Acarbohydrate (/ˌkɑːrboʊˈhaɪdreɪt/) is abiomolecule consisting ofcarbon (C),hydrogen (H) andoxygen (O) atoms, usually with a hydrogen–oxygenatom ratio of 2:1 (as in water) and thus with theempirical formulaCm(H2O)n (wherem may or may not be different fromn), which does not mean the H hascovalent bonds with O (for example withCH2O, H has a covalent bond with C but not with O). However, not all carbohydrates conform to this precisestoichiometric definition (e.g.,uronic acids,deoxy-sugars such asfucose), nor are all chemicals that do conform to this definition automatically classified as carbohydrates (e.g.,formaldehyde andacetic acid).
Carbohydrates perform numerous roles in living organisms.[3] Polysaccharides serve as anenergy store (e.g.,starch andglycogen) and as structural components (e.g., cellulose in plants andchitin in arthropods and fungi). The 5-carbon monosaccharideribose is an important component ofcoenzymes (e.g.,ATP,FAD andNAD) and the backbone of the genetic molecule known asRNA. The relateddeoxyribose is a component of DNA. Saccharides and their derivatives include many other importantbiomolecules that play key roles in theimmune system,fertilization, preventingpathogenesis,blood clotting, anddevelopment.[4]
Carbohydrates are central tonutrition 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 cerealflour, such asbread, pizza or pasta. Sugars appear in human diet mainly as table sugar (sucrose, extracted fromsugarcane orsugar beets), lactose (abundant in milk), glucose and fructose, both of which occur naturally inhoney, manyfruits, and some vegetables. Table sugar, milk, or honey is often added to drinks and many prepared foods such as jam, biscuits and cakes.
Inscientific literature, the term "carbohydrate" has many synonyms, like "sugar" (in the broad sense), "saccharide", "ose",[1] "glucide",[8] "hydrate of carbon" or "polyhydroxy compounds withaldehyde orketone". Some of these terms, especially "carbohydrate" and "sugar", are also used with other meanings.
Infood science and in many informal contexts, the term "carbohydrate" often means any food that is particularly rich in the complex carbohydratestarch (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.
The term "carbohydrate" (or "carbohydrate by difference") refers also todietary fiber, which is a carbohydrate, but, unlike sugars and starches, fibers are not hydrolyzed by human digestive enzymes.[5] Fiber generally contributes littlefood 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.[5]
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The history of the discovery regarding carbohydrates dates back around 10,000 years ago inPapua New Guinea during the cultivation ofsugarcane during the Neolithic agricultural revolution.[9] The term "carbohydrate" was first proposed by German chemistCarl Schmidt (chemist) in 1844. In 1856,glycogen, a form of carbohydrate storage in animal livers, was discovered by French physiologistClaude Bernard.[10]
Formerly the name "carbohydrate" was used inchemistry for any compound with the formula Cm (H2O)n. Following this definition, some chemists consideredformaldehyde (CH2O) to be the simplest carbohydrate,[11] while others claimed that title forglycolaldehyde.[12] 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.,glycosaminoglycans),carboxylic acid and deoxy modifications (e.g.,fucose andsialic acid).
Natural saccharides are generally built of simple carbohydrates calledmonosaccharides with general formula (CH2O)n wheren is three or more. A typical monosaccharide has the structure H–(CHOH)x(C=O)–(CHOH)y–H, that is, analdehyde orketone with manyhydroxyl groups added, usually one on eachcarbonatom that is not part of the aldehyde or ketonefunctional group. Examples of monosaccharides areglucose,fructose, andglyceraldehydes. However, some biological substances commonly called "monosaccharides" do not conform to this formula (e.g.,uronic acids and deoxy-sugars such asfucose) and there are many chemicals that do conform to this formula but are not considered to be monosaccharides (e.g., formaldehyde CH2O andinositol (CH2O)6).[13]
Monosaccharides can be linked together into what are calledpolysaccharides (oroligosaccharides) 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 ofDNA, is a modified version ofribose;chitin is composed of repeating units ofN-acetyl glucosamine, anitrogen-containing form of glucose.
