View source for Chemistry:Carbohydrate

{{short description|Organic compound that consists only of carbon, hydrogen, and oxygen}}
[[File:Lactose.svg|thumb|upright=1.25|Lactose is a disaccharide found in animal milk. It consists of a molecule of D-galactose and a molecule of D-glucose bonded by beta-1-4 glycosidic linkage.]]

A '''carbohydrate''' ({{IPAc-en|ˌ|k|ɑːr|b|oʊ|ˈ|h|aɪ|d|r|eɪ|t}}) is a [[Physics:Biomolecule|biomolecule]] consisting of [[Chemistry:Carbon|carbon]] (C), [[Chemistry:Hydrogen|hydrogen]] (H) and [[Chemistry:Oxygen|oxygen]] (O) atoms, usually with a hydrogen–oxygen [[Physics:Atom|atom]] ratio of 2:1 (as in water) and thus with the [[Chemistry:Empirical formula|empirical formula]] {{chem2|C_{''m''}(H2O)_{''n''} }} (where ''m'' may or may not be different from ''n''), which does not mean the H has covalent bonds with O (for example with {{chem2|CH2O}}, H has a covalent bond with C but not with O). However, not all carbohydrates conform to this precise stoichiometric definition (e.g., [[Chemistry:Uronic acid|uronic acid]]s, deoxy-sugars such as [[Chemistry:Fucose|fucose]]), nor are all chemicals that do conform to this definition automatically classified as carbohydrates (e.g. [[Physics:Formaldehyde|formaldehyde]] and [[Chemistry:Acetic acid|acetic acid]]).

The term is most common in [[Biology:Biochemistry|biochemistry]], where it is a synonym of '''saccharide''' ({{ety|grc|''σάκχαρον'' ({{grc-transl|σάκχαρον}})|sugar}}<ref name="avenas">{{cite book |vauthors=Avenas P |year=2012 |chapter=Etymology of main polysaccharide names |veditors=Navard P |title=The European Polysaccharide Network of Excellence (EPNOE) |publisher=[[Company:Springer Science+Business Media|Springer-Verlag]] |location=Wien |chapter-url=https://www.springer.com/cda/content/document/cda_downloaddocument/9783709104200-c1.pdf?SGWID=0-0-45-1364512-p174060193 |access-date=January 28, 2018 |archive-date=February 9, 2018 |archive-url=https://web.archive.org/web/20180209064118/https://www.springer.com/cda/content/document/cda_downloaddocument/9783709104200-c1.pdf?SGWID=0-0-45-1364512-p174060193 |url-status=dead}}</ref>), a group that includes [[Chemistry:Sugar|sugar]]s, [[Chemistry:Starch|starch]], and [[Chemistry:Cellulose|cellulose]]. The saccharides are divided into four chemical groups: [[Chemistry:Monosaccharide|monosaccharide]]s, [[Chemistry:Disaccharide|disaccharide]]s, [[Chemistry:Oligosaccharide|oligosaccharide]]s, and [[Chemistry:Polysaccharide|polysaccharide]]s. Monosaccharides and disaccharides, the smallest (lower [[Physics:Molecular weight|molecular weight]]) carbohydrates, are  commonly referred to as sugars.<ref>{{cite journal | vauthors = Flitsch SL, Ulijn RV | title = Sugars tied to the spot | journal = Nature | volume = 421 | issue = 6920 | pages = 219–220 | date = January 2003 | pmid = 12529622 | doi = 10.1038/421219a | s2cid = 4421938 | bibcode = 2003Natur.421..219F }}</ref> While the scientific nomenclature of carbohydrates is complex, the names of the monosaccharides and disaccharides very often end in the suffix ''-ose'', which was originally taken from the word [[Chemistry:Glucose|glucose]] ({{ety|grc|''{{wikt-lang|grc|γλεῦκος}}'' ({{grc-transl|γλεῦκος}})|wine, [[Religion:Must|must]]}}), and is used for almost all sugars, e.g. [[Chemistry:Fructose|fructose]] (fruit sugar), [[Chemistry:Sucrose|sucrose]] (cane or [[Biology:Sugar beet|beet]] sugar), [[Chemistry:Ribose|ribose]], [[Chemistry:Lactose|lactose]] (milk sugar), etc.

Carbohydrates perform numerous roles in living organisms.<ref>{{cite journal | vauthors = Carroll GT, Wang D, Turro NJ, Koberstein JT | title = Photons to illuminate the universe of sugar diversity through bioarrays | journal = Glycoconjugate Journal | volume = 25 | issue = 1 | pages = 5–10 | date = January 2008 | pmid = 17610157 | pmc = 7088275 | doi = 10.1007/s10719-007-9052-1 }}</ref> Polysaccharides serve as an [[Physics:Energy|energy]] store (e.g. [[Chemistry:Starch|starch]] and [[Biology:Glycogen|glycogen]]) and as structural components (e.g. cellulose in plants and [[Chemistry:Chitin|chitin]] in arthropods). The 5-carbon monosaccharide [[Chemistry:Ribose|ribose]] is an important component of coenzymes (e.g. [[Chemistry:Adenosine triphosphate|ATP]], [[Chemistry:Flavin adenine dinucleotide|FAD]] and [[Chemistry:Nicotinamide adenine dinucleotide|NAD]]) and the backbone of the genetic molecule known as [[Biology:RNA|RNA]]. The related [[Chemistry:Deoxyribose|deoxyribose]] is a component of DNA. Saccharides and their derivatives include many other important biomolecules that play key roles in the [[Biology:Immune system|immune system]], fertilization, preventing [[Medicine:Pathogenesis|pathogenesis]], blood clotting, and [[Biology:Developmental biology|development]].<ref>{{cite book |  vauthors = Maton A, Hopkins J, McLaughlin CW, Johnson S, Warner MQ, LaHart D, Wright JD | title = Human Biology and Health | publisher = Prentice Hall | year = 1993 | location = Englewood Cliffs, New Jersey | pages = [https://archive.org/details/humanbiologyheal00scho/page/52 52–59] | isbn = 978-0-13-981176-0 | url-access = registration | url = https://archive.org/details/humanbiologyheal00scho/page/52 }}</ref>

Carbohydrates are central to [[Biology:Nutrition|nutrition]] and are found in a wide variety of natural and processed foods. Starch is a polysaccharide and is abundant in cereals (wheat, maize, rice), potatoes, and processed food based on cereal [[Biology:Flour|flour]], such as bread, pizza or pasta. Sugars appear in human diet mainly as table sugar (sucrose, extracted from [[Biology:Sugarcane|sugarcane]] or [[Biology:Sugar beet|sugar beet]]s), lactose (abundant in milk), glucose and fructose, both of which occur naturally in [[Chemistry:Honey|honey]], many [[Biology:Fruit|fruit]]s, and some vegetables. Table sugar, milk, or honey are often added to drinks and many prepared foods such as jam, biscuits and cakes.

[[Chemistry:Cellulose|Cellulose]], a polysaccharide found in the cell walls of all plants, is one of the main components of insoluble dietary fiber. Although it is not digestible by humans, cellulose and insoluble dietary fiber generally help maintain a healthy digestive system<ref>USDA National Nutrient Database, 2015, p. 14</ref> by facilitating bowel movements. Other polysaccharides contained in dietary fiber include [[Chemistry:Resistant starch|resistant starch]] and [[Chemistry:Inulin|inulin]], which feed some bacteria in the [[Biology:Microbiota|microbiota]] of the [[Biology:Large intestine|large intestine]], and are [[Biology:Metabolism|metabolized]] by these bacteria to yield short-chain fatty acids.<ref name="CRC Handbook of Dietary Fiber in Human Nutrition">{{cite book| vauthors = Cummings JH | title=The Effect of Dietary Fiber on Fecal Weight and Composition| date=2001| publisher=CRC Press| location=Boca Raton, Florida| isbn=978-0-8493-2387-4| pages=184| edition=3rd| url=https://www.crcpress.com/CRC-Handbook-of-Dietary-Fiber-in-Human-Nutrition-Third-Edition/Spiller/p/book/9780849323874| access-date=April 24, 2022| archive-date=April 2, 2019| archive-url=https://web.archive.org/web/20190402203003/https://www.crcpress.com/CRC-Handbook-of-Dietary-Fiber-in-Human-Nutrition-Third-Edition/Spiller/p/book/9780849323874| url-status=live}}</ref><ref>{{cite journal | vauthors = Byrne CS, Chambers ES, Morrison DJ, Frost G | title = The role of short chain fatty acids in appetite regulation and energy homeostasis | journal = International Journal of Obesity | volume = 39 | issue = 9 | pages = 1331–1338 | date = September 2015 | pmid = 25971927 | pmc = 4564526 | doi = 10.1038/ijo.2015.84 }}</ref>

