View source for Carbohydrate

{{short description|Organic compound that consists only of carbon, hydrogen, and oxygen}}
[[File:Lactose.svg|thumb|class=skin-invert|upright=1.25|[[Lactose]] is a [[disaccharide]] found in animal milk. It consists of a molecule of [[galactose|D-galactose]] and a molecule of [[glucose|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 [[biomolecule]] composed of [[carbon]] (C), [[hydrogen]] (H), and [[oxygen]] (O) [[atom]]s. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the [[empirical formula]] {{chem2|C_{''m''}(H2O)_{''n''} }} (where ''m'' and ''n'' may differ). This formula does not imply direct covalent bonding between hydrogen and oxygen atoms; for example, in {{chem2|CH2O}}, hydrogen is covalently bonded to carbon, not oxygen. While the 2:1 hydrogen-to-oxygen ratio is characteristic of many carbohydrates, exceptions exist. For instance, [[uronic acid]]s and [[deoxy-sugar]]s like [[fucose]] deviate from this precise [[Stoichiometry|stoichiometric]] definition. Conversely, some compounds conforming to this definition, such as [[formaldehyde]] and [[acetic acid]], are not classified as carbohydrates.

The term is predominantly used in [[biochemistry]], functioning as a synonym for '''saccharide''' ({{ety|grc|''σάκχαρον'' ({{grc-transl|σάκχαρον}})|sugar}}), a group that includes [[sugar]]s, [[starch]], and [[cellulose]]. The saccharides are divided into four chemical groups: [[monosaccharide]]s, [[disaccharide]]s, [[oligosaccharide]]s, and [[polysaccharide]]s. Monosaccharides and disaccharides, the smallest (lower [[molecular weight]]) carbohydrates, are  commonly referred to as sugars. 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 [[glucose]] ({{ety|grc|''{{wikt-lang|grc|γλεῦκος}}'' ({{grc-transl|γλεῦκος}})|wine, [[must]]}}), and is used for almost all sugars (e.g., [[fructose]] (fruit sugar), [[sucrose]] ([[Sugar cane|cane]] or [[Sugar beet|beet]] sugar), [[ribose]], [[lactose]] (milk sugar)).

Carbohydrates perform numerous roles in living organisms. Polysaccharides serve as an [[energy]] store (e.g., [[starch]] and [[glycogen]]) and as structural components (e.g., cellulose in plants and [[chitin]] in arthropods and fungi). The 5-carbon monosaccharide [[ribose]] is an important component of [[coenzyme]]s (e.g., [[Adenosine triphosphate|ATP]], [[Flavin adenine dinucleotide|FAD]] and [[Nicotinamide adenine dinucleotide|NAD]]) and the backbone of the genetic molecule known as [[RNA]]. The related [[deoxyribose]] is a component of DNA. Saccharides and their derivatives include many other important [[biomolecules]] that play key roles in the [[immune system]], [[fertilization]], preventing [[pathogenesis]], [[blood clotting]], and [[developmental biology|development]].

Carbohydrates are central to [[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 [[flour]], such as [[bread]], pizza or pasta. Sugars appear in human diet mainly as table sugar (sucrose, extracted from [[sugarcane]] or [[sugar beet]]s), lactose (abundant in milk), glucose and fructose, both of which occur naturally in [[honey]], many [[fruit]]s, and some vegetables. Table sugar, milk, or honey is often added to drinks and many prepared foods such as jam, biscuits and cakes.

[[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 by facilitating [[bowel movements]]. Other polysaccharides contained in dietary fiber include [[resistant starch]] and [[inulin]], which feed some bacteria in the [[microbiota]] of the [[large intestine]], and are [[metabolism|metabolized]] by these bacteria to yield [[short-chain fatty acid]]s.

==Terminology==
In [[scientific literature]], the term "carbohydrate" has many synonyms, like "sugar" (in the broad sense), "saccharide", "ose", "glucide", "hydrate of carbon" or "[[Hydroxy group|polyhydroxy]] compounds with [[aldehyde]] or [[ketone]]". Some of these terms, especially "carbohydrate" and "sugar", are also used with other meanings.

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

The term "carbohydrate" (or "carbohydrate by difference") refers also to [[dietary fiber]], which is a carbohydrate, but, unlike sugars and starches, fibers are not hydrolyzed by human digestive enzymes. Fiber generally contributes little [[food energy]] in humans, but is often included in the calculation of total food energy. The fermentation of soluble fibers by gut microflora can yield short-chain fatty acids, and soluble fiber is estimated to provide about 2 kcal/g.

