Carbohydrates

Carbohydrates are chemical compounds that contain varying chain lengths (polymers) of oxygen (O), hydrogen (H), and carbon (C) atoms arranged by the chemical formula C_n(H₂O)_m ("hydrates of carbon").   These chains are called polysaccharides.   Each carbohydrate (a polysaccharide) is composed of single (simple) sugars called monosaccharides arranged in 3 different isomers (patterns) called sugar, starch and cellulose. ^{Wiki n.p.}

Monosaccharides

Monosaccharides are the simpest molecules that comprise all carbohydrates, whether sugars, starches or cellulose.   The 3 monosaccharides all have the same chemical formula, C₆H₁₂O₆. However, they have different structures, called isomers.   These are glucose, galactose, and fructose.   Because each of these sugars has 6 carbon atoms, it is called a hexose.   (There are also sugars with 3 (triose), 4 (tetrose) and 5 (pentose) carbon atoms.) ^{Wiki n.p.}   All monosacchardes are soluble in water and cannot be stored in the body.   Not much glucose and galastose exist naturally in foods, but fructose does occur naturally in many fruits, honey, and some vegetables (See high fructose corn syrup).

Only glucose can be converted into energy by body cells in a series of chemical reactions called cellular respiration.   The net reaction is

C₆H₁₂O₆ + 6O₂ ——> 6CO₂ + 6H₂O + 2830 kJ / mol energy released

Therefore, carbohydrate fructose and galactose, protein amino acids, and fat glycogen and fatty acids must be converted into glucose before they can be converted into energy.   Glycogen also must be hydrolyzed into glucose before it can be used for energy.

Disaccharides

When 2 monosaccharides combine chemically, they form a disaccharide.   The 3 most common disaccharides are the following:

(1) sucrose = glucose + fructose (refined common table sugar),
(2) lactose = glucose + galactose (sugar in milk),
(3) maltose = glucose + glucose (sugar in grain & a product of starch digestion).

The chemical formula for a disaccharide is C₁₂H₂₂O₁₁ = 2C₆H₁₂O₆ - H₂O.   The glycosidic bond between the 2 monosaccharides results when one loses a hydrogen ( H) atom and the other loses a hydroxide (OH) group to form water (H₂O) in a chemical reaction called condensation.   Like monosacchirides, disacchirides are soluble in water and cannot be stored in the body.   The glycosidic bonds of a disaccharide must be broken by a chemical reaction called hydrolysis to form simple sugars for release of energy in cell respiration .   After digestion and absorption, the principal monosaccharides present in the blood, cells and internal tissues are glucose, fructose, and galactose.   Galactose is synthesized in the body into glycoproteins and glycolipids and is also a source of energy in cells because galactose can be transformed into glucose by hydrolysis.   Fructose also can be converted in glucose by hydrolysis.   Maltose can be broken down into two glucose molecules by hydrolysis.   The enzyme maltase can achieve this conversion. ^{Wiki n.p.}

Lactase is an enzyme that breaks down lactose into two simpler forms of sugar called glucose and galactose, which are then absorbed into the bloodstream.   When the body cannot form enough lactase, it becomes intolerant of lactose.   Not all people deficient in lactase have the symptoms commonly associated with lactose intolerance, but those who do are said to have lactose intolerance.   Lactose intolerance and cow's milk intolerance are not related.   Being intolerant to cow's milk is an allergic reaction triggered by the immune system.   Lactose intolerance is a problem caused by the digestive system.

Polysaccharides

Carbohydrates are mostly polysaccharides, i.e., long chains (in hundreds or thousands) of simple glucose molecules with the chemical formula (C₆H₁₀O₅)_n.   Carbohydrates also contain some mono- and di-saccharides.   There are 3 kinds of carbohydrates used by the human body: sugar, starch, and cellulose.   These forms all are polymers of glucose in different molecular arrangements.   Another important polysaccharide is glycogen, which is a polymer of glucose formed in the body for storage. ^{Wiki n.p.}

Sugar

Natural sugar found in sugar cane, sugar beets and maple trees is a carbohydrate and polysaccharide.   Sugar is a polymer of glucose and is found in all fruits and vegetables.   It is the major product of plant photosynthesis used in plant respiration to provide energy and structure for life and growth.   Sugar cane, sugar beets, and maple trees contain large amounts of sugar that is separated for commercial use.   Sugar is composed of long chains of saccharides, which cannot be stored by the body directly.   Instead, sugar is broken down into glucose in a chemical reaction called hydrolysis.   Sugar is stored in muscles and liver in the form of glycogen. ^{Wiki n.p.}

