: Stereochemistry of the Monosaccharides". in Pigman and Horton. The Carbohydrates: Chemistry and Biochemistry Vol 1A (2nd ed. ed.). San Diego: Academic Press. pp. 1-67. 
7. ^ ^{a b} Pigman, Ward; Anet, E.F.L.J. (1972). "Chapter 4: Mutarotations and Actions of Acids and Bases". in Pigman and Horton. The Carbohydrates: Chemistry and Biochemistry Vol 1A (2nd ed. ed.). San Diego: Academic Press. pp. 165–194. 
8. ^ N.A.Campbell (1996) Biology (4th edition). Benjamin Cummings NY. p.23 ISBN 0-8053-1957-3
9. ^ Is dietary carbohydrate essential for human nutrition? - Westman 75 (5): 951 - American Journal of Clinical Nutrition
10. ^ A High-Protein, High-Fat, Carbohydrate-Free Diet Reduces Energy Intake, Hepatic Lipogenesis, and Adiposity in Rats - Pichon et al. 136 (5): 1256 - Journal of Nutrition
11. ^ Food and Nutrition Board (2002/2005). Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. Washington, DC: The National Academies Press. Page 769. ISBN 0-309-08537-3
12. ^ Joint WHO/FAO expert consultation (2003). Diet, Nutrition and the Prevention of Chronic Diseases (PDF). Geneva: World Health Organization. Pages 55-56. ISBN 92-4-120916-X
13. ^ Joint WHO/FAO expert consultation (1998), Carbohydrates in human nutrition, chapter 1. ISBN 92-5-104114-8.
14. ^ DHHS and USDA, Dietary Guidelines for Americans 2005, Chapter 7 Carbohydrates

External links

Wikimedia Commons has media related to: Carbohydrates

• Carbohydrates, including interactive models and animations (Requires MDL Chime)
• IUPAC-IUBMB Joint Commission on Biochemical Nomenclature (JCBN): Carbohydrate Nomenclature
• Carbohydrates detailed
• Carbohydrates and Glycosylation - The Virtual Library of Biochemistry and Cell Biology
• Consortium for Functional Glycomics
• Wine Carbohydrates

v • d • e Metabolism (Catabolism, Anabolism) General Metabolic pathway · Metabolic network · Cellular respiration (Anaerobic/Aerobic) Specific paths Protein metabolism Protein synthesis · Amino acid synthesis · Catabolism Nucleotide metabolism: Purine metabolism  · Nucleotide salvage · Pyrimidine metabolism Carbohydrate metabolism Anabolism Gluconeogenesis · Glycogenesis · Photosynthesis (Carbon fixation) Carbohydrate catabolism Glycolysis · Glycogenolysis · Fermentation (Ethanol, Lactic acid)  · Cellular respiration · Xylose metabolism Other Pentose phosphate pathway Lipid metabolism Fatty acid metabolism Fatty acid degradation (Lipolysis, Beta oxidation) · Fatty acid synthesis Other Eicosanoid metabolism · Sphingolipid metabolism · Steroid metabolism Other Metal metabolism (Iron metabolism) · Ethanol metabolism

v • d • e Food chemistry Additives · Carbohydrates · Coloring · Enzymes · Essential fatty acids · Flavors · Lipids · Minerals · Proteins · Vitamins · Water

v • d • e Types of Carbohydrates General: Aldose · Ketose · Pyranose · Furanose Geometry Cyclohexane conformation · Anomer · Mutarotation Monosaccharides Trioses Ketotriose (Dihydroxyacetone) · Aldotriose (Glyceraldehyde) Tetroses Ketotetrose (Erythrulose) · Aldotetroses (Erythrose, Threose) Pentoses Ketopentose (Ribulose, Xylulose) Aldopentose (Ribose, Arabinose, Xylose, Lyxose) Deoxy sugar (Deoxyribose) Hexoses Ketohexose (Psicose, Fructose, Sorbose, Tagatose) Aldohexose (Allose, Altrose, Glucose, Mannose, Gulose, Idose, Galactose, Talose) Deoxy sugar (Fucose, Fuculose, Rhamnose) >6 Heptose (Sedoheptulose) · Octose · Nonose (Neuraminic acid) Multiple Disaccharides Sucrose · Lactose · Maltose  · Trehalose · Turanose · Cellobiose Trisaccharides Raffinose · Melezitose · Maltotriose Tetrasaccharides Acarbose · Stachyose Other oligosaccharides Fructooligosaccharide (FOS) · Galactooligosaccharides (GOS) · Mannan-oligosaccharides (MOS) Polysaccharides Glucose/Glucan: Glycogen · Starch (Amylose, Amylopectin) · Cellulose · Dextrin/Dextran · Beta-glucan (Zymosan, Lentinan, Sizofiran)  · Maltodextrin Fructose/Fructan: Inulin · Levan beta 2→6 Mannose/Mannan Galactose/Galactan N-Acetylglucosamine: Chitin Major families of biochemicals Saccharides/Carbohydrates/Glycosides · Amino acids/Peptides/Proteins/Glycoproteins · Lipids/Terpenes/Steroids/Carotenoids · Alkaloids/Nucleobases/Nucleic acids · Cofactors/Phenylpropanoids/Polyketides/Tetrapyrroles

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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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