==Terminology==
In [[Scientific literature|scientific literature]], the term "carbohydrate" has many synonyms, like "sugar" (in the broad sense), "saccharide", "ose",<ref name="avenas"/> "glucide",<ref>{{cite book | vauthors = Fearon WF | year = 1949 | title = Introduction to Biochemistry | edition = 2nd | location = London | publisher = Heinemann | url = https://books.google.com/books?id=YkOaBQAAQBAJ | isbn = 978-1483225395 | access-date = November 30, 2017 | archive-date = July 27, 2020 | archive-url = https://web.archive.org/web/20200727175530/https://books.google.com/books?id=YkOaBQAAQBAJ | url-status = live }}</ref> "hydrate of carbon" or "[[Chemistry:Hydroxy group|polyhydroxy]] compounds with [[Chemistry:Aldehyde|aldehyde]] or [[Chemistry:Ketone|ketone]]". Some of these terms, especially "carbohydrate" and "sugar", are also used with other meanings.

In [[Engineering:Food science|food science]] and in many informal contexts, the term "carbohydrate" often means any food that is particularly rich in the complex carbohydrate [[Chemistry:Starch|starch]] (such as cereals, bread and pasta) or simple carbohydrates, such as sugar (found in candy, jams, and desserts).  This informality is sometimes confusing since it confounds chemical structure and digestibility in humans.

Often in lists of nutritional information, such as the [[USDA National Nutrient Database]], the term "carbohydrate" (or "carbohydrate by difference") is used for everything other than water, protein, fat, ash, and ethanol.<ref>USDA National Nutrient Database, 2015, p. 13</ref> This includes chemical compounds such as [[Chemistry:Acetic acid|acetic]] or [[Chemistry:Lactic acid|lactic acid]], which are not normally considered carbohydrates. It also includes dietary fiber which is a carbohydrate but which does not contribute  food energy in humans, even though it is often included in the calculation of total food energy just as though it did (i.e., as if it were a digestible and absorbable carbohydrate such as a sugar).
In the strict sense, "[[Chemistry:Sugar|sugar]]" is applied for sweet, soluble carbohydrates, many of which are used in human food.

== History ==
The history of the discovery regarding carbohydrates dates back around 10,000 years ago in Papua New Guinea during the cultivation of [[Biology:Sugarcane|Sugarcane]] during the Neolithic agricultural revolution {{Citation needed|date=August 2023}}. The term "carbohydrate" was first proposed by German chemist Carl Schmidt (chemist) in 1844. In 1856, [[Biology:Glycogen|glycogen]], a form of carbohydrate storage in animal livers, was discovered by French physiologist Claude Bernard.{{Citation needed|date=July 2023}}

== Structure ==
Formerly the name "carbohydrate" was used in [[Chemistry:Chemistry|chemistry]] for any compound with the formula C<sub>''m''</sub> (H<sub>2</sub>O)<sub>''n''</sub>. Following this definition, some chemists considered [[Physics:Formaldehyde|formaldehyde]] (CH<sub>2</sub>O) to be the simplest carbohydrate,<ref name="coulter">{{cite book | vauthors = Coulter JM, Barnes CR, Cowles HC | year = 1930 | url = https://books.google.com/books?id=WyZnVpCiTHIC&q=simplest+carbohydrate&pg=PA375 | title = A Textbook of Botany for Colleges and Universities | publisher = BiblioBazaar | isbn = 978-1113909954 | access-date = April 24, 2022 | archive-date = April 17, 2022 | archive-url = https://web.archive.org/web/20220417005854/https://books.google.com/books?id=WyZnVpCiTHIC&q=simplest+carbohydrate&pg=PA375 | url-status = live }}</ref> while others claimed that title for [[Chemistry:Glycolaldehyde|glycolaldehyde]].<ref name="tietz">{{cite book | vauthors = Burtis CA, Ashwood ER, Tietz NW | year = 2000 | url = https://books.google.com/books?id=l5hqAAAAMAAJ&q=simplest+carbohydrate | title = Tietz fundamentals of clinical chemistry | publisher = W.B. Saunders | isbn = 9780721686349 | access-date = January 8, 2016 | archive-date = June 24, 2016 | archive-url = https://web.archive.org/web/20160624073749/https://books.google.com/books?id=l5hqAAAAMAAJ&q=simplest+carbohydrate | url-status = live }}</ref> Today, the term is generally understood in the biochemistry sense, which excludes compounds with only one or two carbons and includes many biological carbohydrates which deviate from this formula. For example, while the above representative formulas would seem to capture the commonly known carbohydrates, ubiquitous and abundant carbohydrates often deviate from this. For example, carbohydrates often display chemical groups such as: ''N''-acetyl (e.g. [[Chemistry:Chitin|chitin]]), [[Chemistry:Sulfate|sulfate]] (e.g. [[Chemistry:Glycosaminoglycan|glycosaminoglycan]]s), [[Chemistry:Carboxylic acid|carboxylic acid]]  and deoxy modifications (e.g. [[Chemistry:Fucose|fucose]] and [[Chemistry:Sialic acid|sialic acid]]).

Natural saccharides are generally built of simple carbohydrates called [[Chemistry:Monosaccharide|monosaccharide]]s with general formula (CH<sub>2</sub>O)<sub>''n''</sub> where ''n'' is three or more. A typical monosaccharide has the structure H–(CHOH)<sub>''x''</sub>(C=O)–(CHOH)<sub>''y''</sub>–H, that is, an [[Chemistry:Aldehyde|aldehyde]] or [[Chemistry:Ketone|ketone]] with many hydroxyl groups added, usually one on each [[Chemistry:Carbon|carbon]] [[Physics:Atom|atom]] that is not part of the aldehyde or ketone [[Chemistry:Functional group|functional group]]. Examples of monosaccharides are [[Chemistry:Glucose|glucose]], [[Chemistry:Fructose|fructose]], and [[Chemistry:Glyceraldehyde|glyceraldehyde]]s. However, some biological substances commonly called "monosaccharides" do not conform to this formula (e.g. [[Chemistry:Uronic acid|uronic acid]]s and deoxy-sugars such as [[Chemistry:Fucose|fucose]]) and there are many chemicals that do conform to this formula but are not considered to be monosaccharides (e.g. formaldehyde CH<sub>2</sub>O and [[Chemistry:Inositol|inositol]] (CH<sub>2</sub>O)<sub>6</sub>).<ref>{{cite book | vauthors = Matthews CE, Van Holde KE, Ahern KG | year = 1999 | title = Biochemistry | edition = 3rd | publisher = Benjamin Cummings | isbn = 978-0-8053-3066-3 }}{{page needed|date=January 2018}}</ref>

The open-chain form of a monosaccharide often coexists with a [[Chemistry:Heterocyclic compound|closed ring form]] where the [[Chemistry:Aldehyde|aldehyde]]/[[Chemistry:Ketone|ketone]] carbonyl group carbon (C=O) and hydroxyl group (–OH) react forming a [[Chemistry:Hemiacetal|hemiacetal]] with a new C–O–C bridge.

Monosaccharides can be linked together into what are called [[Chemistry:Polysaccharide|polysaccharide]]s (or [[Chemistry:Oligosaccharide|oligosaccharide]]s) 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, [[Chemistry:Deoxyribose|deoxyribose]], a component of [[Biology:DNA|DNA]], is a modified version of [[Chemistry:Ribose|ribose]]; [[Chemistry:Chitin|chitin]] is composed of repeating units of N-acetyl glucosamine, a [[Chemistry:Nitrogen|nitrogen]]-containing form of glucose.

