Digestion and Absorption (Magill’s Medical Guide, Sixth Edition)
Dietary carbohydrates include monosaccharides, like glucose, fructose, and lactose; disaccharides (two monosaccharides linked together), like sucrose and lactose; and polysaccharides (many monosaccharides linked together in polymers), like starches and fiber.
Starches are first broken down in the mouth by salivary alpha-amylase, then in the small intestine, by alpha-amylases of both salivary and pancreatic origin. The resulting simpler sugars are further digested by enzymes linked to the inner lining of the small intestine: maltase, sucrase, and trehalase, which yield absorbable monosaccharides. These sugars cross the cells lining the small intestine via specialized molecular transport mechanisms, then diffuse into the intestinal capillaries and reach the bloodstream.
(The entire section is 104 words.)
Metabolism (Magill’s Medical Guide, Sixth Edition)
In the body, the main role of carbohydrates is energy production and storage. Carbohydrates can also be joined to proteins (glycoproteins, for cell-cell interactions) or fatty acids (glycolipids, which provide energy and can be markers for cellular recognition).
The body converts most digestible carbohydrates into glucose, which is a universal energy source for cells. Excess glucose is stored as glycogen (glycogenesis), which can then be broken down (glycogenolysis) when energy is needed. Glucose is maintained at a constant level in the blood by the interplay of insulin, gulcagon, and other hormones.
Carbohydrate-related diseases are often genetic in nature, linked to inborn errors in enzymes or cellular transporters. Examples are galactosemia, glycogen storage diseases, and lactose intolerance. Diabetes mellitus is a metabolic disorder characterized by excessive blood glucose. Type 1 diabetes is caused by insulin deficiency; type 2 can be the result of insulin resistance, impaired insulin secretion, and increased glucose production.
According to current recommendations, carbohydrates should represent 40 to 60 percent of total calorie intake, preferably as starches and natural sugars. Refined simple sugars provide calories but very little nutrition, and thus their intake should be limited.
(The entire section is 184 words.)
Perspective and Prospects (Magill’s Medical Guide, Sixth Edition)
Food availability in the developed countries has reached unprecedented levels, and the per capita consumption of carbohydrates, particularly in the form of refined sugars, has increased dramatically in the last two decades. Since the 1990’s, the incidence of obesity has been climbing steadily, and so has the incidence of diabetes and related health problems. Current research in nutrition and carbohydrate metabolism is addressing the problem, which has reached epidemic proportions. Great progress is being made in dietary manipulations and drug development.
(The entire section is 81 words.)
For Further Information: (Magill’s Medical Guide, Sixth Edition)
McGraw-Hill Encyclopedia of Science and Technology. 10th ed. 20 vols. New York: McGraw-Hill, 2007.
Sherwood, Lauralee. “The Digestive System.” In Human Physiology: From Cells to Systems. 7th ed. Pacific Grove, Calif.: Brooks/Cole/Cengage Learning, 2010.
Stanhope, K. L., and P. J. Havel. “Fructose Consumption: Considerations for Future Research on Its Effects on Adipose Distribution, Lipid Metabolism, and Insulin Sensitivity in Humans.” Journal of Nutrition 139, no. 6 (June, 2009): 1236S-1241S.
(The entire section is 66 words.)
Carbohydrates (Encyclopedia of Nursing & Allied Health)
Carbohydrates are compounds that consist of carbon, hydrogen, and oxygen, linked together by energy- containing bonds. There are two types of carbohydrates: complex and simple. The complex carbohydrates, such as starch and fiber, are classified as polysaccharides. Simple carbohydrates are known as sugars and they are classified as mono- or disaccharides, depending on the number of sugars present. Monosaccharides consist of only one sugar; disaccharides have two sugar molecules bonded together.
In the digestive tract, carbohydrates are broken down into the monosaccharide glucose, which provides energy for the body's cells and tissues. Glucose is the body's primary source of fuel.
A common concern among consumers is that a high intake of carbohydrate-rich foods will cause weight gain. Consuming too much of any particular food can cause an increase in weight, but eating a balanced diet with plenty of fruits, vegetables, and grains will help promote weight
management. General guidelines recommend that about 55 to 60% of daily calories come from carbohydrates.
Carbohydrates are either simple or complex. Both have four calories per gram, and both are further reduced by the body to glucose, but complex carbohydrates, which undergo most of their digestion in the large intestine, take longer to digest. Carbohydrates come almost exclusively from plants, vegetables, and grains. Milk is the only animal-based product that contains a significant amount of carbohydrate.
Simple carbohydrates include the single sugars, or monosaccharides, and the double sugars, or disaccharides. The monosaccharides include glucose, fructose, and galactose. Disaccharides include lactose, which is made of glucose and galactose; maltose, made of two glucose units; and sucrose, made of glucose and fructose. Monosaccharides can be absorbed directly into the bloodstream, but disaccharides need to be broken down into their monosaccharide components before they can be absorbed.
