| Carbohydrates are chemical compounds that contain | | | | glycosidic bond. The binding between the two sugars |
| oxygen, hydrogen, and carbon atoms, and no other | | | | results in the loss of a hydrogen atom (H) from one |
| elements. They consist of monosaccharide sugars of | | | | molecule and a hydroxyl group (OH) from the other. |
| varying chain lengths and that have the general | | | | The most common disaccharides are sucrose (cane |
| chemical formula Cn(H2O)n or are derivatives of such. | | | | or beet sugar - made from one glucose and one |
| The smallest value for "n" is 3. A 3-carbon sugar is | | | | fructose), lactose (milk sugar - made from one glucose |
| referred to as a triose, whereas a 6-carbon sugar is | | | | and one galactose) and maltose (made of two |
| called a hexose (see monosaccharides below). | | | | glucoses). The formula of these disaccharides is |
| Certain carbohydrates are an important storage and | | | | C12H22O11. |
| transport form of energy in most organisms, including | | | | Oligosaccharides and polysaccharides |
| plants and animals. Carbohydrates are classified by | | | | Oligosaccharides and polysaccharides are composed |
| their number of sugar units: monosaccharides (such as | | | | of longer chains of monosaccharide units bound |
| glucose and fructose), disaccharides (such as sucrose | | | | together by glycosidic bonds. The distinction between |
| and lactose), oligosaccharides, and polysaccharides | | | | the two is based upon the number of monosaccharide |
| (such as starch, glycogen, and cellulose). | | | | units present in the chain. Oligosaccharides typically |
| Pure carbohydrates contain carbon, hydrogen, and | | | | contain between three and nine monosaccharide units, |
| oxygen atoms, in a 1:2:1 molar ratio, giving the general | | | | and polysaccharides contain greater than ten |
| formula Cn(H2O)n. (This applies only to | | | | monosaccharide units. |
| monosaccharides, see below, although all | | | | Definitions of how large a carbohydrate must be to fall |
| carbohydrates have the more general formula | | | | into each category vary however. |
| Cn(H2O)m.) However, many important "carbohydrates" | | | | Oligosaccharides are found as a common form of |
| deviate from this, such as deoxyribose and glycerol, | | | | protein posttranslational modification. Polysaccharides |
| although they are not, in the strict sense, | | | | represent an important class of biological polymer. |
| carbohydrates. Sometimes compounds containing | | | | Examples include starch, cellulose, chitin and glycogen. |
| other elements are also counted as carbohydrates | | | | Nutrition Strictly speaking, carbohydrates are not |
| (e.g. chitin, which contains nitrogen). | | | | necessary for human nutrition because proteins can |
| The simplest carbohydrates are monosaccharides, | | | | be converted to carbohydrates. The traditional diet of |
| which are small straight-chain aldehydes and ketones | | | | some cultures consists of very little carbohydrate, and |
| with many hydroxyl groups added, usually one on each | | | | these people remain relatively healthy.[citation needed] |
| carbon except the functional group. Other | | | | However, carbohydrates require less water to digest |
| carbohydrates are composed of monosaccharide | | | | than proteins or fats and are the most common |
| units and break down under hydrolysis. These may be | | | | source of energy. |
| classified as disaccharides, oligosaccharides, or | | | | Proteins and fat are vital building components for body |
| polysaccharides, depending on whether they have two, | | | | tissue and cells, and thus it could be considered |
| several, or many monosaccharide units. | | | | advisable not to deplete such resources by |
| Monosaccharides Monosaccharides may be divided | | | | necessitating their use in energy production. |
| into aldoses, which have an aldehyde group on the first | | | | Carbohydrates, like proteins, contain 4 kilocalories per |
| carbon atom, and ketoses, which typically have a | | | | gram while fats contain 9 kilocalories and alcohol |
| ketone group on the second. They may also be | | | | contains 7 kilocalories per gram. |
| divided into trioses, tetroses, pentoses, hexoses, and | | | | Based on evidence for risk of heart disease and |
| so forth, depending on how many carbon atoms they | | | | obesity, the Institute of Medicine recommends that |
| contain. For instance, glucose is an aldohexose, | | | | American and Canadian adults get between 40-65% |
| fructose a ketohexose, and ribose an aldopentose. | | | | of dietary energy from carbohydrates.[1] The Food |
| Further, each carbon atom that supports a hydroxyl | | | | and Agriculture Organization and World Health |
| group (except for the first and last) is optically active, | | | | Organization jointly recommend that national dietary |
| allowing a number of different carbohydrates with the | | | | guidelines set a goal of 55-75% of total energy from |
| same basic structure. For instance, galactose is an | | | | carbohydrates.[2] Foods high in carbohydrates Breads, |
| aldohexose but has different properties from glucose | | | | pastas, potatoes, bran and cereals are all high in |
| because the atoms are arranged differently. | | | | carbohydrates. |
| The straight-chain structure described here is only one | | | | Classification Dietitians and nutritionists commonly |
| of the forms a monosaccharide may take. The | | | | classify carbohydrates as simple (monosaccharides |
| aldehyde or ketone group may react with a hydroxyl | | | | and disaccharides) or complex (oligosaccharides and |
| group on a different carbon atom to form a | | | | polysaccharides), depending on their chemical structure. |
| hemiacetal or hemiketal, in which case there is an | | | | The term complex carbohydrate was first used in the |
| oxygen bridge between the two carbon atoms, | | | | Senate Select Committee publication Dietary Goals for |
| forming a heterocyclic ring. Rings with five and six | | | | the United States (1977), where it denoted "fruit, |
| atoms are called furanose and pyranose forms and | | | | vegetables and whole-grains". Dietary guidelines |
| exist in equilibrium with the straight-chain form. | | | | generally recommend that complex carbohydrates |
| It should be noted that the ring form has one more | | | | and nutrient-rich simple carbohydrates such as fruit and |
| optically active carbon than the straight-chain form, and | | | | dairy products should make up the bulk of |
| so has both an alpha and a beta form, which | | | | carbohydrate consumption. The USDA's Dietary |
| interconvert in equilibrium. However, the carbohydrate | | | | Guidelines for Americans 2005 dispenses with the |
| may further react with an alcohol to form an acetal or | | | | simple/complex distinction, instead recommending |
| ketal, in which case the two forms become distinct. | | | | fiber-rich foods and whole grains. |
| This is the basic type of link between the | | | | The glycemic index and glycemic load systems are |
| monosaccharide units of larger carbohydrates. | | | | popular alternative classification methods which rank |
| Disaccharides Disaccharides are composed of two | | | | carbohydrates based on their effect on blood glucose |
| monosaccharide units bound together by a covalent | | | | levels. |