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 theirdegree of polymerization, and may be divided initially into three principal groups, namely sugars, oligosaccharides and polysaccharides.[14]
D-glucose is an aldohexose with the formula (C·H2O)6. The red atoms highlight thealdehyde group and the blue atoms highlight theasymmetric center furthest from the aldehyde; because this -OH is on the right of theFischer projection, this is a D sugar.
Monosaccharides are the simplest carbohydrates in that they cannot behydrolyzed to smaller carbohydrates. They are aldehydes or ketones with two or more hydroxyl groups. The generalchemical formula of an unmodified monosaccharide is (C•H2O)n, literally a "carbon hydrate". Monosaccharides are important fuel molecules as well as building blocks for nucleic acids. The smallest monosaccharides, for which n=3, are dihydroxyacetone and D- and L-glyceraldehydes.
Theα andβanomers of glucose. Note the position of the hydroxyl group (red or green) on the anomeric carbon relative to the CH2OH 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).[15]
Monosaccharides are classified according to three different characteristics: the placement of itscarbonyl group, the number ofcarbon atoms it contains, and itschiral handedness. If the carbonyl group is analdehyde, the monosaccharide is analdose; if the carbonyl group is aketone, the monosaccharide is aketose. Monosaccharides with three carbon atoms are calledtrioses, those with four are calledtetroses, five are calledpentoses, six arehexoses, and so on.[16] These two systems of classification are often combined. For example,glucose is analdohexose (a six-carbon aldehyde),ribose is analdopentose (a five-carbon aldehyde), andfructose is aketohexose (a six-carbon ketone).
Each carbon atom bearing ahydroxyl group (-OH), with the exception of the first and last carbons, areasymmetric, making themstereo centers with two possible configurations each (R or S). Because of this asymmetry, a number ofisomers may exist for any given monosaccharide formula. UsingLe Bel-van't Hoff rule, the aldohexose D-glucose, for example, has the formula (C·H2O)6, of which four of its six carbons atoms are stereogenic, making D-glucose one of 24=16 possiblestereoisomers. In the case ofglyceraldehydes, an aldotriose, there is one pair of possible stereoisomers, which areenantiomers andepimers.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 sugarrotates planepolarized light. This usage of "d-" and "l-" is no longer followed in carbohydrate chemistry.[17]
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 ahemiacetal orhemiketal, forming aheterocyclic ring with an oxygen bridge between two carbon atoms. Rings with five and six atoms are calledfuranose andpyranose forms, respectively, and exist in equilibrium with the straight-chain form.[18]
During the conversion from straight-chain form to the cyclic form, the carbon atom containing the carbonyl oxygen, called theanomeric 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 calledanomers. In theα anomer, the -OH substituent on the anomeric carbon rests on the opposite side (trans) of the ring from the CH2OH side branch. The alternative form, in which the CH2OH substituent and the anomeric hydroxyl are on the same side (cis) of the plane of the ring, is called theβ anomer.
Monosaccharides are the major fuel source formetabolism, 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, oftenpolysaccharides. 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 isglycogen, 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 thecell wall of plants and many forms of algae.Ribose is a component ofRNA.Deoxyribose is a component ofDNA.Lyxose is a component of lyxoflavin found in the humanheart.[19]Ribulose andxylulose occur in thepentose phosphate pathway.Galactose, a component of milk sugarlactose, is found ingalactolipids inplant cell membranes and inglycoproteins in manytissues.Mannose occurs in human metabolism, especially in theglycosylation 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 duringdigestion, and found insemen.Trehalose, a major sugar of insects, is rapidly hydrolyzed into two glucose molecules to support continuous flight.
Two joined monosaccharides are called adisaccharide, the simplest kind of polysaccharide. Examples includesucrose andlactose. They are composed of two monosaccharide units bound together by acovalent bond known as aglycosidic linkage formed via adehydration reaction, resulting in the loss of ahydrogen atom from one monosaccharide and ahydroxyl group from the other. Theformula of unmodified disaccharides is C12H22O11. Although there are numerous kinds of disaccharides, a handful of disaccharides are particularly notable.
Sucrose, pictured to the right, is the most abundant disaccharide, and the main form in which carbohydrates are transported in plants. It is composed of oneD-glucose molecule and oneD-fructose molecule. Thesystematic name for sucrose,O-α-D-glucopyranosyl-(1→2)-D-fructofuranoside, indicates four things:
Its monosaccharides: glucose and fructose
Their ring types: glucose is apyranose and fructose is afuranose
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 theanomeric carbon of both monosaccharides participates in the glycosidic bond.