When food is consumed, the digestion of carbohydrates begins in the mouth, where an enzyme in saliva breaks down starch molecules into the disaccharide maltose. The food then moves into the stomach where it mixes with the stomach's acid and other juices. In the small intestine, starch is further broken down into disaccharides and small polysaccharides by an enzyme released from the pancreas. Cells lining the small intestine then secrete an enzyme that further splits these disaccharides and polysaccharides into monosaccharides. The cells lining the small intestine can absorb these monosaccharides, which are then taken to the liver. The liver converts fructose and galactose to glucose. If there is an excess of fructose or galactose, it may also be converted to fat. Lastly, the glucose is transported to the body's cells by the circulatory system, where it can be used for energy.
When there is an excess of glucose, the muscle and liver cells often convert it to glycogen, which is the storage form of glucose. The muscles store two thirds of the body's glycogen solely for themselves, and the liver stores the other one third, which can be used by the brain or other organs. When blood glucose levels decline, the body breaks down some of its glycogen stores, and uses the glucose for energy. If blood glucose levels are too high, the excess glucose is taken to the liver where it is converted to glycogen and stored for future use.
One of the complex carbohydrates, fiber, is a polysaccharide in which the bonds holding it together cannot be digested by humans. Fiber can be either water-soluble or water-insoluble. Even though these compounds cannot be digested by humans, they serve several important functions. The main function of insoluble fiber is to bind bile acids, which reduces fat and cholesterol absorption. Sources of insoluble fiber include wheat bran, whole grains, and brown rice. Soluble fiber, which helps decrease low-density lipoprotein (LDL) cholesterol, can be found in barley, fruit, legumes, and oats.
Fiber is an extremely important part of the diet. It aids in weight control by displacing calorie-dense fats in the diet. Fiber also absorbs water and slows the movement of food through the digestive tract, promoting a feeling of fullness. Recommended intakes of fiber should be about 27 to 40 grams per day.
The food guide pyramid was designed by health professionals to help consumers make nutritious food choices. The bottom and largest portion of the pyramid represents the bread, cereal, rice, and pasta group, and it is recommended that a healthy diet includes six to 11 servings from this food group daily. Three to five servings from the vegetable group and two to four servings from the fruit group are also recommended. These amounts will provide sufficient carbohydrates (including fiber) in the diet.
When carbohydrate intake is low, there is insufficient glucose production, which then causes the body to use its protein for energy. This ultimately prevents the body's protein from performing its more important functions, such as maintaining the body's immune system.
Without carbohydrate, the body also goes into a state of ketosis, in which by-products of fat breakdown, called ketones, accumulate in the blood. This causes a shift in the acid-base balance of the blood, which can be fatal.
Diabetes is a disease in which the body cannot metabolize carbohydrates, and either doesn't make or doesn't respond to insulin, a hormone secreted by the pancreas that is used to transport glucose to the body's cells. In individuals with type 1 diabetes, the pancreas fails to produce insulin, thus causing blood glucose levels to remain the same after meals. This condition is known as hyperglycemia. These individuals must receive daily injections of insulin to control their blood glucose levels. In type 2 diabetes, there may be sufficient insulin, but the body's cells may be resistant to it. Once again, this causes blood glucose levels to rise. Type 2 diabetes can be treated through oral medication and proper diet, although the need for insulin injections may develop later on.
Health care team roles
Registered dietitians and nutritionists are the professionals most qualified to educate individuals on the role of carbohydrates in a healthy diet, as well as the complications associated with low-carbohydrate intakes. Medical doctors and nursing professionals also play an important role in treating carbohydrate-related conditions such as diabetes, while dietitians serve to make recommendations concerning the nutritional needs of these individuals.
Diabetes condition characterized by inadequate use of insulin preventing a person from controlling blood sugar levels.
Fructoseonosaccharide known as fruit sugar.
Galactoseonosaccharide known as milk sugar.
Glucoseonosaccharide used for energy; also known as blood sugar.
Lactoseisaccharide known as milk sugar.
Low-density lipoprotein cholesterol (LDL)Lipoproteins containing a large amount of cholesterol; transport lipids to other tissues in the body.
Maltoseisaccharide known as malt sugar.
Sucroseisaccharide commonly known as table sugar.
Polysaccharidesong chains of glucose units linked together.
Sizer, Francis, and Eleanor Whitney. Nutrition: Concepts and Controversies. 7th ed. Wadsworth Publishing Company, 1997.
American Dietetic Association. 216 West Jackson Blvd., Chicago, Ill., 60606. <<a href="http://www.eatright.org">http://www.eatright.org>.