==Division==
Carbohydrates are polyhydroxy aldehydes, ketones, alcohols, acids, their simple derivatives and their polymers having linkages of the acetal type. They may be classified according to their [[Chemistry:Degree of polymerization|degree of polymerization]], and may be divided initially into three principal groups, namely sugars, oligosaccharides and polysaccharides.<ref>{{cite book | title = Carbohydrates in human nutrition | series = FAO Food and Nutrition Paper – 66 | chapter = Chapter 1 – The role of carbohydrates in nutrition | chapter-url = http://www.fao.org/docrep/w8079e/w8079e07.htm | publisher = Food and Agriculture Organization of the United Nations | access-date = December 21, 2015 | archive-date = December 22, 2015 | archive-url = https://web.archive.org/web/20151222095451/http://www.fao.org/docrep/w8079e/w8079e07.htm | url-status = live }}</ref>
{| class="wikitable"
|+ The major dietary carbohydrates
|-
! Class<br>(degree of polymerization) !! Subgroup !! Components
|-
! rowspan=3 | [[Chemistry:Sugar|Sugar]]s (1–2)
|| [[Chemistry:Monosaccharide|Monosaccharide]]s || [[Chemistry:Glucose|Glucose]], [[Chemistry:Galactose|galactose]], [[Chemistry:Fructose|fructose]], [[Chemistry:Xylose|xylose]]
|-
| [[Chemistry:Disaccharide|Disaccharide]]s || [[Chemistry:Sucrose|Sucrose]], [[Chemistry:Lactose|lactose]], [[Chemistry:Maltose|maltose]], [[Chemistry:Isomaltulose|isomaltulose]], [[Chemistry:Trehalose|trehalose]]
|-
| [[Chemistry:Polyol|Polyol]]s || [[Chemistry:Sorbitol|Sorbitol]], [[Chemistry:Mannitol|mannitol]]
|-
! rowspan=2 | [[Chemistry:Oligosaccharide|Oligosaccharide]]s (3–9)
|| Malto-oligosaccharides || [[Chemistry:Maltodextrin|Maltodextrin]]s
|-
| Other oligosaccharides || [[Chemistry:Raffinose|Raffinose]], [[Chemistry:Stachyose|stachyose]], fructo-oligosaccharides
|-
! rowspan=2 | [[Chemistry:Polysaccharide|Polysaccharide]]s (>9)
|| [[Chemistry:Starch|Starch]] || [[Chemistry:Amylose|Amylose]], [[Chemistry:Amylopectin|amylopectin]], modified starches
|-
| Non-starch polysaccharides || [[Biology:Glycogen|Glycogen]], [[Chemistry:Cellulose|Cellulose]], [[Biology:Hemicellulose|Hemicellulose]], [[Chemistry:Pectin|Pectin]]s, [[Chemistry:Hydrocolloid|Hydrocolloid]]s
|}

==Monosaccharides==
{{Main|Chemistry:Monosaccharide}}
[[Image:D-glucose color coded.png|upright=0.5|thumb|[[Chemistry:Glucose|D-glucose]] is an aldohexose with the formula (C·H<sub>2</sub>O)<sub>6</sub>. 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 [[Chemistry:Hydrolysis|hydrolyzed]] to smaller carbohydrates. They are aldehydes or ketones with two or more hydroxyl groups. The general [[Chemistry:Chemical formula|chemical formula]] of an unmodified monosaccharide is (C•H<sub>2</sub>O)<sub>n</sub>, 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-glyceraldehydes.

===Classification of monosaccharides===
<div class="thumb tleft">
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[[Image:Alpha-D-glucopyranose-2D-skeletal.svg|185px]]
[[Image:Beta-D-glucopyranose-2D-skeletal.svg|185px]]
<div class="thumbcaption">
The α and β [[Chemistry:Anomer|anomer]]s of glucose. Note the position of the hydroxyl group (red or green) on the anomeric carbon relative to the CH<sub>2</sub>OH group bound to carbon 5: they either have identical absolute configurations (R,R or S,S) (α), or opposite absolute configurations (R,S or S,R) (β).<ref>{{cite book | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK1955/#_ch2_s4_ | chapter = Structural Basis of Glycan Diversity | title = Essentials of Glycobiology | edition = 3rd | publisher = Cold Spring Harbor Laboratory Press | location = Cold Spring Harbor (NY) | vauthors = Bertozzi CR, Rabuka D | isbn = 978-1-621821-32-8 | year = 2017 | pmid = 20301274 | access-date = August 30, 2017 | archive-date = May 19, 2020 | archive-url = https://web.archive.org/web/20200519081218/https://www.ncbi.nlm.nih.gov/books/NBK1955/#_ch2_s4_ | url-status = live }}</ref>

</div>
</div>
</div>
Monosaccharides are classified according to three different characteristics: the placement of its carbonyl group, the number of [[Chemistry:Carbon|carbon]] atoms it contains, and its [[Biology:Chirality|chiral]] handedness. If the carbonyl group is an [[Chemistry:Aldehyde|aldehyde]], the monosaccharide is an [[Chemistry:Aldose|aldose]]; if the carbonyl group is a [[Chemistry:Ketone|ketone]], the monosaccharide is a [[Chemistry:Ketose|ketose]]. Monosaccharides with three carbon atoms are called [[Chemistry:Triose|triose]]s, those with four are called [[Chemistry:Tetrose|tetrose]]s, five are called [[Chemistry:Pentose|pentose]]s, six are [[Chemistry:Hexose|hexose]]s, and so on.<ref>{{cite book | vauthors = Campbell NA, Williamson B, Heyden RJ | title = Biology: Exploring Life | publisher = Pearson Prentice Hall | year = 2006 | location = Boston, Massachusetts | url = http://www.phschool.com/el_marketing.html | isbn = 978-0-13-250882-7 | access-date = December 2, 2008 | archive-date = November 2, 2014 | archive-url = https://web.archive.org/web/20141102041816/http://www.phschool.com/el_marketing.html | url-status = live }}</ref> These two systems of classification are often combined. For example, [[Chemistry:Glucose|glucose]] is an [[Chemistry:Aldohexose|aldohexose]] (a six-carbon aldehyde), [[Chemistry:Ribose|ribose]] is an [[Chemistry:Aldopentose|aldopentose]] (a five-carbon aldehyde), and [[Chemistry:Fructose|fructose]] is a [[Chemistry:Ketohexose|ketohexose]] (a six-carbon ketone).

Each carbon atom bearing a [[Chemistry:Hydroxyl group|hydroxyl group]] (-OH), with the exception of the first and last carbons, are [[Biology:Chirality|asymmetric]], making them stereo centers with two possible configurations each (R or S). Because of this asymmetry, a number of [[Chemistry:Isomer|isomer]]s may exist for any given monosaccharide formula. Using Le Bel-van't Hoff rule, the aldohexose D-glucose, for example, has the formula (C·H<sub>2</sub>O)<sub>6</sub>, of which four of its six carbons atoms are stereogenic, making D-glucose one of 2<sup>4</sup>=16 possible stereoisomers. In the case of [[Chemistry:Glyceraldehyde|glyceraldehyde]]s, an aldotriose, there is one pair of possible stereoisomers, which are enantiomers and [[Chemistry:Epimer|epimer]]s. [[Chemistry:Dihydroxyacetone|1, 3-dihydroxyacetone]], the ketose corresponding to the aldose glyceraldehydes, is a symmetric molecule with no stereo centers. 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 [[Physics:Polarization (waves)|polarized light]]. This usage of "d-" and "l-" is no longer followed in carbohydrate chemistry.<ref>{{cite book | vauthors = Pigman W, Horton D | title=The Carbohydrates: Chemistry and Biochemistry Vol 1A| veditors = Pigman W, Horton D |edition=2nd|year=1972|publisher=Academic Press|location=San Diego|pages=1–67|chapter=Chapter 1: Stereochemistry of the Monosaccharides|isbn=978-0323138338}}</ref>

===Ring-straight chain isomerism===
[[Image:Glucose Fisher to Haworth.gif|thumb|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 [[Chemistry:Hemiacetal|hemiacetal]] or hemiketal, forming a heterocyclic ring with an oxygen bridge between two carbon atoms. Rings with five and six atoms are called [[Chemistry:Furanose|furanose]] and [[Chemistry:Pyranose|pyranose]] forms, respectively, and exist in equilibrium with the straight-chain form.<ref name=pigman>{{cite book | vauthors = Pigman W, Anet EF |title=The Carbohydrates: Chemistry and Biochemistry Vol 1A| veditors = Pigman W, Horton D  |edition=2nd|year=1972|publisher=Academic Press|location=San Diego|pages=165–194|chapter=Chapter 4: Mutarotations and Actions of Acids and Bases|isbn=978-0323138338}}</ref>

During the conversion from straight-chain form to the 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 is called [[Chemistry:Anomer|anomer]]s. In the ''α anomer'', the -OH substituent on the anomeric carbon rests on the opposite side (trans) of the ring from the CH<sub>2</sub>OH side branch. The alternative form, in which the CH<sub>2</sub>OH substituent and the anomeric hydroxyl are on the same side (cis) of the plane of the ring, is called the ''β anomer''.

