Carbohydrates

Carbohydrates are organic compounds of carbon, hydrogen, and oxygen, which include starches, cellulose, and sugars. Carbohydrates are the most abundant class of organic compounds found in living organisms. They originate as products of photosynthesis, an endothermic reductive condensation of carbon dioxide requiring light energy and the pigment chlorophyll. Carbohydrates consist of starch, sugars and some related substances such as sugar alcohols and organic acids. Carbohydrates are carbon compounds that contain large quantities of hydroxyl groups. The simplest carbohydrates also contain either an aldehyde moiety (polyhydroxyaldehydes) or a ketone moiety (polyhydroxyketones). All carbohydrates can be classified as either monosaccharides, oligosaccharides or polysaccharides.

Carbohydrates are chemical compounds that act as the primary biological means of storing or consuming energy, other forms being fat and protein. Carbohydrates are the main source of energy for all body functions and are necessary for the assimilation of other nutrients. The primary function of carbohydrates is for short-term energy storage. A secondary function is intermediate-term energy storage. Other carbohydrates are involved as structural components in cells, such as cellulose which is found in the cell walls of plants.

Carbohydrates, as a class, are the most abundant organic compounds found in nature. They are produced by green plants and by bacteria using the process known as photosynthesis, in which carbon dioxide is taken from the air by means of solar energy to yield the carbohydrates as well as all the other chemicals needed by the organisms to survive and grow. Carbohydrates come from a wide array of foods - bread, beans, milk, popcorn, potatoes, cookies, spaghetti, corn, and cherry pie. They also come in a variety of forms. They may be in simple (sugars) or complex (starches and fibre) form. The most common and abundant are sugars, fibers, and starches.

Monosaccharides

Monosaccharides are simple carbohydrates that consist of a single sugar molecule. The largest group of monosaccharides are the hexoses with six carbon atoms in the molecule (eg glucose, fructose, mannose, galactose). Other monosaccharide categories are the heptoses with seven carbon atoms (eg xylose), the pentoses with five carbon atoms, and tetroses with four carbon atoms. Monosaccharides are classified by the number of carbon atoms they contain (triose, tetrose, pentose, hexose and heptose) and by the active group, which is either an aldehyde or a ketone.

Monosaccharides contain either a ketone or aldehyde functional group, plus hydroxyl groups on most or all of the non-carbonyl carbon atoms. Most monosaccharides form cyclic structures, which predominate in aqueous solution, by forming hemiacetals or hemiketals (depending on whether they are aldoses or ketoses) with themselves.

Glucose

The most common monosaccharide is glucose. and this is the most important one for living organisms. Glucose is the main sugar metabolized by the body for energy. The body digests carbohydrates in foods, transforming them into glucose, which serves as the primary fuel for the brain and muscles. Glucose is also called blood sugar as it circulates in the blood at a concentration of 65-110 mg/mL of blood. The most common form of this sugar is called dextroglucose, commonly referred to as dextrose. Glucose is absorbed into the bloodstream through the intestinal wall. Only the mono-saccharides glucose, fructose and galactose are absorbed in humans; these are the end-products of the digestion of carbohydrates.

Galactose

Galactose (also called brain sugar) is a type of sugar found in dairy products, in sugar beets and other gums and mucilages. It is also synthesized by the body, where it forms part of glycolipids and glycoproteins in several tissues. It is considered a nutritive sweetener because it has food energy. Galactose is less sweet than glucose and not very water-soluble. Galactose is one of the hexoses in lactose. Galactose resembles glucose in chemical structure. Glucose is the main sugar metabolized by the body for energy. Galactose can easily be converted into glucose when needed for energy and can be formed from glucose.

Fructose

Commonly known as fruit sugar, fructose is a simple carbohydrate widely distributed in organism, plants, and animals. Fructose in the body may be changed into glucose by the liver and intestines. Fructose is often recommended for, and consumed by, people with diabetes mellitus or hypoglycemia, because it has a very low Glycemic Index (GI 32) relative to cane sugar (sucrose). Honey, tree fruits, berries, melons, and some root vegetables such as: beets, sweet potatoes, parsnips and onions, contain fructose; usually in combination with sucrose and glucose.

Glucosamine

Glucosamine is an amino monosaccharide found in chitin, glycoproteins and glycosaminoglycans such as hyaluronic acid and heparan sulfate. Glucosamine and other amino sugars are important constituents of many natural polysaccharides. Glucosamine is an amino derivative of the simple sugar, glucose. Glucosamine is commonly used for the relief of pain and symptoms associated with osteoarthritis and other joint disorders. Glucosamine provides the primary substrate for both collagen and proteoglycan synthesis.