In 1971, Japanese scientists produced a cheap sweetener made from corn that was six times sweeter than natural sugar.   It consists of glucose + fructose, but with a high proportion of fructose.   They called it high-fructose corn syrup (HFCS).   This sweetener costs less than sugar, prevents products from freezer burn, keeps long shelf-life products (e.g., those in vending machines) tasting fresh, and makes bakery products, many of which contain no sugar, look more natural and fresh.   NFCS was brought to market in the late 1970s.   Thereafter, many convenience food and beverage prices fell by as much as 20% as HFCS replaced the more expensive sweetener, natural sugar. ^{Critser 10}

Starch

Starch is a white, tasteless, solid carbohydrate composed of glucose chain of molecules.   The arrangement of the starch polymer is different from that of the sugar polymer, so each is digested differently.   There are 2 kinds of starch: amylose, which is a linear, unbranched chain, and amylopectin, a highly branched chain.   Like sugar, starch is broken down into glucose for use by the body   It is synthesized into glycogen for storage.   Examples of foods containing considerable starch are rice, wheat, corn and potatoes.   Starch is digested by hydrolysis and catalyzed by enzymes called amylases, which break the glycosidic bonds between the glucoses in the starch polysaccharide.   Humans and other animals have amylases, so they can digest starch.   Digestion of starches consists of the process of the cleavage of the starch molecules back into their constituent glucose units by the action of the amylases.   The resulting sugars are then processed by further enzymes (such as maltase) in the body, in the same manner as other sugars in the diet. ^{Wiki n.p.}

Glycogen

Glycogen is a polymer of glucose, which is similar to amylopectin in structure, except that the chains are shorter and more frequent.   It is insoluble in water.   The human body combines glucose into glycogen for storage, mainly in liver and muscle cells for daily use and in adipose cells and tissues as body fat for long term use.   Glycogen also can be made by the body from fat glycerol and fatty acids and also from protein in chemical reactions called glyconeogenesis when there are insufficient amounts of carbohydrates available in the body.   These alternate sources of glycogen are desirable when a person desires to lose weight, but undesirable when a person requires large amounts of energy for exertion in work or play.   Whatever the source of glycogen, when energy is needed by the cells, they break down the glycogen into glucose.   The glucose provides energy to cells in cell respiration.   Since glycogen is created exclusively in the body, it is not an essential nutrient. ^{Wiki n.p.}

Cellulose

Cellulose is a carbohydrate and a polymer of glucose.   However, it is different from starch in that the glucose chains are straight with no side branches and the glycosidic bonds are different and cannot be broken down into individual glucose molecules through hydrolysis by humans.   Thus, most cellulose is indigestible to humans.   The cellulose structure allows for long, straight, and rigid molecules that lie close together, thus providing stiffness to plant walls and fruit.   Cellulose accounts for most of the structure of wood and cotton, for examples.   Although humans cannot digest cellulose, animals called ruminants can.   In humans, indigestible cellulose is beneficial as "fiber".   Commercially, Wood and wood derivates, such as paper, are important cellulose materials.

Fiber (dietary)

Fiber ("roughage", "bulk") is the cellulose and other materials found in plants.   Fiber contains no energy for humans because the body cannot digest it.   Fiber can be divided into two categories according to their physical characteristics and effects on the body:   water insoluble and water soluble.   Each form functions differently and provides different health benefits.   Insoluble fibers, such as cellulose, hemicellulose and lignin, do not dissolve in water.   Soluble fibers, such as gum and pectin, do dissolve in water to form a gel. ^{Hopkins n.p.}   Both types of fiber "bulk up" waste and move it through the colon more rapidly, preventing constipation, hemorrhoids and diverticulosis.   (Diverticula are pouches of the intestinal wall that can become inflamed and painful.)   Fiber possibly reduces colon cancer.   In additional to the digestive system benefits, soluble fiber reduces blood cholesterol levels, which may help reduce risk of heart disease.   Fiber is a weight reducer because the fibers called cellulose and hemicelluloses take up space in the stomach, thus making the body feel full; therefore, food intake is less. ^{Hopkins n.p.}

Insoluble fiber sources: Fruits, vegetables, dried beans, wheat bran, seeds, popcorn, brown rice, and whole grain products such as breads, cereals, and pasta.   Soluble fiber sources: Fruits, such as prunes, apples, oranges, pears, peaches, and grapes; vegetables, seeds, oat bran, dried beans, oatmeal, barley and rye. ^{Hopkins n.p.}

Although fiber is not considered an essential nutrient, most nutrition scientists recommend a diet containing 20-35 grams of fiber a day.   The average American diet barely consumes half of this amount with an intake of 10-15 grams daily.   Increasing the consumption of complex carbohydrates is the best way to increase fiber intake. ^{Hopkins n.p.}