===Use in living organisms===
Monosaccharides are the major fuel source for [[Biology:Metabolism|metabolism]], being used both as an energy source ([[Chemistry:Glucose|glucose]] being the most important in nature as it is the product of photosynthesis in plants) and in [[Biology:Biosynthesis|biosynthesis]]. When monosaccharides are not immediately needed, they are often converted to more space-efficient (i.e., less water-soluble) forms, often [[Chemistry:Polysaccharide|polysaccharide]]s. In many animals, including humans, this storage form is [[Biology:Glycogen|glycogen]], especially in liver and muscle cells. In plants, [[Chemistry:Starch|starch]] is used for the same purpose. The most abundant carbohydrate, [[Chemistry:Cellulose|cellulose]], is a structural component of the [[Biology:Cell wall#plant cell walls|cell wall]] of plants and many forms of algae. [[Chemistry:Ribose|Ribose]] is a component of [[Biology:RNA|RNA]]. [[Chemistry:Deoxyribose|Deoxyribose]] is a component of [[Biology:DNA|DNA]]. [[Chemistry:Lyxose|Lyxose]] is a component of lyxoflavin found in the human heart.<ref>{{cite encyclopedia |title=lyxoflavin |url=http://www.merriam-webster.com/medical/lyxoflavin |dictionary=Merriam-Webster |access-date=February 26, 2014 |archive-date=October 31, 2014 |archive-url=https://web.archive.org/web/20141031135041/http://www.merriam-webster.com/medical/lyxoflavin |url-status=live }}</ref> [[Chemistry:Ribulose|Ribulose]] and [[Chemistry:Xylulose|xylulose]] occur in the [[Biology:Pentose phosphate pathway|pentose phosphate pathway]]. [[Chemistry:Galactose|Galactose]], a component of milk sugar [[Chemistry:Lactose|lactose]], is found in [[Biology:Galactolipid|galactolipid]]s in [[Biology:Cell membrane#lipids|plant cell membranes]] and in [[Chemistry:Glycoprotein|glycoprotein]]s in many tissues. [[Chemistry:Mannose|Mannose]] occurs in human metabolism, especially in the [[Biology:Glycosylation|glycosylation]] of certain proteins. [[Chemistry:Fructose|Fructose]], or fruit sugar, is found in many plants and humans, it is metabolized in the liver, absorbed directly into the intestines during [[Biology:Digestion|digestion]], and found in [[Biology:Semen|semen]]. [[Chemistry:Trehalose|Trehalose]], a major sugar of insects, is rapidly hydrolyzed into two glucose molecules to support continuous flight.

==Disaccharides==
[[Image:sucrose 3Dprojection.png|thumb|Sucrose, also known as table sugar, is a common disaccharide. It is composed of two monosaccharides: D-glucose (left) and D-fructose (right).]]
{{Main|Chemistry:Disaccharide}}
Two joined monosaccharides are called a [[Chemistry:Disaccharide|disaccharide]], the simplest kind of polysaccharide. Examples include [[Chemistry:Sucrose|sucrose]] and [[Chemistry:Lactose|lactose]]. They are composed of two monosaccharide units bound together by a [[Chemistry:Covalent|covalent]] bond known as a glycosidic linkage formed via a [[Chemistry:Dehydration reaction|dehydration reaction]], resulting in the loss of a [[Chemistry:Hydrogen|hydrogen]] atom from one monosaccharide and a [[Chemistry:Hydroxyl group|hydroxyl group]] from the other. The [[Chemistry:Chemical formula|formula]] of unmodified disaccharides is C<sub>12</sub>H<sub>22</sub>O<sub>11</sub>. Although there are numerous kinds of disaccharides, a handful of disaccharides are particularly notable.

[[Chemistry:Sucrose|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 [[Chemistry:Glucose|D-glucose]] molecule and one [[Chemistry:Fructose|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 [[Chemistry:Pyranose|pyranose]] and fructose is a [[Chemistry:Furanose|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.

[[Chemistry:Lactose|Lactose]], a disaccharide composed of one [[Chemistry:Galactose|D-galactose]] molecule and one [[Chemistry:Glucose|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 [[Chemistry:Maltose|maltose]] (two D-glucoses linked α-1,4) and [[Chemistry:Cellobiose|cellobiose]] (two D-glucoses linked β-1,4). Disaccharides can be classified into two types: reducing and non-reducing disaccharides. If the functional group is present in bonding with another sugar unit, it is called a reducing disaccharide or biose.

==Nutrition==
[[File:GrainProducts.jpg|thumb|upright|[[Biology:Cereal|Grain]] products: rich sources of carbohydrates]]

Carbohydrate consumed in food yields 3.87 kilocalories of energy per [[Gram|gram]] for simple sugars,<ref>{{cite web|url=http://ndb.nal.usda.gov/ndb/foods/show/6202|title=Show Foods|work=usda.gov|access-date=June 4, 2014|archive-date=October 3, 2017|archive-url=https://web.archive.org/web/20171003224558/https://ndb.nal.usda.gov/ndb/foods/show/6202|url-status=dead}}</ref> and 3.57 to 4.12 kilocalories per gram for complex carbohydrate in most other foods.<ref>{{cite web|url=http://www.fao.org/docrep/006/y5022e/y5022e04.htm|title=Calculation of the Energy Content of Foods – Energy Conversion Factors|work=fao.org|access-date=August 2, 2013|archive-date=May 24, 2010|archive-url=https://web.archive.org/web/20100524003622/http://www.fao.org/DOCREP/006/Y5022E/y5022e04.htm|url-status=live}}</ref> Relatively high levels of carbohydrate are associated with processed foods or refined foods made from plants, including sweets, cookies and candy, table sugar, honey, soft drinks, breads and crackers, jams and fruit products, pastas and breakfast cereals. Lower amounts of digestible carbohydrate are usually associated with unrefined foods as these foods have more fiber, including beans, tubers, rice, and unrefined fruit{{dubious|Typo, fruit instead of food?What is meant by "unrefined fruit"? Not some posh down-one's-nose remark on low quality fruit, I presume.|date=September 2021}}.<ref>{{cite web |url=https://www.diabetes.org.uk/upload/How%20we%20help/catalogue/carb-reference-list-0511.pdf |title=Carbohydrate reference list |website=www.diabetes.org.uk |access-date=October 30, 2016 |archive-date=March 14, 2016 |archive-url=https://web.archive.org/web/20160314193016/https://www.diabetes.org.uk/upload/how%20we%20help/catalogue/carb-reference-list-0511.pdf |url-status=dead }}</ref>  Animal-based foods generally have the lowest carbohydrate levels, although milk does contain a high proportion of [[Chemistry:Lactose|lactose]].