Disaccharides

Disaccharides consist of two monosaccharides linked together by a dehydration synthesis. The most common disaccharides are sucrose (cane or beet sugar - made from one glucose and one fructose), lactose (milk sugar - made from one glucose and one galactose) and maltose (made of two glucoses).

Lactose

Lactose is a disaccharide of milk which on hydrolysis yields glucose and galactose. Lactose is broken down by lactase to form galactose and glucose which are then absorbed by the small intestine. Lactose is formed in the mammary glands of all lactating animals and is present in their milk. Lactose is broken down in digestive system by the help of an enzyme protein called lactase. It yields the simple sugars d-glucose and d-galactose on hydrolysis. Lactose intolerance is the inability to digest significant amounts of lactose, or milk sugar, resulting from a shortage of the enzyme lactase, which is normally produced by the cells that line the small intestine.

Maltose

Maltose is a fermentable sugar produced by conversion of the starch of sprouting barley grains by malt enzymes, principally diastase. It is the fundamental structural unit of glycogen and starch and is used as a nutrient and sweetener. Maltose is the primary starch degradation product and will be further processed to alcohol and carbon dioxide during fermentation. Through a process called fermentation, glucose, maltose and other sugars are converted to ethanol by yeast cells in the absence of oxygen.

Sucrose

Sucrose is common disaccharide which functions as a transport sugar in plants. Sucrose is the common chemical name for table sugar. Sucrose is a disaccharide; each molecule of sucrose consists of two "simple sugars" (monosaccharides). Sucrose is broken down in the gut by acidic hydrolysis into its component sugars, fructose and glucose, which are then absorbed into the bloodstream through the intestine. Sucrose occurs naturally in many green plants as a product of photosynthesis.

Oligosaccharides and polysaccharides

Oligosaccharides and polysaccharides are composed of longer chains of monosaccharide units bound together by glycosidic bonds. The distinction between the two is based upon the number of monosaccharide units present in the chain. Oligosaccharides typically contain between three and nine monosaccharide units, and polysaccharides contain greater than ten monosaccharide units. Definitions of how large a carbohydrate must be to fall into each category vary however.

Polysaccharides are complex carbohydrates, made up of multiple sugar molecules. This term is commonly used only for those containing more than ten monosaccharide residues. Examples of polysaccharides include starch, dextrin, glycogen, cellulose and chitin. Glycosaminoglycan (GAG) is the polysaccharide unit that makes up proteoglycans, a molecule made of saccharides and proteins. GAGs are extracellular matrix molecules that help give tissues like cartilage their rigid structure. GAGs include chondroitin sulphate, dermatan sulphate, keratan sulphate, heparan sulphate, heparin, and hyaluronan.

Starches are polymers of glucose in which glucopyranose units are bonded by alpha-linkages. Amylose consists of a linear chain of several hundred glucose molecules. Amylopectine is a branched molecule made of several thousand of glucose units. The body must convert starch into glucose which can be utilized for immediate energy or converted to glycogen and stored in the liver for later energy needs. Potato, rice, wheat, and maize are major sources of starch in the human diet.

Glycogen is a polysaccharide made up of repeated glucose units. When carbohydrate energy is needed, glycogen is converted into glucose for use by the muscle cells. Glycogen is the chief source of stored fuel in the body. In humans and other vertebrates, most glycogen is found in the skeletal muscles, but it is found in the highest concentration in the liver. Muscle cell glycogen appears to function as an immediate reserve source of available glucose for muscle cells. Other cells that contain small amounts use it locally as well.

Cellulose is a long-chain polymer polysaccharide carbohydrate, of beta-glucose. It forms the primary structural component of plants and is not digestible by humans. Cellulose is a carbohydrate that comprises much of a plant's cell, especially the cell wall. Cellulose is a polymer made with repeated glucose units bonded together by beta-linkages. Humans and many other animals lack an enzyme to break the beta-linkages, so they do not digest cellulose. Certain animals can digest cellulose, because bacteria possesing the enzyme are present in their gut.

Oligosaccharides are corroborates that contain 3-10 saccharide units. The important oligosaccharides are raffinose and stachyose. They contain a few repeating units of glucose, fructose and galactose. Oligosaccharide can be found on cell membranes and surfaces and they function as cell markers. Oligosaccharides are considered as functional foods, since non-digestible oligosaccharides seem to be useful as prebiotics, which stimulate the growth of beneficial bacteria in the intestine. Oligosaccharides are often manufactured through enzymatic synthesis. Fructo-oligosaccharides (FOS) and inulin, which are found in many vegetables, consist of short chains of fructose molecules. Galacto-oligosaccharides (GOS), which also occur naturally, consist of short chains of galactose molecules. These compounds can be only partially digested by humans.