Kennedy, Ron. "Carbohydrates in Nutrition." The Doctor's Medical Library. <<a href="http://www.medical-library.net/sites/carbohydrates_in%20nutrition.html">http://www.medical-library.net/sites/carbohydrates_in%20nut... > (April 18,2001).
Marlett, Judith A., and Joanne L. Slavin. "Health Implications of Dietary Fiberosition of ADA." Journal of the American Dietetic Association 97 (1997):1157-1159. <<a href="http://www.eatright.org/adap1097.html">http://www.eatright.org/adap1097.html>. (April 18,2001).
"Pasta Power! Debunking the Myths about Pasta." Journal of the American Dietetic Association Online 1997. <<a href="http://www.eatright.org/nfs/nfs73.html">http://www.eatright.org/nfs/nfs73.html> (April 18,2001).
Lisa M. Gourley
Carbohydrates (Encyclopedia of Food & Culture)
CARBOHYDRATES. Plants manufacture and store carbohydrates as their main source of energy through photosynthesis. Once consumed, these organic compounds can be digested, absorbed, and metabolized, supplying humans or animals with energy. Carbohydrates provide roughly half of the total caloric intake of the average human diet. These calories may be used immediately for energy metabolism or may be transformed and stored as glycogen or fat to be used as an energy source as demanded. Dietary carbohydrates are comprised of a wide array of compounds ranging from the simple oneor two-unit sugars to the long chain starches, glycogen and cellulose. Carbohydrates can be classified as monosaccharides, di-and oligosaccharides, and polysaccharides.
|Classification||Number of sugar units**||Examples|
|Monosaccharides||1||Glucose, galactose, fructose|
|Disaccharides||2||Sucrose, lactose, maltose|
|Oligosaccharides||20||Includes the disaccharides|
|Polysaccharides||> 10||Glycogen, starch, cellulose|
|**A "sugar unit" is one monosaccharideach unit is not necessarily the same monosaccharide. For example, sucrose consists of one glucose uni and one fructose unit.|
Monosaccharides, often referred to as simple sugars, are the simplest form of carbohydrates and are seldom found free in nature. The three that can be absorbed by the human body include glucose, galactose, and fructose. Glucose is the most abundant of the monosaccharides and the most important nutritionally. It is the repeating monosaccharide unit in starch, glycogen, and cellulose, and is found in all edible disaccharides.
Oligosaccharides are short chains of monosaccharide units that are joined by glycosidic bonds. They generally have between two to ten units, with the disaccharides, those chains containing two units, being the most abundant. The most common disaccharides include:
Sucrose (from table, cane, and beet sugars), consisting of glucose and fructose
Lactose (from milk sugar), consisting of glucose and galactose
Maltose (from malt sugar), consisting of two glucose units
Polysaccharides are long chains of monosaccharide units. The major polysaccharides include the digestible forms (glycogen and starch) and nondigestible forms (cellulose, hemicellulose, lignin, pectin, and gums).
Starch is the most common digestible polysaccharide found in plants. It can be found in two formsmylose and amylopectin. Amylose is a linear, unbranched molecule that is bound solely by a-1,4 glycosidic bonds. Amylopectin, which makes up the greatest percent of the total starch content, is branched with a-1,6 bonds at the branch points.
Glycogen is the major storage form of carbohydrates in animals, found primarily in the liver and skeletal muscle. When energy intake exceeds energy expenditure, excess calories from fat, protein, and carbohydrate can be used to form glycogen. It is made up of repeating glucose units and is highly branched. During times of fasting or in between meals, these chains can be broken down to single glucose units and used as an energy source for the body. Although found in animal tissue, animal products do not contain large amounts of glycogen because it is depleted at the time of slaughter due to stress hormones.
Cellulose is the major component of cell walls in plants. Just as starch and glycogen, it too is made up of repeating glucose molecules. However, the glycosidic bonds connecting the units are b-1,4. These bonds are resistant to mammalian digestive enzymes rendering cellulose, and other substances containing these bonds, indigestible. Thus, cellulose is not considered to be a significant source of energy for the body. However, as a fiber, it is important for intestinal bacteria.
Since cellulose is a major part of the plant cell wall, it also encases some of the starch, preventing the digestive enzymes from reaching it and decreasing the digestibility of some raw foods such as potatoes and grains. Cooking causes the granules to swell and also softens and ruptures the cellulose wall, allowing the starch to be digested.