Lactose, a disaccharide composed of oneD-galactose molecule and oneD-glucose molecule, occurs naturally in mammalian milk. Thesystematic name for lactose isO-β-D-galactopyranosyl-(1→4)-D-glucopyranose. Other notable disaccharides includemaltose (two D-glucoses linked α-1,4) andcellobiose (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.
Carbohydrate consumed in food yields 3.87 kilocalories of energy pergram for simple sugars,[20] and 3.57 to 4.12 kilocalories per gram for complex carbohydrate in most other foods.[21] 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.[22] Animal-based foods generally have the lowest carbohydrate levels, although milk does contain a high proportion oflactose.
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 othermonosaccharides anddisaccharides but glucose is often metabolized first. InEscherichia coli, for example, thelac operon will express enzymes for the digestion of lactose when it is present, but if both lactose and glucose are present, thelac operon is repressed, resulting in the glucose being used first (see:Diauxie).Polysaccharides are also common sources of energy. Many organisms can easily break down starches into glucose; most organisms, however, cannot metabolizecellulose or other polysaccharides such aschitin andarabinoxylans. These carbohydrate types can be metabolized by some bacteria and protists.Ruminants andtermites, 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.[23]
TheInstitute of Medicine recommends that American and Canadian adults get between 45 and 65% ofdietary energy from whole-grain carbohydrates.[24] TheFood and Agriculture Organization andWorld 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).[25] A 2017Cochrane Systematic Review concluded that there was insufficient evidence to support the claim that whole grain diets can affect cardiovascular disease.[26]
The termcomplex carbohydrate was first used in theU.S. Senate Select Committee on Nutrition and Human Needs publicationDietary Goals for the United States (1977) where it was intended to distinguish sugars from other carbohydrates (which were perceived to be nutritionally superior).[27] 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 (monosaccharides anddisaccharides) andcomplex if they arepolysaccharides (oroligosaccharides).[5][28] Carbohydrates are sometimes divided into "available carbohydrates", which are absorbed in thesmall intestine and "unavailable carbohydrates", which pass to thelarge intestine, where they are subject tofermentation by thegastrointestinal microbiota.[5]
Theglycemic index (GI) andglycemic load concepts characterize the potential for carbohydrates in food to raiseblood glucose compared to a reference food (generally pure glucose).[29] 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).[29] 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.[29]
Glycemic load is a measure relating the quality of carbohydrates in a food (low- vs. high-carbohydrate content – the GI) by the amount of carbohydrates in a single serving of that food.[29]
Health effects of dietary carbohydrate restriction
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.[33] AnEndocrine 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."[33] 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 ofcalorie restriction,[33] not the ratio ofmacronutrients in a diet.[34] The reasoning of diet advocates that carbohydrates cause undue fat accumulation by increasing bloodinsulin 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.[35] However, as far as energy expenditure itself is concerned, the claim that low-carbohydrate diets have a "metabolic advantage" is not supported byclinical evidence.[33][36] Further, it is not clear how low-carbohydrate dieting affectscardiovascular health, although two reviews showed that carbohydrate restriction may improve lipid markers ofcardiovascular disease risk.[37][38]
Carbohydrate-restricted diets are no more effective than a conventionalhealthy diet in preventing the onset oftype 2 diabetes, but for people with type 2 diabetes, they are a viable option for losing weight or helping withglycemic control.[39][40][41] There is limited evidence to support routine use of low-carbohydrate dieting in managingtype 1 diabetes.[42] TheAmerican Diabetes Association recommends that people with diabetes should adopt a generally healthy diet, rather than a diet focused on carbohydrate or other macronutrients.[41]
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).[44] 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,[45] the roots of Ilex asprella plants in China,[46] and straws from rice in California.[47]
Sugar content of selected common plant foods (in grams per 100 g)[48]
^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".