Sugars and carbohydrates

In biochemistry, a sugar is the simplest molecule that can be identified as a carbohydrate. These include monosaccharides and disaccharides, trisaccharides and the oligosaccharides; these being sugars composed of 1, 2, 3 or more units. In general use, "sugar" is taken to mean Sucrose, also called "table sugar" or saccharose, a disaccharide which is a white crystalline solid. Some other sugars are fructose, which is found in fruits; lactose, which is found in milk; and glucose, which is the most common sugar in the bodies of animals and plants. Table sugar or sucrose is extracted from plant sources. The most important two sugar crops are sugarcane (Saccharum spp.) and sugar beets (Beta vulgaris). Raw sugars are yellow to brown sugars made from clarified cane juice, boiled down to a crystalline solid with minimal chemical processing.

Mill white sugar, also called plantation white, crystal sugar, or superior sugar, is raw sugar whose colored impurities have not been removed, but rather bleached white by exposure to sulfur dioxide. White refined sugar is the most common form of sugar in North America and Europe. Refined sugar can be made by dissolving raw sugar and purifying it with a phosphoric acid method similar to that used for blanco directo, a carbonatation process involving calcium hydroxide and carbon dioxide, or by various filtration strategies. It is then further decolorized by filtration through a bed of activated carbon or bone char depending on where the processing takes place. Brown sugars are obtained in the late stages of sugar refining (stopping the refinement before sugar becomes white and free of molasses), or by coating white refined sugar with a cane molasses syrup.

Refined sugars provide calories, but lack vitamins, minerals, and fiber. Such simple sugars are often called "empty calories" and can lead to weight gain. Also, many refined foods, such as white flour, sugar, and polished rice, lack B vitamins and other important nutrients unless they are marked "enriched." It is healthiest to obtain carbohydrates, vitamins, and other nutrients in as natural a form as possible -- for example, from fruit instead of table sugar.

Simple carbohydrates and complex carbohydrates

Carbohydrates are classified as simple or complex. The classification depends on the chemical structure of the particular food source and reflects how quickly the sugar is digested and absorbed. Simple carbohydrates have one (single) or two (double) sugars while complex carbohydrates have three or more. Complex carbohydrates are made up of sugar molecules that are strung together in long, complex chains. Complex carbohydrates may be classified as either starches, which have alpha glycosidic linkages, which are readily digested by intestinal amylases or as dietary fiber which have beta linkages which are resistant to these enzymes. Complex carbohydrates are found in foods such as peas, beans, whole grains, and vegetables. Both simple and complex carbohydrates are turned to glucose (blood sugar) in the body and are used as energy.

Complex Carbohydrates also tend to be better sources of other important nutrients such as vitamins and minerals plus some protein and fibre. Complex carbohydrate foods provide vitamins, minerals, and fiber that are important to the health of an individual. The majority of carbohydrates should come from complex carbohydrates (starches) and naturally occurring sugars, rather than processed or refined sugars, which do not have the vitamins, minerals, and fiber found in complex and natural carbohydrates. Refined sugars are often called "empty calories" because they have little to no nutritional value. Simple carbohydrates or sugars should be eaten in moderation, since they are high in calories but low in nutrients.

Fructose is a simple carbohydrate widely distributed in organism, plants, and animals. Fructose in the body may be changed into glucose by the liver and intestines. As glucose it is used by the body in several ways, including as a source of energy. Fructose is the sweetest of sugars. It is much sweeter than sucrose (cane sugar). It is best obtained by hydrolysis of inulin, a polysaccharide found in dahlia bulbs and the Jerusalem artichoke. Fructose is classified as a monosaccharide, the most important ketose sugar, a hexose, and is a reducing sugar. An older common name for fructose is levulose, after its levorotatory property of rotating plane polarized light to the left.

Fructose is used to sweeten some diet foods. It is considered a nutritive sweetener because it has calories. Fructose is one of the three most important blood sugars along with glucose and galactose. Fructose is implicated in producing obesity, elevated LDL cholesterol and triglycerides, leading to metabolic syndrome. It is used therapeutically as a fluid and nutrient replenisher. Fructose also chelates minerals in the blood. This effect is especially important with micronutrients such as copper, chromium and zinc.

