arbohydrates are one of three macronutrients that provide the body with energy ( protein and fats being the other two). The chemical compounds in carbohydrates are found in both simple and complex forms, and in order for the body to use carbohydrates for energy, food must undergo digestion, absorption , and glycolysis . It is recommended that 55 to 60 percent of caloric intake come from carbohydrates.

Chemical Structure

Carbohydrates are a main source of energy for the body and are made of carbon, hydrogen, and oxygen . Chlorophyll in plants absorbs light energy from the sun. This energy is used in the process of photosynthesis, which allows green plants to take in carbon dioxide and release oxygen and allows for the production of carbohydrates. This process converts the sun's light energy into a form of chemical energy useful to humans. Plants transform carbon dioxide (CO ₂ ) from the air, water (H ₂ O) from the ground, and energy from the sun into oxygen (O ₂ ) and carbohydrates (C ₆ H ₁₂ O ₆ ) (6 CO ₂ + 6 H ₂ O + energy = C ₆ H ₁₂ O ₆ + 6 O ₂ ). Most carbohydrates have a ratio of 1:2:1 of carbon, hydrogen, and oxygen, respectively.

Humans and other animals obtain carbohydrates by eating foods that contain them. In order to use the energy contained in the carbohydrates, humans must metabolize , or break down, the structure of the molecule in a process that is opposite that of photosynthesis. It starts with the carbohydrate and oxygen and produces carbon dioxide, water, and energy. The body utilizes the energy and water and rids itself of the carbon dioxide.

Simple Carbohydrates

Simple carbohydrates, or simple sugars, are composed of monosaccharide or disaccharide units. Common monosaccharides (carbohydrates composed of single sugar units) include glucose , fructose, and galactose. Glucose is the most common type of sugar and the primary form of sugar that is stored in the body for energy. It sometimes is referred to as blood sugar or dextrose and is of particular importance to individuals who have diabetes or hypoglycemia . Fructose, the primary sugar found in fruits, also is found in honey and high-fructose corn syrup (in soft drinks) and is a major source of sugar in the diet of Americans. Galactose is less likely than glucose or fructose to be found in nature. Instead, it often combines with glucose to form the disaccharide lactose, often referred to as milk sugar. Both fructose and galactose are metabolized to glucose for use by the body.

Oligosaccharides are carbohydrates made of two to ten monosaccharides. Those composed of two sugars are specifically referred to as disaccharides, or double sugars. They contain two monosaccharides bound by either an alpha bond or a beta bond. Alpha bonds are digestible by the human body, whereas beta bonds are more difficult for the body to break down.

There are three particularly important disaccharides: sucrose , maltose, and lactose. Sucrose is formed when glucose and fructose are held together by an alpha bond. It is found in sugar cane or sugar beets and is refined to make granulated table sugar. Varying the degree of purification alters the

SUGAR COMPARISON

Sugar	Carbohydrate	Monosaccharide or disaccharide	Additional information
Beet sugar (cane sugar)	Sucrose	Disaccharide (fructose and glucose)	Similar to white and powdered sugar, but varied degree of purification
Brown sugar	Sucrose	Disaccharide (fructose and glucose)	Similar to white and powdered sugar, but varied degree of purification
Corn syrup	Glucose	Monosaccharide
Fruit sugar	Fructose	Monosaccharide	Very sweet
High-fructose corn syrup	Fructose	Monosaccharide	Very sweet and inexpensive  Added to soft drinks and canned or frozen fruits
Honey	Fructose and glucose	Monosaccharides
Malt sugar	Maltose	Disaccharide (glucose and glucose)	Formed by the hydrolysis of starch, but sweeter than starch
Maple syrup	Sucrose	Disaccharide (fructose and glucose)
Milk sugar	Lactose	Disaccharide (glucose and galactose)	Made in mammary glands of most lactating animals
Powdered sugar	Sucrose	Disaccharide (fructose and glucose)	Similar to white and brown sugar, but varied degree of purification
White sugar	Sucrose	Disaccharide (fructose and glucose)	Similar to brown and powdered sugar, but varied degree of purification
SOURCE: Mahan and Escott-Stump, 2000; Northwestern University; Sizer and Whitney, 1997; and Wardlaw and Kessel, 2002.

final product, but white, brown, and powdered sugars all are forms of sucrose. Maltose, or malt sugar, is composed of two glucose units linked by an alpha bond. It is produced from the chemical decomposition of starch, which occurs during the germination of seeds and the production of alcohol. Lactose is a combination of glucose and galactose. Because it contains a beta bond, it is hard for some individuals to digest in large quantities. Effective digestion requires sufficient amounts of the enzyme lactase.

