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