The most important carbohydrate isglucose, 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 byphotosynthesis storing the absorbed energy internally, often in the form ofstarch orlipids. Plant components are consumed by animals andfungi, and used as fuel forcellular respiration. Oxidation of one gram of carbohydrate yields approximately 16 kJ (4 kcal) ofenergy, while the oxidation of one gram of lipids yields about 38 kJ (9 kcal). The human body stores between 300 and 500 g of carbohydrates depending on body weight, with the skeletal muscle contributing to a large portion of the storage.[49] Energy obtained from metabolism (e.g., oxidation of glucose) is usually stored temporarily within cells in the form ofATP.[50] Organisms capable of anaerobic andaerobic respiration metabolize glucose andoxygen (aerobic) to release energy, withcarbon dioxide andwater as byproducts.
In glycolysis, oligo- and polysaccharides are cleaved first to smaller monosaccharides by enzymes calledglycoside hydrolases. The monosaccharide units can then enter into monosaccharide catabolism. A 2 ATP investment is required in the early steps of glycolysis to phosphorylate Glucose toGlucose 6-Phosphate (G6P) andFructose 6-Phosphate (F6P) toFructose 1,6-biphosphate (FBP), thereby pushing the reaction forward irreversibly.[49] In some cases, as with humans, not all carbohydrate types are usable as the digestive and metabolic enzymes necessary are not present.
Carbohydrate chemistry is a large and economically important branch of organic chemistry. Some of the mainorganic reactions that involve carbohydrates are:
^"Chapter 1 – The role of carbohydrates in nutrition".Carbohydrates in human nutrition. FAO Food and Nutrition Paper – 66. Food and Agriculture Organization of the United Nations.Archived from the original on December 22, 2015. RetrievedDecember 21, 2015.
^Pigman W, Horton D (1972). "Chapter 1: Stereochemistry of the Monosaccharides". In Pigman W, Horton D (eds.).The Carbohydrates: Chemistry and Biochemistry Vol 1A (2nd ed.). San Diego: Academic Press. pp. 1–67.ISBN978-0323138338.
^Pigman W, Anet EF (1972). "Chapter 4: Mutarotations and Actions of Acids and Bases". In Pigman W, Horton D (eds.).The Carbohydrates: Chemistry and Biochemistry Vol 1A (2nd ed.). San Diego: Academic Press. pp. 165–194.ISBN978-0323138338.
^"lyxoflavin".Merriam-Webster.Archived from the original on October 31, 2014. RetrievedFebruary 26, 2014.
^"Carbohydrates".The Nutrition Source. Harvard School of Public Health. September 18, 2012.Archived from the original on May 7, 2013. RetrievedApril 3, 2013.
^abcd"Glycemic Index and Glycemic Load". Micronutrient Information Center, Linus Pauling Institute, Oregon State University. 2025. RetrievedJanuary 19, 2025.
^Butryn ML, Clark VL, Coletta MC (2012). "Behavioral approaches to the treatment of obesity". In Akabas SR, Lederman SA, Moore BJ (eds.).Textbook of Obesity. New York: John Wiley & Sons. p. 259.ISBN978-0-470-65588-7.Taken together, these findings indicate that calorie intake, not macronutrient composition, determines long-term weight loss maintenance.
^Lopes da Silva MV, de Cassia Goncalves Alfenas R (2011). "Effect of the glycemic index on lipid oxidation and body composition".Nutrición Hospitalaria.26 (1):48–55.doi:10.3305/nh.2011.26.1.5008.PMID21519729.
^Meng Y, Bai H, Wang S, Li Z, Wang Q, Chen L (September 2017). "Efficacy of low carbohydrate diet for type 2 diabetes mellitus management: A systematic review and meta-analysis of randomized controlled trials".Diabetes Research and Clinical Practice.131:124–131.doi:10.1016/j.diabres.2017.07.006.PMID28750216.
^Seckold R, Fisher E, de Bock M, King BR, Smart CE (March 2019). "The ups and downs of low-carbohydrate diets in the management of Type 1 diabetes: a review of clinical outcomes".Diabetic Medicine (Review).36 (3):326–334.doi:10.1111/dme.13845.PMID30362180.S2CID53102654.
^abc"Top 5 worst celeb diets to avoid in 2018". British Dietetic Association. December 7, 2017.Archived from the original on July 31, 2020. RetrievedDecember 1, 2020.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.
^"Carbohydrates and Blood Sugar".The Nutrition Source. August 5, 2013.Archived from the original on January 30, 2017. RetrievedJanuary 30, 2017 – via Harvard T.H. Chan School of Public Health.
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