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

== Structure ==
Formerly the name "carbohydrate" was used in [[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 [[formaldehyde]] (CH<sub>2</sub>O) to be the simplest carbohydrate, while others claimed that title for [[glycolaldehyde]]. 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., [[chitin]]), [[sulfate]] (e.g., [[glycosaminoglycan]]s), [[carboxylic acid]]  and deoxy modifications (e.g., [[fucose]] and [[sialic acid]]).

Natural saccharides are generally built of simple carbohydrates called [[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 [[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]]s. However, some biological substances commonly called "monosaccharides" do not conform to this formula (e.g., [[uronic acid]]s 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 CH<sub>2</sub>O and [[inositol]] (CH<sub>2</sub>O)<sub>6</sub>).

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

Monosaccharides can be linked together into what are called [[polysaccharide]]s (or [[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, [[deoxyribose]], a component of [[DNA]], is a modified version of [[ribose]]; [[chitin]] is composed of repeating units of [[N-acetyl glucosamine]], a [[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 [[degree of polymerization]], and may be divided initially into three principal groups, namely sugars, oligosaccharides and polysaccharides.
{| class="wikitable"
|+ The major dietary carbohydrates
|-
! Class<br>(degree of polymerization) !! Subgroup !! Components
|-
! rowspan=3 | [[Sugar]]s (1–2)
|| [[Monosaccharide]]s || [[Glucose]], [[galactose]], [[fructose]], [[xylose]]
|-
| [[Disaccharide]]s || [[Sucrose]], [[lactose]], [[maltose]], [[isomaltulose]], [[trehalose]]
|-
| [[Polyol]]s || [[Sorbitol]], [[mannitol]]
|-
! rowspan=2 | [[Oligosaccharide]]s (3–9)
|| Malto-oligosaccharides || [[Maltodextrin]]s
|-
| Other oligosaccharides || [[Raffinose]], [[stachyose]], fructo-oligosaccharides
|-
! rowspan=2 | [[Polysaccharide]]s (>9)
|| [[Starch]] || [[Amylose]], [[amylopectin]], modified starches
|-
| Non-starch polysaccharides || [[Glycogen]], [[Cellulose]], [[Hemicellulose]], [[Pectin]]s, [[Hydrocolloid]]s
|}

==Monosaccharides==
{{Main|Monosaccharide}}

[[Image:D-glucose color coded.png|upright=0.5|thumb|class=skin-invert|[[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 [[chirality (chemistry)|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 [[hydrolysis|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•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">
<div class="thumbinner" style="width:187px;">
[[Image:Alpha-D-glucopyranose-2D-skeletal.svg|185px|class=skin-invert]]
[[Image:Beta-D-glucopyranose-2D-skeletal.svg|185px|class=skin-invert]]
<div class="thumbcaption">
The [[α]] and [[Beta (letter)|β]] [[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).
</div>
</div>
</div>

Monosaccharides are classified according to three different characteristics: the placement of its [[carbonyl]] group, the number of [[carbon]] atoms it contains, and its [[chirality (chemistry)|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 [[triose]]s, those with four are called [[tetrose]]s, five are called [[pentose]]s, six are [[hexose]]s, and so on. 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 [[Chirality (chemistry)|asymmetric]], making them [[Stereogenic|stereo center]]s with two possible configurations each (R or S). Because of this asymmetry, a number of [[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 [[stereoisomer]]s. In the case of [[glyceraldehyde]]s, an aldotriose, there is one pair of possible stereoisomers, which are [[enantiomers]] and [[epimer]]s. [[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  [[Levorotation and dextrorotation|rotates]] plane [[Polarization (waves)|polarized light]]. This usage of "d-" and "l-" is no longer followed in carbohydrate chemistry.

===Ring-straight chain isomerism===
[[Image:Glucose Fisher to Haworth.gif|thumb|class=skin-invert|[[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.