Organisms typically cannot metabolize all types of carbohydrate to yield energy. Glucose is a nearly universal and accessible source of energy. Many organisms also have the ability to metabolize other [[Chemistry:Monosaccharide|monosaccharide]]s and [[Chemistry:Disaccharide|disaccharide]]s but glucose is often metabolized first. In ''[[Biology:Escherichia coli|Escherichia coli]]'', for example, the [[Biology:Lac operon|lac operon]] will express enzymes for the digestion of lactose when it is present, but if both lactose and glucose are present the ''lac'' operon is repressed, resulting in the glucose being used first (see: [[Biology:Diauxie|Diauxie]]). [[Chemistry:Polysaccharide|Polysaccharide]]s are also common sources of energy. Many organisms can easily break down starches into glucose; most organisms, however, cannot metabolize [[Chemistry:Cellulose|cellulose]] or other polysaccharides like [[Chemistry:Chitin|chitin]] and arabinoxylans. These carbohydrate types can be metabolized by some bacteria and protists. [[Biology:Ruminant|Ruminant]]s and [[Biology:Termite|termite]]s, for example, use microorganisms to process cellulose. Even though these complex carbohydrates are not very digestible, they represent an important dietary element for humans, called dietary fiber. Fiber enhances digestion, among other benefits.<ref>{{cite journal | vauthors = Pichon L, Huneau JF, Fromentin G, Tomé D | title = A high-protein, high-fat, carbohydrate-free diet reduces energy intake, hepatic lipogenesis, and adiposity in rats | journal = The Journal of Nutrition | volume = 136 | issue = 5 | pages = 1256–1260 | date = May 2006 | pmid = 16614413 | doi = 10.1093/jn/136.5.1256 | doi-access = free }}</ref>

The Institute of Medicine recommends that American and Canadian adults get between 45 and 65% of dietary energy from whole-grain carbohydrates.<ref>Food and Nutrition Board (2002/2005). ''[https://archive.today/20070210182833/http://newton.nap.edu/books/0309085373/html Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein and Amino Acids]''. Washington, D.C.: The National Academies Press. Page [http://newton.nap.edu/books/0309085373/html/769.html 769] {{Webarchive|url=https://web.archive.org/web/20060912060636/http://newton.nap.edu/books/0309085373/html/769.html |date=September 12, 2006 }}. {{ISBN|0-309-08537-3}}.</ref> The Food and Agriculture Organization and [[Organization:World Health Organization|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).<ref>Joint WHO/FAO expert consultation (2003). ''[https://web.archive.org/web/20110423051140/http://www.who.int/hpr/NPH/docs/who_fao_expert_report.pdf]'' (PDF). Geneva: [[Organization:World Health Organization|World Health Organization]]. pp. 55–56. {{ISBN|92-4-120916-X}}.</ref>  A 2017 Cochrane Systematic Review concluded that there was insufficient evidence to support the claim that whole grain diets can affect cardiovascular disease.<ref name="pmid28836672">{{cite journal | vauthors = Kelly SA, Hartley L, Loveman E, Colquitt JL, Jones HM, Al-Khudairy L, Clar C, Germanò R, Lunn HR, Frost G, Rees K | display-authors = 6 | title = Whole grain cereals for the primary or secondary prevention of cardiovascular disease | journal = The Cochrane Database of Systematic Reviews | volume = 8 | issue = 8 | pages = CD005051 | date = August 2017 | pmid = 28836672 | pmc = 6484378 | doi = 10.1002/14651858.CD005051.pub3 | url = https://spiral.imperial.ac.uk:8443/bitstream/10044/1/54579/2/Kelly_et_al-2017-.pdf | access-date = September 27, 2018 | url-status = dead | archive-url = https://web.archive.org/web/20180928044051/https://spiral.imperial.ac.uk:8443/bitstream/10044/1/54579/2/Kelly_et_al-2017-.pdf | archive-date = September 28, 2018 }}</ref>

===Classification===<!-- This title is used as a redirect target -->
[[Biology:Nutrition|Nutrition]]ists often refer to carbohydrates as either simple or complex. However, the exact distinction between these groups can be ambiguous. 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 was intended to distinguish sugars from other carbohydrates (which were perceived to be nutritionally superior).<ref>Joint WHO/FAO expert consultation (1998), ''Carbohydrates in human nutrition'', [http://www.fao.org/docrep/W8079E/w8079e07.htm chapter 1] {{Webarchive|url=https://web.archive.org/web/20070115102707/http://www.fao.org/docrep/w8079e/w8079e07.htm |date=January 15, 2007 }}. {{ISBN|92-5-104114-8}}.</ref> However, the report put "fruit, vegetables and whole-grains" in the complex carbohydrate column, despite the fact that these may contain sugars as well as polysaccharides. This confusion persists as today some nutritionists use the term complex carbohydrate to refer to any sort of digestible saccharide present in a whole food, where fiber, vitamins and minerals are also found (as opposed to processed carbohydrates, which provide energy but few other nutrients). The standard usage, however, is to classify carbohydrates chemically: simple if they are sugars ([[Chemistry:Monosaccharide|monosaccharide]]s and [[Chemistry:Disaccharide|disaccharide]]s) and complex if they are [[Chemistry:Polysaccharide|polysaccharide]]s (or [[Chemistry:Oligosaccharide|oligosaccharide]]s).<ref name=NutSource>{{cite web|title=Carbohydrates|url=http://www.hsph.harvard.edu/nutritionsource/carbohydrates/|work=The Nutrition Source|publisher=Harvard School of Public Health|access-date=April 3, 2013|date=September 18, 2012|archive-date=May 7, 2013|archive-url=https://web.archive.org/web/20130507074502/http://www.hsph.harvard.edu/nutritionsource/carbohydrates/|url-status=live}}</ref>

In any case, the simple vs. complex chemical distinction has little value for determining the nutritional quality of carbohydrates.<ref name=NutSource /> Some simple carbohydrates (e.g. [[Chemistry:Fructose|fructose]]) raise blood glucose rapidly, while some complex carbohydrates (starches), raise blood sugar slowly. The speed of digestion is determined by a variety of factors including which other nutrients are consumed with the carbohydrate, how the food is prepared, individual differences in metabolism, and the chemistry of the carbohydrate.<ref name=Jenkins>{{cite journal | vauthors = Jenkins DJ, Jenkins AL, Wolever TM, Thompson LH, Rao AV | title = Simple and complex carbohydrates | journal = Nutrition Reviews | volume = 44 | issue = 2 | pages = 44–49 | date = February 1986 | pmid = 3703387 | doi = 10.1111/j.1753-4887.1986.tb07585.x }}</ref> Carbohydrates are sometimes divided into "available carbohydrates", which are absorbed in the [[Biology:Small intestine|small intestine]] and "unavailable carbohydrates", which pass to the [[Biology:Large intestine|large intestine]], where they are subject to [[Biology:Fermentation|fermentation]] by the [[Biology:Human gastrointestinal microbiota|gastrointestinal microbiota]].<ref>{{Cite book| vauthors = Hedley CL |url=https://books.google.com/books?id=9ljO7LtX72AC&pg=PA79|title=Carbohydrates in Grain Legume Seeds: Improving Nutritional Quality and Agronomic Characteristics|date=2001|publisher=CABI|isbn=978-0-85199-944-9|pages=79|language=en|access-date=April 24, 2022|archive-date=April 24, 2022|archive-url=https://web.archive.org/web/20220424025211/https://books.google.com/books?id=9ljO7LtX72AC&pg=PA79|url-status=live}}</ref>

The USDA's ''Dietary Guidelines for Americans 2010'' call for moderate- to high-carbohydrate consumption from a balanced diet that includes six one-ounce servings of grain foods each day, at least half from [[Biology:Whole grain|whole grain]] sources and the rest are  from enriched.<ref>DHHS and USDA, [http://www.cnpp.usda.gov/DietaryGuidelines.htm ''Dietary Guidelines for Americans 2010''] {{Webarchive|url=https://web.archive.org/web/20140820230816/http://www.cnpp.usda.gov/DietaryGuidelines.htm |date=August 20, 2014 }}.</ref>