Complex Carbohydrates

Complex carbohydrates, or polysaccharides, are composed of simple sugar units in long chains called polymers. Three polysaccharides are of particular importance in human nutrition : starch, glycogen , and dietary fiber .

Starch and glycogen are digestible forms of complex carbohydrates made of strands of glucose units linked by alpha bonds. Starch, often contained in seeds, is the form in which plants store energy, and there are two types: amylose and amylopectin. Starch represents the main type of digestible complex carbohydrate. Humans use an enzyme to break down the bonds linking glucose units, thereby releasing the sugar to be absorbed into the bloodstream. At that point, the body can distribute glucose to areas that need energy, or it can store the glucose in the form of glycogen.

Glycogen is the polysaccharide used to store energy in animals, including humans. Like starch, glycogen is made up of chains of glucose linked by alpha bonds; but glycogen chains are more highly branched than starch. It is this highly branched structure that allows the bonds to be more quickly broken down by enzymes in the body. The primary storage sites for glycogen in the human body are the liver and the muscles.

Another type of complex carbohydrate is dietary fiber. In general, dietary fiber is considered to be polysaccharides that have not been digested at the point of entry into the large intestine. Fiber contains sugars linked by bonds that cannot be broken down by human enzymes, and are therefore

Pastas and whole-grain breads contain complex carbohydrates, which are long strands of glucose molecules. Nutritionists recommend that 55–60 percent of calories come from carbohydrates, and especially complex carbohydrates.

[Photograph by James Noble. Corbis. Reproduced by permission.]

labeled as indigestible. Because of this, most fibers do not provide energy for the body. Fiber is derived from plant sources and contains polysaccharides such as cellulose , hemicellulose, pectin, gums, mucilages, and lignins.

The indigestible fibers cellulose, hemicellulose, and lignin make up the structural part of plants and are classified as insoluble fiber because they usually do not dissolve in water. Cellulose is a nonstarch carbohydrate polymer made of a straight chain of glucose molecules linked by beta bonds and can be found in whole-wheat flour, bran, and vegetables. Hemicellulose is a nonstarch carbohydrate polymer made of glucose, galactose, xylose, and other monosaccharides; it can be found in bran and whole grains. Lignin, a noncarbohydrate polymer containing alcohols and acids, is a woody fiber found in wheat bran and the seeds of fruits and vegetables.

In contrast, pectins, mucilages, and gums are classified as soluble fibers because they dissolve or swell in water. They are not broken down by human enzymes, but instead can be metabolized (or fermented) by bacteria present in the large intestine. Pectin is a fiber made of galacturonic acid and other monosaccharides. Because it absorbs water and forms a gel, it is often used in jams and jellies. Sources of pectin include citrus fruits, apples, strawberries, and carrots. Mucilages and gums are similar in structure. Mucilages are dietary fibers that contain galactose, manose, and other monosaccharides; and gums are dietary fibers that contain galactose, glucuronic acid, and other monosaccharides. Sources of gums include oats, legumes , guar, and barley.

Digestion and Absorption

Carbohydrates must be digested and absorbed in order to transform them into energy that can be used by the body. Food preparation often aids in the digestion process. When starches are heated, they swell and become easier for the body to break down. In the mouth, the enzyme amylase, which is contained in saliva, mixes with food products and breaks some starches into smaller units. However, once the carbohydrates reach the acidic environment of the stomach, the amylase is inactivated. After the carbohydrates have passed through the stomach and into the small intestine, key digestive enzymes are secreted from the pancreas and the small intestine where most digestion and absorption occurs. Pancreatic amylase breaks starch into disaccharides and small polysaccharides, and enzymes from the cells of the small-intestinal wall break any remaining disaccharides into their monosaccharide components. Dietary fiber is not digested by the small intestine; instead, it passes to the colon unchanged.

Sugars such as galactose, glucose, and fructose that are found naturally in foods or are produced by the breakdown of polysaccharides enter into absorptive intestinal cells. After absorption, they are transported to the liver where galactose and fructose are converted to glucose and released into the bloodstream. The glucose may be sent directly to organs that need energy, it may be transformed into glycogen (in a process called glycogenesis) for storage in the liver or muscles, or it may be converted to and stored as fat.

Glycolysis

The molecular bonds in food products do not yield high amounts of energy when broken down. Therefore, the energy contained in food is released within cells and stored in the form of adenosine triphosphate (ATP), a high-energy compound created by cellular energy-production systems. Carbohydrates are metabolized and used to produce ATP molecules through a process called glycolysis.