Fructose is found in fruits, honey, and the sole sugar in bull and human semen. Fructose is more commonly found together with glucose and sucrose in honey and fruit juices. Bees gather nectar from flowers which contains sucrose. They then use an enzyme to hydrolyze or break apart the sucrose into its component parts of glucose and fructose. Glucose and fructose are formed in equal amounts when sucrose is hydrolyzed by the enzyme invertase or by heating with dilute acid; the resulting equimolar mixture of fructose and glucose, called invert sugar, is the major component of honey.

Fructose depends on glucose to carry it into the blood stream. Absorption of fructose without glucose present is very poor, and excess fructose is carried into the lower intestine where it provides nutrients for the existing flora, which produce gas.

Hereditary fructose intolerance is an inherited condition where the body does not produce the chemical needed to break down fructose (fruit sugar). Fructose intolerance is a disorder caused by the body's inability to produce an enzyme called aldolase B that is necessary for absorption of fructose. The undigested fructose collects in the liver and kidneys, eventually causing liver and kidney failure. In fructose-intolerant people, ingestion of fructose (fruit sugar) and sucrose (cane or beet sugar, table sugar) produces complicated chemical changes that cannot be corrected because of the absence of the enzyme aldolase B.

Ingestion of fructose causes profound hypoglycemia (low blood sugar) and progressive liver damage. The body is unable to convert its energy storage material, glycogen, into glucose. Urine tests can be used to detect fructose sugar in the urine. Blood tests can also be used to detect hyperbilirubinemia and high levels of liver enzymes in the blood. Fructose intolerance can be successfully treated by eliminating fructose from the diet.

Galactose is an aldohexose epimeric with glucose at the 4 carbon but less soluble and less sweet, occurring naturally in both D- and L- forms (the latter in plants), it is a component of lactose and other oligosaccharides, cerebrosides and gangliosides, and various glycolipids and glycoproteins. Galactose is more commonly found in the disaccharide, lactose or milk sugar. It is found as the monosaccharide in peas.

Galactose is an essential sugar found in abundance in the diet. Galactose is a monosaccharide constituent, together with glucose, of the disaccharide lactose. The hydrolysis of lactose to glucose and galactose is catalyzed by the enzyme beta-galactosidase, a lactase. In the human body, glucose is changed into galactose in order to enable the mammary glands to secrete lactose. Individuals who lack this enzyme are 'lactose intolerant' and unable to realize the nutritive potential of milk sugars.

The lactose then passes to the large intestine where it is digested by bacteria, producing gas and flatulence. Galactose is mainly used as an energy source. Its interconversion to glucose is prevented by an enzyme deficiency in the condition galactosaemia. Galactose has been widely used to assess liver function based on its enzymatic biotransformation to glucose, and as a nutrient for glucose-intolerant neonates.

Galactose appears to help correct many disorders, including enhancing wound healing, decreasing inflammation, and stimulating calcium absorption. It also appears to help lower the risk of developing cataracts. Dietary galactose is also important in maintaining normal bacterial flora in the intestines. Prolonged use of Galactose has proven to increase the number of Bifidobacteria while providing the proper environment for other beneficial bacteria in the human gut.

Lactose is a disaccharide of milk which on hydrolysis yields glucose and galactose. Lactose is the sugar making up around 2-8% of the solids in milk. The name comes from the Latin word for milk, plus the -ose ending used to name sugars. Lactose is a disaccharide consisting of two subunits, a galactose and a glucose linked together. Lactose has the same empirical formula (C12H22O11) as sucrose (cane sugar) and maltose but differs from both in structure. Lactose is formed in the mammary glands of all lactating animals and is present in their milk.

It is produced commercially as a byproduct of milk processing. Lactose is a unique carbohydrate found in the milk of mammals. This disaccharide requires breakdown into glucose and galactose before it can be absorbed. Lactose is broken down in digestive system by the help of an enzyme protein called lactase. It yields the simple sugars d-glucose and d-galactose on hydrolysis.

Lactose intolerance

Lactose intolerance is the inability to digest significant amounts of lactose, or milk sugar, resulting from a shortage of the enzyme lactase, which is normally produced by the cells that line the small intestine. Lactose is a disaccharide that must be split into two monosaccharides to be used by the body. This operation occurs in the small intestine under the influence of an enzyme called lactase. Most individuals produce sufficient quantities of lactase at birth and during early childhood to digest the lactose in milk. With lactose intolerance, the result of consuming lactose or a lactose-containing food is excess gas production and often diarrhea. This inability results from a shortage of the enzyme lactase, which is normally produced by the cells that line the small intestine.