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

=== Health effects of dietary carbohydrate restriction ===

{{Main|Chemistry:Low-carbohydrate diet}}
<!-- safety / AEs -->
Low-carbohydrate diets may miss the health advantages – such as increased intake of dietary fiber – afforded by high-quality carbohydrates found in [[Biology:Legume|legume]]s and pulses, [[Biology:Whole grain|whole grain]]s, fruits, and vegetables.<ref name=mort>{{cite journal | vauthors = Seidelmann SB, Claggett B, Cheng S, Henglin M, Shah A, Steffen LM, Folsom AR, Rimm EB, Willett WC, Solomon SD | display-authors = 6 | title = Dietary carbohydrate intake and mortality: a prospective cohort study and meta-analysis | journal = The Lancet. Public Health | volume = 3 | issue = 9 | pages = e419–e428 | date = September 2018 | pmid = 30122560 | pmc = 6339822 | doi = 10.1016/s2468-2667(18)30135-x | type = Meta-analysis }}</ref><ref name=fibre>{{cite journal | vauthors = Reynolds A, Mann J, Cummings J, Winter N, Mete E, Te Morenga L | title = Carbohydrate quality and human health: a series of systematic reviews and meta-analyses | journal = Lancet | volume = 393 | issue = 10170 | pages = 434–445 | date = February 2019 | pmid = 30638909 | doi = 10.1016/S0140-6736(18)31809-9 | url = http://discovery.dundee.ac.uk/ws/files/30375889/Final_Lancet_for_John.pdf | access-date = April 24, 2022 | url-status = live | s2cid = 58632705 | doi-access = free | archive-url = https://web.archive.org/web/20210811080032/https://discovery.dundee.ac.uk/ws/files/30375889/Final_Lancet_for_John.pdf | archive-date = August 11, 2021 | type = Review }}</ref> A "meta-analysis, of moderate quality," included as adverse effects of the diet halitosis, [[Medicine:Headache|headache]] and [[Medicine:Constipation|constipation]].<ref name=obes>{{cite journal | vauthors = Churuangsuk C, Kherouf M, Combet E, Lean M | title = Low-carbohydrate diets for overweight and obesity: a systematic review of the systematic reviews | journal = Obesity Reviews | volume = 19 | issue = 12 | pages = 1700–1718 | date = December 2018 | pmid = 30194696 | doi = 10.1111/obr.12744 | url = http://eprints.gla.ac.uk/168899/1/168899.pdf | access-date = April 24, 2022 | url-status = live | type = Systematic review | s2cid = 52174104 | archive-url = https://web.archive.org/web/20190923071822/http://eprints.gla.ac.uk/168899/1/168899.pdf | archive-date = September 23, 2019 }}</ref>{{Better source needed|reason=Quoting source: "Only one meta-analysis, of moderate quality, reported adverse effects of LCDs [...]"|date=August 2022}}

<!-- weight -->
Carbohydrate-restricted diets can be as effective as low-fat diets in helping achieve weight loss over the short term when overall calorie intake is reduced.<ref name=endo>{{cite journal | vauthors = Schwartz MW, Seeley RJ, Zeltser LM, Drewnowski A, Ravussin E, Redman LM, Leibel RL | title = Obesity Pathogenesis: An Endocrine Society Scientific Statement | journal = Endocrine Reviews | volume = 38 | issue = 4 | pages = 267–296 | date = August 2017 | pmid = 28898979 | pmc = 5546881 | doi = 10.1210/er.2017-00111 }}</ref> An Endocrine Society scientific statement said that "when calorie intake is held constant [...] body-fat accumulation does not appear to be affected by even very pronounced changes in the amount of fat vs carbohydrate in the diet."<ref name=endo/> In the long term, effective weight loss or maintenance depends on [[Biology:Calorie restriction|calorie restriction]],<ref name=endo/> not the ratio of macronutrients in a diet.<ref name=tob>{{cite book |title=Behavioral approaches to the treatment of obesity |vauthors=Butryn ML, Clark VL, Coletta MC |work=Textbook of Obesity | veditors = Akabas SR, Lederman SA, Moore BJ |publisher=John Wiley & Sons|year=2012 |quote=Taken together, these findings indicate that calorie intake, not macronutrient composition, determines long-term weight loss maintenance.|isbn=978-0-470-65588-7|page=259}}</ref> The reasoning of diet advocates that carbohydrates cause undue fat accumulation by increasing blood [[Biology:Insulin|insulin]] levels, and that low-carbohydrate diets have a "metabolic advantage", is not supported by [[Medicine:Evidence-based medicine|clinical evidence]].<ref name=endo/><ref name=hall>{{cite journal | vauthors = Hall KD | title = A review of the carbohydrate-insulin model of obesity | journal = European Journal of Clinical Nutrition | volume = 71 | issue = 3 | pages = 323–326 | date = March 2017 | pmid = 28074888 | doi = 10.1038/ejcn.2016.260 | type = Review | s2cid = 54484172 }}</ref> Further, it is not clear how low-carbohydrate dieting affects cardiovascular health, although two reviews showed that carbohydrate restriction may improve lipid markers of [[Medicine:Cardiovascular disease|cardiovascular disease]] risk.<ref name=man>{{cite journal | vauthors = Mansoor N, Vinknes KJ, Veierød MB, Retterstøl K | title = Effects of low-carbohydrate diets v. low-fat diets on body weight and cardiovascular risk factors: a meta-analysis of randomised controlled trials | journal = The British Journal of Nutrition | volume = 115 | issue = 3 | pages = 466–479 | date = February 2016 | pmid = 26768850 | doi = 10.1017/S0007114515004699 | s2cid = 21670516 | doi-access = free }}</ref><ref name=ght>{{cite journal | vauthors = Gjuladin-Hellon T, Davies IG, Penson P, Amiri Baghbadorani R | title = Effects of carbohydrate-restricted diets on low-density lipoprotein cholesterol levels in overweight and obese adults: a systematic review and meta-analysis | journal = Nutrition Reviews | volume = 77 | issue = 3 | pages = 161–180 | date = March 2019 | pmid = 30544168 | doi = 10.1093/nutrit/nuy049 | url = http://researchonline.ljmu.ac.uk/id/eprint/8898/1/nutr-rev%20corrected%20version%2007072018.pdf | access-date = April 24, 2022 | url-status = live | type = Systematic review | s2cid = 56488132 | doi-access = free | archive-url = https://web.archive.org/web/20200506070047/http://researchonline.ljmu.ac.uk/id/eprint/8898/1/nutr-rev%20corrected%20version%2007072018.pdf | archive-date = May 6, 2020 }}</ref>

<!-- diabetes -->
Carbohydrate-restricted diets are no more effective than a conventional healthy diet in preventing the onset of [[Medicine:Type 2 diabetes|type 2 diabetes]], but for people with type 2 diabetes, they are a viable option for losing weight or helping with glycemic control.<ref name=brouns>{{cite journal | vauthors = Brouns F | title = Overweight and diabetes prevention: is a low-carbohydrate-high-fat diet recommendable? | journal = European Journal of Nutrition | volume = 57 | issue = 4 | pages = 1301–1312 | date = June 2018 | pmid = 29541907 | pmc = 5959976 | doi = 10.1007/s00394-018-1636-y | type = Review }}</ref><ref name=meng>{{cite journal | vauthors = Meng Y, Bai H, Wang S, Li Z, Wang Q, Chen L | title = Efficacy of low carbohydrate diet for type 2 diabetes mellitus management: A systematic review and meta-analysis of randomized controlled trials | journal = Diabetes Research and Clinical Practice | volume = 131 | pages = 124–131 | date = September 2017 | pmid = 28750216 | doi = 10.1016/j.diabres.2017.07.006 }}</ref><ref name=ada/> There is limited evidence to support routine use of low-carbohydrate dieting in managing [[Medicine:Type 1 diabetes|type 1 diabetes]].<ref name=ups>{{cite journal | vauthors = Seckold R, Fisher E, de Bock M, King BR, Smart CE | title = The ups and downs of low-carbohydrate diets in the management of Type 1 diabetes: a review of clinical outcomes | journal = Diabetic Medicine | volume = 36 | issue = 3 | pages = 326–334 | date = March 2019 | pmid = 30362180 | doi = 10.1111/dme.13845 | type = Review | s2cid = 53102654 }}</ref> The American Diabetes Association recommends that people with diabetes should adopt a generally healthy diet, rather than a diet focused on carbohydrate or other macronutrients.<ref name=ada>{{cite journal | vauthors = ((American Diabetes Association Professional Practice Committee)) | title = 5. Lifestyle Management: ''Standards of Medical Care in Diabetes-2019'' | journal = Diabetes Care | volume = 42 | issue = Suppl 1 | pages = S46–S60 | date = January 2019 | pmid = 30559231 | doi = 10.2337/dc19-S005 | url = http://care.diabetesjournals.org/content/42/Supplement_1/S46 | access-date = April 24, 2022 | url-status = live | doi-access = free | archive-url = https://web.archive.org/web/20181218145626/http://care.diabetesjournals.org/content/42/Supplement_1/S46 | archive-date = December 18, 2018 }}</ref>