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 [[anomer]]s. In the ''α anomer'', the -OH substituent on the anomeric carbon rests on the opposite side ([[Cis-trans isomerism|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 [[metabolism]], and glucose is an energy-rich molecule utilized to generate ATP in almost all living organisms.  Glucose is a high-energy substrate produced in plants through photosynthesis by combining energy-poor water and carbon dioxide in an endothermic reaction fueled by solar energy.  When monosaccharides are not immediately needed, they are often converted to more space-efficient (i.e., less water-soluble) forms, often [[polysaccharide]]s.  In animals, glucose circulating the blood is a major metabolic substrate and is oxidized in the mitochondria to produce ATP for performing useful cellular work.  In humans and other animals, serum glucose levels must be regulated carefully to maintain glucose within acceptable limits and prevent the deleterious effects of hypo- or hyperglycemia.  Hormones such as insulin and glucagon serve to keep glucose levels in balance: insulin stimulates glucose uptake into the muscle and fat cells when glucose levels are high, whereas glucagon helps to raise glucose levels if they dip too low by stimulating hepatic glucose synthesis.  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. The most abundant carbohydrate, [[cellulose]], is a structural component of the [[cell wall#plant cell walls|cell wall]] of plants and many forms of algae. [[Ribose]] is a component of [[RNA]]. [[Deoxyribose]] is a component of [[DNA]]. [[Lyxose]] is a component of lyxoflavin found in the human [[heart]]. [[Ribulose]] and [[xylulose]] occur in the [[pentose phosphate pathway]]. [[Galactose]], a component of milk sugar [[lactose]], is found in [[galactolipid]]s in [[cell membrane#lipids|plant cell membranes]] and in [[glycoprotein]]s in many [[biological tissue|tissues]]. [[Mannose]] occurs in human metabolism, especially in the [[glycosylation]] of certain proteins. [[Fructose]], or fruit sugar, is found in many plants and humans, it is metabolized in the liver, absorbed directly into the intestines during [[digestion]], and found in [[semen]]. [[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: [[glucose|D-glucose]] (left) and [[fructose|D-fructose]] (right).]]
{{Main|Disaccharide}}
Two joined monosaccharides are called a [[disaccharide]], the simplest kind of polysaccharide. 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 [[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.

[[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 [[glucose|D-glucose]] molecule and one [[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 [[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 [[galactose|D-galactose]] molecule and one [[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 [[maltose]] (two D-glucoses linked α-1,4) and [[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.

==Oligosaccharides and polysaccharides==

===Oligosaccharides===
{{main|Oligosaccharide}}
Oligosaccharides are saccharide polymers composed of three to ten units of monosaccharides, connected via [[Glycosidic bond|glycosidic linkages]], similar to [[disaccharide]]s. They are usually linked to lipids or amino acids glycosic linkage with oxygen or nitrogen to form [[glycolipid]]s and [[glycoprotein]]s, though some, like the [[raffinose]] series and the [[fructooligosaccharide]]s, do not. They have roles in [[Cell–cell recognition|cell recognition]] and [[cell adhesion]].
[[File:FOS.svg|thumb|The structure of [[fructooligosaccharide]]]]

===Polysaccharides===
{{main|Polysaccharides}}

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

Carbohydrate consumed in food yields 3.87 kilocalories of energy per [[gram]] for simple sugars, and 3.57 to 4.12 kilocalories per gram for complex carbohydrate in most other foods. 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. Refined carbohydrates from processed foods such as white bread or rice, soft drinks, and desserts are readily digestible, and many are known to have a high glycemic index, which reflects a rapid assimilation of glucose.  By contrast, the digestion of whole, unprocessed, fiber-rich foods such as beans, peas, and whole grains produces a slower and steadier release of glucose and energy into the body.  Animal-based foods generally have the lowest carbohydrate levels, although milk does contain a high proportion of [[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 [[monosaccharide]]s and [[disaccharide]]s but glucose is often metabolized first. In ''[[Escherichia coli]]'', for example, the [[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: [[Diauxie]]). [[Polysaccharide]]s are also common sources of energy. Many organisms can easily break down starches into glucose; most organisms, however, cannot metabolize [[cellulose]] or other polysaccharides such as [[chitin]] and [[arabinoxylans]]. These carbohydrate types can be metabolized by some bacteria and protists. [[Ruminant]]s and [[termite]]s, for example, use microorganisms to process cellulose, fermenting it to caloric short-chain fatty acids. Even though humans lack the enzymes to digest fiber, dietary fiber represents an important dietary element for humans.  Fibers promote healthy digestion, help regulate postprandial glucose and insulin levels, reduce cholesterol levels, and promote satiety.

The [[Institute of Medicine]] recommends that American and Canadian adults get between 45 and 65% of [[food energy|dietary energy]] from whole-grain carbohydrates. 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).  A 2017 [[The Cochrane Database of Systematic Reviews|Cochrane Systematic Review]] concluded that there was insufficient evidence to support the claim that whole grain diets can affect cardiovascular disease.