Lactase breaks down the lactose, milk sugar, into glucose and galactose that can then be absorbed into the bloodstream. Digestive diseases or injuries to the small intestine can reduce the amount of enzymes produced, and is the usual cause of lactose intolerance in young children. Adults with lactose intolerance cannot break down lactose (milk sugar) into glucose and galactose for absorption by the intestine. Many adults who have primary lactose intolerance can drink 100 mL –200 mL of milk without having symptoms. Yogurt has lactase activity. Lactase can be added to milk, allowing predigestion of lactose. In the case of total intolerance, use lactase in tablet or liquid form or drink lactose-free milk.

Maltase is one enzyme produced by the cells lining the small intestine to break down disaccharides. Maltose is the final disaccharide and consists of two glucose molecules joined by an alpha glycosidic bond. It is the fundamental structural unit of glycogen and starch and is used as a nutrient and sweetener. Maltose is formed from two glucose molecules joined together at carbons one and four by a glycosidic bond. Maltose is produced from starch by hydrolysis in the presence of diastase, an enzyme present in malt. Maltose is hydrolyzed to glucose by maltase, an enzyme present in yeast. Maltose has a molecular formula of C12H22O11.

Maltose is a reducing disaccharide. Maltase is the enzyme that splits/hydrolyses maltose, malt sugar, into two separate α-glucose molecules. Maltose is the primary starch degradation product and will be further processed to alcohol and carbon dioxide during fermentation. Through a process called fermentation, glucose, maltose and other sugars are converted to ethanol by yeast cells in the absence of oxygen. Through an analogous process, muscle cells convert glucose into lactic acid to obtain energy while the body operates under anaerobic conditions. Failure to break down maltose in the intestine will lead to diarrhea, excessive gases, and other symptoms.

Mannose is a hexose monosaccharide (6 carbon sugar) with a structure very similar to glucose. Mannose is formed from glucose in the body and used in the formation of short chain sugars naturally attached to certain proteins. Mannose can be formed by the oxidation of mannitol. The root of both these words is manna, the food supplied to the Israelites during their journey through Arabia, which is the sweet secretion of several trees and shrubs, such as fraxinus ornus.

D-mannose is a simple sugar structurally related to glucose. It is absorbed slowly from the gastrointestinal tract, and then a large proportion of it is excreted into the urine. Mannose enters the carbohydrate metabolism stream by phosphorylation and conversion to fructose-6-phosphate. Mannose is a key sugar used for N-linked oligosaccharide and GPI-anchor synthesis. D-mannose is in many fruits, including peaches, apples, oranges, cranberries, and blueberries.

Mannose is one of the most important sugars in the body. It is an important part of globulins, and is found frequently in the polysaccharides of glycoproteins. D-mannose has been widely used in the feed industry to treat and prevent urinary tract infections by inhibiting the adherence of bacteria to membranes or cell walls. In the bladder, d-mannose can adhere to bacterial lectins, preventing them from sticking to the lining of the bladder. Bacteria can then be flushed away during urination, thereby precluding the formation of colonies within the urinary tract.

Mannose prompts anti-inflammatory activity and tissue regeneration. It appears to have an active role in the activation of macrophages, whose function it is to clean up debris which can cause inflammation. Mannose is a dietary supplement that may be used to treat stomach and bowel infections. Mannose has been used to treat carbohydrate-deficient glycoprotein syndrome. Mannose, like another essential sugar (glucosamine), is also crucial for joint protection, especially in cases of rheumatoid arthritis.

Also known as white or table sugar, sucrose is a sweet crystalline dextrorotatory non-reducing disaccharide that occurs naturally in most land plants and is the simple carbohydrate obtained from sugarcane, sugar beet and other sources. Sucrose is broken down in the gut by acidic hydrolysis into its component sugars, fructose and glucose, which are then absorbed into the bloodstream through the intestine. Sucrose is the first storage molecule produced in all green plants. It is also the main carrier of energy from one part of the plant to another, in all plant life.

Sucrose is the most common sweetener in the modern, industrialized world, although it has been displaced in industrial food production by some other sweeteners such as glucose syrups or combinations of functional ingredients and high intensity sweeteners. Its ubiquity is due to the combination of sweetness and functional properties. It is important to the structure of many foods including biscuits (cookies), ice cream and sorbets, and also gives preserving qualities to foods. As such it is common in many processed and "junk" foods. Sucrose is an excellent preservative because it has no "reducing end" or reactive group like the other sugars. Sucrose is an excellent natural preservative and is found in many jarred foods including jams.

Sucrose occurs naturally in many green plants as a product of photosynthesis. Sucrose is present in limited quantities in many plants, including various palms and the sugar maple, but the sugar beet and the sugar cane are the only commercially important sources. Other natural sources of sucrose are found in many fruits, seeds, roots, and honey.