<!-- keto -->
An extreme form of low-carbohydrate diet – the [[Chemistry:Ketogenic diet|ketogenic diet]] – is established as a medical diet for treating [[Medicine:Epilepsy|epilepsy]].<ref name=bda-2018/> Through celebrity endorsement during the early 21st century, it became a fad diet as a means of weight loss, but with risks of undesirable [[Medicine:Side effect|side effect]]s, such as low energy levels and increased hunger, [[Medicine:Insomnia|insomnia]], nausea, and gastrointestinal discomfort.{{scientific citation needed|date=May 2023}}<ref name=bda-2018>{{cite web |publisher=British Dietetic Association |title=Top 5 worst celeb diets to avoid in 2018 |date=7 December 2017 |url=https://www.bda.uk.com/resource/top-5-worst-celeb-diets-to-avoid-in-2018.html |quote=The British Dietetic Association (BDA) today revealed its much-anticipated annual list of celebrity diets to avoid in 2018. The line-up this year includes Raw Vegan, Alkaline, Pioppi and Ketogenic diets as well as Katie Price's Nutritional Supplements. |access-date=1 December 2020 |archive-date=July 31, 2020 |archive-url=https://web.archive.org/web/20200731182316/https://www.bda.uk.com/resource/top-5-worst-celeb-diets-to-avoid-in-2018.html |url-status=live }}</ref> The British Dietetic Association named it one of the "top 5 worst celeb diets to avoid in 2018".<ref name=bda-2018/>

==Sources==
[[File:Glucose 2.jpg|thumb|Glucose tablets]]
Most dietary carbohydrates contain glucose, either as their only building block (as in the polysaccharides starch and glycogen), or together with another monosaccharide (as in the hetero-polysaccharides sucrose and lactose).<ref>{{Cite news|url=https://www.hsph.harvard.edu/nutritionsource/carbohydrates/carbohydrates-and-blood-sugar/|title=Carbohydrates and Blood Sugar|date=2013-08-05|newspaper=The Nutrition Source|language=en-US|access-date=2017-01-30|via=Harvard T.H. Chan School of Public Health|url-status=live|archive-url=https://web.archive.org/web/20170130010758/https://www.hsph.harvard.edu/nutritionsource/carbohydrates/carbohydrates-and-blood-sugar/|archive-date=2017-01-30}}</ref> Unbound glucose is one of the main ingredients of honey. Glucose is extremely abundant and has been isolated from a variety of natural sources across the world, including male cones of the coniferous tree Wollemia nobilis in Rome,<ref>{{cite journal | vauthors = Venditti A, Frezza C, Vincenti F, Brodella A, Sciubba F, Montesano C, Franceschin M, Sergi M, Foddai S, Di Cocco ME, Curini R, Delfini M, Bianco A, Serafini M | display-authors = 6 | title = A syn-ent-labdadiene derivative with a rare spiro-β-lactone function from the male cones of Wollemia nobilis | journal = Phytochemistry | volume = 158 | pages = 91–95 | date = February 2019 | pmid = 30481664 | doi = 10.1016/j.phytochem.2018.11.012 | bibcode = 2019PChem.158...91V | s2cid = 53757166 }}</ref> the roots of Ilex asprella plants in China,<ref>{{cite journal | vauthors = Lei Y, Shi SP, Song YL, Bi D, Tu PF | title = Triterpene saponins from the roots of Ilex asprella | journal = Chemistry & Biodiversity | volume = 11 | issue = 5 | pages = 767–775 | date = May 2014 | pmid = 24827686 | doi = 10.1002/cbdv.201300155 | s2cid = 40353516 }}</ref> and straws from rice in California.<ref>{{cite book | vauthors = Balan V, Bals B, Chundawat SP, Marshall D, Dale BE | chapter = Lignocellulosic Biomass Pretreatment Using AFEX | title = Biofuels | series = Methods in Molecular Biology | volume = 581 | pages = 61–77 | date = 2009 | pmid = 19768616 | doi = 10.1007/978-1-60761-214-8_5 | publisher = Humana Press | isbn = 978-1-60761-213-1 | bibcode = 2009biof.book...61B | place = Totowa, NJ }}</ref>
{|class="wikitable sortable" style="text-align:center; margin:auto"
|+ Sugar content of selected common plant foods (in grams per 100&nbsp;g)<ref name="www.nal.usda.gov">{{Cite web|url=https://fdc.nal.usda.gov/index.html|title=FoodData Central|website=fdc.nal.usda.gov}}</ref>
|-
! Food <br />item
! Carbohydrate, <br />total,{{ref|2|A}} including <br />dietary fiber
! Total <br />sugars
! Free <br />fructose
! Free <br />glucose
! Sucrose
! Ratio of <br />fructose/<br />glucose
! Sucrose as <br />proportion of <br />total sugars (%)
|-
!colspan=8 style="text-align:left"| Fruits
|-
| style="text-align:left;" | [[Biology:Apple|Apple]] || 13.8|| 10.4|| 5.9|| 2.4|| 2.1|| 2.0|| 19.9
|-
| style="text-align:left;" | [[Biology:Apricot|Apricot]]|| 11.1|| 9.2|| 0.9|| 2.4|| 5.9|| 0.7|| 63.5
|-
| style="text-align:left;" | [[Biology:Banana|Banana]]|| 22.8|| 12.2|| 4.9|| 5.0|| 2.4|| 1.0|| 20.0
|-
| style="text-align:left;" | [[Biology:Ficus|Fig]], dried|| 63.9|| 47.9|| 22.9|| 24.8|| 0.9|| 0.93|| 0.15
|-
| style="text-align:left;" | Grapes|| 18.1|| 15.5|| 8.1|| 7.2|| 0.2|| 1.1|| 1
|-
| style="text-align:left;" | Navel orange|| 12.5|| 8.5|| 2.25|| 2.0|| 4.3|| 1.1|| 50.4
|-
| style="text-align:left;" | [[Biology:Peach|Peach]]|| 9.5|| 8.4|| 1.5|| 2.0|| 4.8|| 0.9|| 56.7
|-
| style="text-align:left;" | [[Biology:Pear|Pear]]|| 15.5|| 9.8|| 6.2|| 2.8|| 0.8|| 2.1|| 8.0
|-
| style="text-align:left;" | [[Biology:Pineapple|Pineapple]]|| 13.1|| 9.9|| 2.1|| 1.7|| 6.0|| 1.1|| 60.8
|-
| style="text-align:left;" | [[Biology:Plum|Plum]]|| 11.4|| 9.9|| 3.1|| 5.1|| 1.6|| 0.66|| 16.2
|-
!colspan=8 style="text-align:left"| Vegetables
|-
| style="text-align:left;" | Beet, red|| 9.6|| 6.8|| 0.1|| 0.1|| 6.5||1.0|| 96.2
|-
| style="text-align:left;" | [[Biology:Carrot|Carrot]]|| 9.6|| 4.7|| 0.6|| 0.6|| 3.6|| 1.0|| 77
|-
| style="text-align:left;" | Red pepper, sweet|| 6.0|| 4.2|| 2.3|| 1.9|| 0.0|| 1.2|| 0.0
|-
| style="text-align:left;" | [[Biology:Onion|Onion]], sweet|| 7.6|| 5.0|| 2.0|| 2.3|| 0.7|| 0.9|| 14.3
|-
| style="text-align:left;" | [[Biology:Sweet potato|Sweet potato]]||20.1|| 4.2|| 0.7|| 1.0|| 2.5|| 0.9|| 60.3
|-
| style="text-align:left;" | [[Biology:Yam (vegetable)|Yam]]|| 27.9|| 0.5|| {{n/a|Traces}}|| {{n/a|Traces}}|| {{n/a|Traces}}|| {{n/a}}|| {{n/a|Traces}}
|-
| style="text-align:left;" | Sugar cane|| || 13–18|| 0.2–1.0|| 0.2–1.0|| 11–16|| 1.0|| high
|-
| style="text-align:left;" | [[Biology:Sugar beet|Sugar beet]]|| || 17–18|| 0.1–0.5|| 0.1–0.5|| 16–17|| 1.0|| high
|-
!colspan=8 style="text-align:left"| Grains
|-
| style="text-align:left;" | [[Biology:Maize|Corn]], sweet|| 19.0|| 6.2|| 1.9|| 3.4|| 0.9|| 0.61|| 15.0
|}
{{note|2|A}} The carbohydrate value is calculated in the USDA database and does not always correspond to the sum of the sugars, the starch, and the "dietary fiber".

