Carbohydrate
Carbohydrates: Sugars, starches, and cellulose. Chemical compound made from carbon, hydrogen, and oxygen.
Sugar: Simple carbohydrates (mono- or disaccharides); In everyday language, used interchangeably with sucrose.
Starch: A polysaccharide. Cellulose: The main constituent of the cell walls of plants.
Glucose: Sugar found naturally in food, usually as a component of food disaccharides and polysaccharides. Glucose is a monosaccharide.
Glycogen: A highly branched glucose chain. The storage form of carbohydrates in humans and animals.
Fiber: An indigestible carbohydrate. Fiber is a polysaccharide.
Monosaccharide: A one-sugar unit. Mono 5 one, saccharide 5 sugar. Glucose, fructose, and galactose are monosaccharides.
Disaccharide: A two-sugar unit. Di 5 two, saccharide 5 sugar. Sucrose, lactose, and maltose are disaccharides.
Polysaccharide: Chains of glucose molecules such as starch. Poly 5 many, saccharide 5 sugar.
Fructose: Sugar found naturally in fruits and vegetables. May also be processed from corn syrup and added to foods. Fructose is a monosaccharide.
Galactose: Sugar found naturally in food only as part of the disaccharide, lactose. Galactose is a monosaccharide.
Lactose: Sugar found naturally in milk. May also be added to processed foods. Lactose is a disaccharide made up of glucose and galactose.
Sucrose: A disaccharide made of glucose and fructose.
Maltose: Sugar produced during the fermentation process that is used to make beer and other alcoholic beverages. Maltose is a disaccharide made up of two glucose molecules.
Fermentation: The breaking down of a substance into a simpler one by a microorganism, such as the production of alcohol from sugar by yeast.
Carbohydrates are compounds that contain carbon, hydrogen, and oxygen. The presence of these three atoms also gives the word carbohydrate its common abbreviation—CHO. Carbohydrates are found in food as sugars, starches, and cellulose. Carbohydrates are found in the body predominately in the form of glucose (mostly in the blood) and in the storage form of glycogen (in many tissues, predominately muscle and liver). Carbohydrates are the primary energy source for moderate to intense exercise and provide approximately 4 kcal/g. The largest amount of carbohydrate in the body is stored in the form of muscle glycogen. Smaller amounts are stored as liver glycogen, which helps to maintain normal concentrations of glucose (a sugar) in the blood.
Carbohydrates found in food replenish the carbohydrates used, although the body has a limited ability to make glucose from other substances. The sugars and starches found in food provide energy because the body can digest and absorb these kinds of carbohydrates. The cellulose found in starchy foods does not provide energy because humans do not possess the enzymes necessary to digest it. However, cellulose and other fibers are important forms of carbohydrates since fiber is needed for good health.
In addition to energy, starchy foods also contain vitamins, minerals, and other nutrients. Sugars only provide energy and do not contain vitamins, minerals, or fiber. Athletes use carbohydrates as a source of energy and nutrients. Training and competition reduces carbohydrate stores, which must be replenished on a daily basis. The timing of carbohydrate intake can be important, especially immediately after exercise when muscle glycogen resynthesize begins.
Forms of Carbohydrates
To understand the differences in the various forms of carbohydrates in food, one must look more closely at their chemical composition. Carbohydrates are generally classified as monosaccharides, disaccharides, or polysaccharides. It helps to know that saccharide means sugar, and mono means one, di means two, and poly means many. Therefore, a monosaccharide consists of one sugar molecule, a disaccharide two sugar molecules, and a polysaccharide is made up of many sugar molecules. Sugar alcohols are derived from mono- and disaccharides and are discussed separately.
The three monosaccharides found in foods are glucose, fructose, and galactose.
Fructose is naturally found in fruits and vegetables but the largest amount of fructose in Indian diets is added to foods when they are processed. If one looks at the food label of a sweet tasting processed food there is a good chance that one of the ingredients is high-fructose corn syrup. Galactose is a monosaccharide but it is found naturally in food only as part of the disaccharide, lactose.