Glycolysis breaks down glucose or glycogen into pyruvic acid through enzymatic reactions within the cytoplasm of the cells. The process results in the formation of three molecules of ATP (two, if the starting product was glucose). Without the presence of oxygen, pyruvic acid is changed to lactic acid , and the energy-production process ends. However, in the presence of oxygen, larger amounts of ATP can be produced. In that situation, pyruvic acid is transformed into a chemical compound called acetyle coenzyme A, a compound that begins a complex series of reactions in the Krebs Cycle and the electron transport system. The end result is a net gain of up to thirty-nine molecules of ATP from one molecule of glycogen (thirty-eight molecules of ATP if glucose was used). Thus, through certain systems, glucose can be used very efficiently in the production of energy for the body.

Recommended Intake

At times, carbohydrates have been incorrectly labeled as "fattening." Evidence actually supports the consumption of more, rather than less, starchy foods. Carbohydrates have four calories per gram, while dietary fats contribute nine per gram, so diets high in complex carbohydrates are likely to provide fewer calories than diets high in fat. Recommendations are for 55 to 60 percent of total calories to come from carbohydrates (approximately 275 to 300 grams for a 2,000-calorie diet). The majority of carbohydrate calories should come from complex rather than simple carbohydrates. Of total caloric intake, approximately 45 to 50 percent of calories should be from complex carbohydrates, and 10 percent or less from simple carbohydrates.

Low-Carb Diets

Low-carbohydrate diets, such as the Atkins and South Beach diets, are based on the proposition that it's not fat that makes you fat. Allowing dieters to eat steak, butter, eggs, bacon, and other high-fat foods, these diets instead outlaw starches and refined carbohydrates on the theory that they are metabolized so quickly that they lead to hunger and overeating. This theory, which was first popularized in the nineteenth century, came under scathing criticism from the medical establishment during the early 1970s when Dr. Robert Atkins published the phenomenally popular low-carb diet bearing his name. According to the American Medical Association (AMA), the Atkins diet was a "bizarre regimen" that advocated "an unlimited intake of saturated fats and cholesterol-rich foods" and therefore presented a considerable risk of heart disease. Most doctors recommended instead a diet low in fat and high in carbohydrates, with plenty of grains, fruits, and vegetables and limited red meat or dairy products. This became the received wisdom during the 1980s, at the same time that the U.S. waistline began to expand precipitously. As dieters found that weight loss was difficult to maintain on a low-fat diet, low-carb diets regained popularity—with as many as 30 million people trying a low-carb diet in 2003. Several small-scale studies began to suggest that a low-carb diet may indeed be effective and may not have the deleterious effects its detractors have claimed; other research found that any benefits of a low-carb diet are short-lived, and that the negative effects will take decades to become evident. The National Institutes of Health has pledged $2.5 million for a five-year study of the Atkins diet with 360 subjects. While the results of this and other large-scale studies are awaited, many researchers stress that the key issue in maintaining a healthy weight is the number of calories consumed, not the type of calories. The National Academy of Sciences recommends that adults obtain 45 to 65 percent of their calories from carbohydrates, 20 to 35 percent from fat, and 10 to 35 percent from protein.

—Paula Kepos

It is important to consume a minimum amount of carbohydrates to prevent ketosis , a condition resulting from the breakdown of fat for energy in the absence of carbohydrates. In this situation, products of fat breakdown, called ketone bodies, build up in the blood and alter normal pHbalance. This can be particularly harmful to a fetus. To avoid ketosis, daily carbohydrate intake should include a minimum of 50 to 100 grams. In terms of dietary fiber, a minimum intake of 20 to 35 grams per day is recommended.

Exchange System

The exchange system is composed of lists that describe carbohydrate, fat, and protein content, as well as caloric content, for designated portions of specific foods. This system takes into account the presence of more than one type of nutrient in any given food. Exchange lists are especially useful for individuals who require careful diet planning, such as those who monitor intake of calories or certain nutrients. It is particularly useful for diabetics, for whom carbohydrate intake must be carefully controlled, and was originally developed for planning diabetic diets.

Diabetes, Carbohydrate-Modified Diets, and Carbohydrate Counting

Diabetes is a condition that alters the way the body handles carbohydrates. In terms of diet modifications, diabetics can control blood sugar levels by appropriately managing the carbohydrates, proteins, and fats in their meals. The amount of carbohydrates, not necessarily the source, is the primary issue. Blood glucose levels after a meal can be related to the process of food preparation, the amount of food eaten, fat intake, sugar absorption, and the combination of foods in the meal or snack.

One method of monitoring carbohydrate levels—carbohydrate counting—assigns a certain number of carbohydrate grams or exchanges to specific foods. Calculations are used to determine insulin need, resulting in better control of blood glucose levels with a larger variety of foods. Overall, diabetic diets can include moderate amounts of sugar, as long as they are carefully monitored.