==Metabolism==
{{Main|Biology:Carbohydrate metabolism}}
Carbohydrate metabolism is the series of [[Biology:Biochemistry|biochemical]] processes responsible for the [[Biology:Anabolism|formation]], [[Biology:Catabolism|breakdown]] and interconversion of carbohydrates in living organisms.

The most important carbohydrate is [[Chemistry:Glucose|glucose]], a simple sugar ([[Chemistry:Monosaccharide|monosaccharide]]) that is metabolized by nearly all known organisms. Glucose and other carbohydrates are part of a wide variety of metabolic pathways across species: plants synthesize carbohydrates from carbon dioxide and water by [[Earth:Photosynthesis|photosynthesis]] storing the absorbed energy internally, often in the form of [[Chemistry:Starch|starch]] or [[Chemistry:Lipid|lipid]]s. Plant components are consumed by animals and [[Biology:Fungi|fungi]], and used as fuel for [[Biology:Cellular respiration|cellular respiration]]. Oxidation of one gram of carbohydrate yields approximately 16&nbsp;kJ (4&nbsp;kcal) of [[Chemistry:Chemical energy|energy]], while the oxidation of one gram of lipids yields about 38&nbsp;kJ (9&nbsp;kcal). The human body stores between 300 and 500&nbsp;g of carbohydrates depending on body weight, with the skeletal muscle contributing to a large portion of the storage.<ref name="Maughan">{{Cite web|url=https://onesearch.cuny.edu/primo-explore/fulldisplay?docid=TN_sciversesciencedirect_elsevierS0263-9319(13)00087-2&context=PC&vid=hc&search_scope=everything&tab=default_tab&lang=en_US|title=Surgery Oxford| vauthors = Maughan R |date=June 2013|website=www.onesearch.cuny.edu}}{{Dead link|date=June 2021 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Energy obtained from metabolism (e.g., oxidation of glucose) is usually stored temporarily within cells in the form of [[Chemistry:Adenosine triphosphate|ATP]].<ref name="energetics">{{cite web | title = Energetics of Cellular Respiration (Glucose Metabolism) | vauthors = Mehta S | work = Biochemistry Notes, Notes | date = 9 October 2013 | url = http://pharmaxchange.info/press/2013/10/energetics-of-cellular-respiration-glucose-metabolism/ | access-date = October 15, 2015 | archive-date = January 25, 2018 | archive-url = https://web.archive.org/web/20180125234509/http://pharmaxchange.info/press/2013/10/energetics-of-cellular-respiration-glucose-metabolism/ | url-status = live }}</ref> Organisms capable of anaerobic and aerobic respiration metabolize glucose and [[Chemistry:Oxygen|oxygen]] (aerobic) to release energy, with [[Chemistry:Carbon dioxide|carbon dioxide]] and [[Chemistry:Water|water]] as byproducts.

===Catabolism===
Catabolism is the metabolic reaction which cells undergo to break down larger molecules, extracting energy. There are two major [[Biology:Metabolic pathway|metabolic pathway]]s of monosaccharide [[Biology:Catabolism|catabolism]]: [[Biology:Glycolysis|glycolysis]] and the [[Biology:Citric acid cycle|citric acid cycle]].

In glycolysis, oligo- and polysaccharides are cleaved first to smaller monosaccharides by enzymes called [[Biology:Glycoside hydrolase|glycoside hydrolase]]s. The monosaccharide units can then enter into monosaccharide catabolism. A 2&nbsp;ATP investment is required in the early steps of glycolysis to phosphorylate Glucose to [[Chemistry:Glucose 6-phosphate|Glucose 6-Phosphate]] ([[Chemistry:Glucose 6-phosphate|G6P]]) and [[Chemistry:Fructose 6-phosphate|Fructose 6-Phosphate]] ([[Chemistry:Fructose 6-phosphate|F6P]]) to [[Chemistry:Fructose 1,6-bisphosphate|Fructose 1,6-biphosphate]] ([[Chemistry:Fructose 1,6-bisphosphate|FBP]]), thereby pushing the reaction forward irreversibly.<ref name="Maughan"/> In some cases, as with humans, not all carbohydrate types are usable as the digestive and metabolic enzymes necessary are not present.

==Carbohydrate chemistry==
[[Chemistry:Carbohydrate chemistry|Carbohydrate chemistry]] is a large and economically important branch of organic chemistry. Some of the main [[Chemistry:Organic reaction|organic reaction]]s that involve carbohydrates are:
* [[Biology:Amadori rearrangement|Amadori rearrangement]]
* [[Chemistry:Carbohydrate acetalisation|Carbohydrate acetalisation]]
* [[Chemistry:Carbohydrate digestion|Carbohydrate digestion]]
* [[Chemistry:Cyanohydrin reaction|Cyanohydrin reaction]]
* [[Chemistry:Koenigs–Knorr reaction|Koenigs–Knorr reaction]]
* [[Chemistry:Lobry de Bruyn–Van Ekenstein transformation|Lobry de Bruyn–Van Ekenstein transformation]]
* [[Chemistry:Nef reaction|Nef reaction]]
* [[Chemistry:Wohl degradation|Wohl degradation]]

== See also ==
<!-- Please keep alphabetical -->
* [[Chemistry:Bioplastic|Bioplastic]]
* Carbohydrate NMR
* [[Biology:Gluconeogenesis|Gluconeogenesis]] – A process where glucose can be synthesized by non-carbohydrate sources.
* [[Biology:Glycobiology|Glycobiology]]
* [[Biology:Glycogen|Glycogen]]
* [[Biology:Glycoinformatics|Glycoinformatics]]
* [[Biology:Glycolipid|Glycolipid]]
* Glycome
* [[Biology:Glycomics|Glycomics]]
* [[Chemistry:Glycosyl|Glycosyl]]
* [[Biology:Macromolecule|Macromolecule]]
* [[Chemistry:Saccharic acid|Saccharic acid]]

== References ==
{{Reflist}}

== Further reading ==
* {{cite web |url=https://www.ars.usda.gov/ARSUserFiles/80400525/Data/SR/SR28/sr28_doc.pdf |archive-url=https://web.archive.org/web/20161031150436/https://www.ars.usda.gov/ARSUserFiles/80400525/Data/SR/SR28/sr28_doc.pdf |archive-date=2016-10-31 |url-status=live |title=Compolition of foods raw, processed, prepared |publisher=United States Department of Agriculture|date=September 2015 |access-date=October 30, 2016}}

== External links ==
* [https://web.archive.org/web/20130629185521/http://www2.ufp.pt/~pedros/bq/carb_en.htm Carbohydrates, including interactive models and animations] (Requires [https://web.archive.org/web/20060320002451/http://www.mdl.com/products/framework/chime/ MDL Chime])
* [https://web.archive.org/web/20050124032405/http://www.chem.qmw.ac.uk/iupac/2carb/ IUPAC-IUBMB Joint Commission on Biochemical Nomenclature (JCBN): Carbohydrate Nomenclature]
* [http://arquivo.pt/wayback/20160516074319/http://www.cem.msu.edu/~reusch/VirtualText/carbhyd.htm Carbohydrates detailed]
* [http://biochemweb.fenteany.com/carbohydrates.shtml Carbohydrates and Glycosylation – The Virtual Library of Biochemistry, Molecular Biology and Cell Biology]
* [http://www.functionalglycomics.org/ Functional Glycomics Gateway], a collaboration between the Consortium for Functional Glycomics and [[Company:Nature Publishing Group|Nature Publishing Group]]

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[[Category:Carbohydrates| ]]

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