===Classification===<!-- This title is used as a redirect target -->
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). 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. The standard usage, however, is to classify carbohydrates chemically: ''simple'' if they are sugars ([[monosaccharide]]s and [[disaccharide]]s) and ''complex'' if they are [[polysaccharide]]s (or [[oligosaccharide]]s). Carbohydrates are sometimes divided into "available carbohydrates", which are absorbed in the [[small intestine]] and "unavailable carbohydrates", which pass to the [[large intestine]], where they are subject to [[fermentation]] by the [[Human gastrointestinal microbiota|gastrointestinal microbiota]].

====Glycemic index====
The [[glycemic index]] (GI) and [[glycemic load]] concepts characterize the potential for carbohydrates in food to raise [[blood glucose]] compared to a reference food (generally pure glucose). Expressed numerically as GI, carbohydrate-containing foods can be grouped as high-GI (score more than 70), moderate-GI (56–69), or low-GI (less than 55) relative to pure glucose (GI=100). Consumption of carbohydrate-rich, high-GI foods causes an abrupt increase in blood glucose concentration that declines rapidly following the meal, whereas low-GI foods with lower carbohydrate content produces a lower blood glucose concentration that returns gradually after the meal.

[[Glycemic load]] is a measure relating the quality of carbohydrates in a food (low- vs. high-carbohydrate content &ndash; the GI) by the amount of carbohydrates in a single serving of that food.

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

{{Main|Low-carbohydrate diet}}
<!-- safety / AEs -->
Low-carbohydrate diets may miss the health advantages – such as increased intake of [[dietary fiber]] and [[phytochemical]]s – afforded by high-quality plant foods such as [[legume]]s and [[pulse (legume)|pulses]], [[whole grain]]s, fruits, and vegetables. A "meta-analysis, of moderate quality," included as adverse effects of the diet [[halitosis]], [[headache]] and [[constipation]].

<!-- 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. 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." In the long term, low-carbohydrate diets do not appear to confer a "metabolic advantage," and effective weight loss or maintenance depends on the level of [[calorie restriction]], not the ratio of [[macronutrient]]s in a diet. The reasoning of diet advocates that carbohydrates cause undue fat accumulation by increasing blood [[insulin]] levels, but a more balanced diet that restricts refined carbohydrates can also reduce serum glucose and insulin levels and may also suppress lipogenesis and promote fat oxidation.  However, as far as energy expenditure itself is concerned, the claim that low-carbohydrate diets have a "metabolic advantage" is not supported by [[evidence-based medicine|clinical evidence]]. Further, it is not clear how low-carbohydrate dieting affects [[cardiovascular health]], although two reviews showed that carbohydrate restriction may improve lipid markers of [[cardiovascular disease]] risk.

<!-- diabetes -->
Carbohydrate-restricted diets are no more effective than a conventional [[healthy diet]] in preventing the onset of [[type 2 diabetes]], but for people with type 2 diabetes, they are a viable option for losing weight or helping with [[glycemic control]]. 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.