There are three disaccharides found in food— sucrose, lactose and maltose.
Sucrose is made of one molecule of glucose and one molecule of fructose. Lactose is a combination of one molecule each of glucose and galactose. Maltose is made up of two glucose molecules. Sucrose is found naturally in fruits, vegetables, honey, and maple syrup.
It is also found in sugar beets and sugar cane, which are processed into white and brown sugar. Sucrose is added to many processed foods. Lactose is naturally found in milk. It is often referred to as milk sugar and is sometimes added to processed foods. Maltose is produced during the fermentation process that is used to make maltose syrup (commonly used in Asian cooking), beer and other alcoholic beverages.
Polysaccharides are chains of glucose. These glucose chains are known as starch, fiber, and glycogen. Starches in food may be straight chains (amylose) or branched chains (amylopectin). Enzymes in the digestive tract help to break down these chains into their basic component, glucose. Although glycogen is found in muscle and liver tissue of live animals, it is not considered a food source of carbohydrates for humans, because it degrades rapidly.
Starch is found in many foods, including grains, legumes (beans), and tubers. Grains are grasses that bear seeds and include wheat, corn, rice, rye, oats, barley, and millet and foods that are made from them such as breads, cereals, and pasta. Legumes are plants that have a doubleseamed pod containing a single row of beans. Examples of legumes are lentils, split peas, blackeyed peas, and many kinds of beans such as soy, kidney, lima, and northern beans. Beans are also known by their color—white, pink, red, or black. Tubers, such as white or sweet potatoes and yams, have underground stems and are often referred to as starchy vegetables.
Dietary fiber is found naturally in grains. Whole grains, which contain the endosperm, the germ, and the bran, have more fiber than grains that are highly refined. When whole grains are processed, the germ and the bran are removed. Since these two parts contain most of the fiber, the processing results in a substantial loss of fiber. Other sources of dietary fiber include legumes, seeds, fruits, and vegetables, including the starchy vegetables
Immediate use of Glucose for Energy
As glucose is taken up into a cell, it can be either metabolized or stored for later use, depending upon the current energy state of the cell. If the energy need of the cell is low and the cell has the enzymatic capability, glucose will be stored as glycogen. The energy content of carbohydrate is approximately 4 kcal/g. In glycolysis, glucose is broken down in a series of chemical steps to form pyruvate, often referred to as a glycolytic intermediate. From pyruvate, the completion of the metabolism of glucose follows one of two pathways: conversion to lactate (anaerobic glycolysis) or oxidation of pyruvate in the mitochondria. How glucose is metabolized is dependent upon a variety of factors: the type of cell, the enzymatic capability of the cell, energy state, hormonal status, training history, and intensity of exercise.
Certain types of cells are more likely to use glucose anaerobically and as a result produce lactate. Some cells lack the organelles or enzymatic capability to oxidize glucose. For example, erythrocytes (red blood cells) have no mitochondria and are therefore incapable of metabolizing glucose aerobically; they derive their energy from anaerobic glycolysis. Fasttwitch muscle fibers do contain mitochondria and the inherent oxidative enzymes, yet they are biased to use glucose anaerobically.
Cells in the body that are considered to be highly aerobic (e.g., heart muscle cells, slow-twitch muscle fibers) will predominately metabolize glucose aerobically via oxidative phosphorylation.
Carbohydrate as a source of energy
The fuel source utilized depends on a variety of factors, with exercise intensity playing a major role. Very-high-intensity, very-short-duration anaerobic exercise typically uses creatine phosphate as the energy source. Carbohydrate is used as the predominant source of energy via anaerobic glycolysis during high-intensity, short-duration anaerobic exercise or through oxidative phosphorylation during moderate- to high-intensity aerobic exercise.
Carbohydrate used by exercising muscle can come from stored muscle glycogen or from glucose that is brought into the muscle from the blood. Glucose is made available in the blood from the liver as a result of at least three processes: the breakdown of liver glycogen, the production of glucose from other sources (gluconeogenesis), or the ingestion of carbohydrates as food or fluids, which are absorbed and passed through the live