<!-- keto -->
An extreme form of low-carbohydrate diet – the [[ketogenic diet]] – is established as a medical diet for treating [[epilepsy]]. Through [[celebrity endorsement]] during the early 21st century, it became a [[fad]] diet as a means of weight loss, but with risks of undesirable [[side effect]]s, such as low energy levels and increased hunger, [[insomnia]], nausea, and [[gastrointestinal]] discomfort. The [[British Dietetic Association]] named it one of the "top 5 worst celeb diets to avoid in 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). 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, the roots of Ilex asprella plants in China, and straws from rice in California.
{|class="wikitable sortable" style="text-align:center; margin:auto"
|+ Sugar content of selected common plant foods (in grams per 100&nbsp;g)
|-
! 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;" | [[Apple]] || 13.8|| 10.4|| 5.9|| 2.4|| 2.1|| 2.0|| 19.9
|-
| style="text-align:left;" | [[Apricot]]|| 11.1|| 9.2|| 0.9|| 2.4|| 5.9|| 0.7|| 63.5
|-
| style="text-align:left;" | [[Banana]]|| 22.8|| 12.2|| 4.9|| 5.0|| 2.4|| 1.0|| 20.0
|-
| style="text-align:left;" | [[Ficus|Fig]], dried|| 63.9|| 47.9|| 22.9|| 24.8|| 0.9|| 0.93|| 0.15
|-
| style="text-align:left;" | [[Grape]]s|| 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;" | [[Peach]]|| 9.5|| 8.4|| 1.5|| 2.0|| 4.8|| 0.9|| 56.7
|-
| style="text-align:left;" | [[Pear]]|| 15.5|| 9.8|| 6.2|| 2.8|| 0.8|| 2.1|| 8.0
|-
| style="text-align:left;" | [[Pineapple]]|| 13.1|| 9.9|| 2.1|| 1.7|| 6.0|| 1.1|| 60.8
|-
| style="text-align:left;" | [[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;" | [[Carrot]]|| 9.6|| 4.7|| 0.6|| 0.6|| 3.6|| 1.0|| 77
|-
| style="text-align:left;" | [[Chili pepper|Red pepper]], sweet|| 6.0|| 4.2|| 2.3|| 1.9|| 0.0|| 1.2|| 0.0
|-
| style="text-align:left;" | [[Onion]], sweet|| 7.6|| 5.0|| 2.0|| 2.3|| 0.7|| 0.9|| 14.3
|-
| style="text-align:left;" | [[Sweet potato]]||20.1|| 4.2|| 0.7|| 1.0|| 2.5|| 0.9|| 60.3
|-
| style="text-align:left;" | [[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;" | [[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;" | [[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|Carbohydrate metabolism}}
Carbohydrate metabolism is the series of [[biochemistry|biochemical]] processes responsible for the [[anabolism|formation]], [[catabolism|breakdown]] and interconversion of carbohydrates in [[life|living]] [[organism]]s.

The most important carbohydrate is [[glucose]], a simple sugar ([[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 [[photosynthesis]] storing the absorbed energy internally, often in the form of [[starch]] or [[lipid]]s. Plant components are consumed by animals and [[fungi]], and used as fuel for [[cellular respiration]]. Oxidation of one gram of carbohydrate yields approximately 16&nbsp;kJ (4&nbsp;kcal) of [[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. Energy obtained from metabolism (e.g., oxidation of glucose) is usually stored temporarily within cells in the form of [[adenosine triphosphate|ATP]]. Organisms capable of anaerobic and [[aerobic respiration]] metabolize glucose and [[oxygen]] (aerobic) to release energy, with [[carbon dioxide]] and [[water]] as byproducts.

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

In glycolysis, oligo- and polysaccharides are cleaved first to smaller monosaccharides by enzymes called [[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 [[Glucose 6-phosphate|Glucose 6-Phosphate]] ([[Glucose 6-phosphate|G6P]]) and [[Fructose 6-phosphate|Fructose 6-Phosphate]] ([[Fructose 6-phosphate|F6P]]) to [[Fructose 1,6-bisphosphate|Fructose 1,6-biphosphate]] ([[Fructose 1,6-bisphosphate|FBP]]), thereby pushing the reaction forward irreversibly. In some cases, as with humans, not all carbohydrate types are usable as the digestive and metabolic enzymes necessary are not present.

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

==Chemical synthesis==
{{Main|Carbohydrate synthesis}}

[[Carbohydrate synthesis]] is a sub-field of [[organic chemistry]] concerned specifically with the generation of natural and unnatural carbohydrate structures. This can include the synthesis of [[monosaccharide]] residues or structures containing more than one monosaccharide, known as [[oligosaccharides]]. Selective formation of [[Glycosidic bond|glycosidic linkages]] and selective reactions of [[Hydroxy group|hydroxyl groups]] are very important, and the usage of [[protecting group]]s is extensive.

Common reactions for glycosidic bond formation are as follows:
* [[Chemical glycosylation]]
* [[Fischer glycosidation]]
* [[Koenigs-Knorr reaction]]
* [[Crich beta-mannosylation]]

While some common protection methods are as below:
* [[Carbohydrate acetalisation]]
* [[Trimethylsilyl]]
* [[Benzyl]] ether
* [[P-methoxybenzyl|p-Methoxybenzyl]] ether

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

== 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 ==
{{Commons category|Carbohydrates}}
{{wikiquote}}
* [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 [[Nature Publishing Group]]

{{metabolism}}
{{Food chemistry}}
{{Carbohydrates}}

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[[Category:Carbohydrates| ]]
[[Category:Nutrition]]
{{二次利用|date=7 May 2025, at 03:19}}