Fiber can be classified as soluble and insoluble. Soluble fiber, which includes pectin and gums, dissolves in water to form a gel in the digestive tract. This increases the time the food is in the small intestine, thus increasing the chance of nutrients being absorbed. It is believed that soluble fiber plays a role in lowering blood LDL cholesterol. This could be due to the binding and increased excretion of fat and bile acid (a derivative of cholesterol) or other mechanisms not yet understood. Bacteria in the bowel can use fiber as a food source. These bacteria can degrade the fiber and release some components that can then be absorbed and used by the body. The increased nutrition for the bacteria can increase microbial growth, which can then lead to increased stool bulk, with little of the fiber actually found in the stool.
Insoluble fiber, including cellulose, hemicellulose, and lignin (a noncarbohydrate component of the cell wall that is often included as dietary fiber), absorbs water, thereby increasing the bulk and volume of the stool. It helps to speed the movement through the intestinal tract, preventing constipation, and is prescribed in the treatment of irritable bowel syndrome. It has also been shown that insoluble fibers bind fat-soluble carcinogens and remove them from the gastrointestinal tract, helping to decrease cancer risk.
Refined and processed foods have not only most of the fiber removed, but along with it many of the vitamins, minerals, and phytochemicals (chemicals found in plants believed to contain protective properties) that contribute to the health benefits of whole grain foods. The federal government's Dietary Guidelines for Americans encourage individuals to include whole grain foods in their diet to ensure adequate fiber to promote proper bowel function, as well as to receive other added health benefits.
Digestion, Absorption, and Transportation
In order for carbohydrates to be absorbed by the intestinal mucosal cells, they must first be converted into monosaccharides. The digestive process begins in the mouth with salivary a-amylase that partially breaks down starch by hydrolyzing some of the a-1,4 bonds. However, the digestion that takes place here is of little significance since food remains in the mouth for only a brief period, although this may differ depending on chewing time. The enzyme continues to work for a short time in the stomach until the pH is lowered due to hydrochloric acid that inhibits the enzyme.
|Examples of carbohydrate food sources|
|Fruit||High-fructose corn syrup||Milk|
|Maple sugar||Milk products||Malt liquor|
|Starch (rye, oats, wheat, rice, potatoes, legumes, cereals, bread)|
|Fruits (apples, berries)||Oats, barley|
|Jams and jellies (additive)||Ice cream (additive) Legumes|
|Whole wheat foods||Whole grains||Fruit|
|Leafy vegetables||Bran, wheat Vegetables|
The bulk of carbohydrate digestion occurs in the small intestine by pancreatic a-amylase. The pH of the small intestines is increased due to the addition of bicarbonate and bile, allowing the enzyme activity to occur. Specific disaccharidases located on the intestinal mucosal cells help to further break down the carbohydrates into the monosaccharides: glucose, fructose, and galactose.
Once the carbohydrates have been broken down, the monosaccharides can be absorbed by the mucosal cells. Glucose and galactose enter by active transport, which requires energy as well as specific receptors and carriers. Fructose is absorbed by facilitated diffusion. Like active transport, facilitated diffusion requires a specific carrier, but instead of needing energy, it relies on the low levels of fructose inside the cell to "pull" the fructose inside. Once transported through the intestinal wall, the monosaccharides enter the blood through the capillaries and are carried to the portal circulation and then to the liver.
Metabolism of Carbohydrates
The liver is the major site of galactose and fructose metabolism, where they are taken up, converted to glucose derivatives, and either stored as liver glycogen or used for energy immediately when needed. Although glucose is metabolized extensively in the liver, unlike galactose and fructose, it is also passed into the blood supply to be used by other tissues. Tissues like skeletal muscle and adipose tissue depend on insulin for glucose uptake, whereas the brain and liver do not. This dependence on insulin becomes a problem for diabetics who either cannot make insulin (IDDM) or are resistent to insulin (NIDDM). For individuals left untreated, dietary carbohydrates cause glucose levels to rise, resulting in hyperglycemia, which will lead to serious consequences if steps are not taken to correct it.
Once in the tissues, the fate of glucose depends on the energy demands of the body. Glucose can be metabolized through the glycolysis pathway to pyruvate where it is either converted to lactate or completely oxidized to CO2, H2O, and energy. Liver and skeletal muscle can convert excess glucose to glycogen through a pathway known as glycogenesis. The glycogen is stored after meals to be used as an energy source when energy demands are higher than intake. At this time the glycogen is broken down into individual glucose units, a process known as glycogenolysis, and the glucose can be metabolized further. Excess carbohydrates also can be used as a substrate for fat synthesis.
Carbohydrates are an essential part of a healthy diet. They provide an easily available energy source, are an important vehicle for micronutrients and phytochemicals, help to maintain adequate blood glucose, and are important in maintaining the integrity and function of the gastrointestinal tract. Table 2 contains examples of foods that contain the various types of carbohydrates.
See also Digestion; Fiber, Dietary; Starch.
Ettinger, Susan. "Macronutrients: Carbohydrates, Proteins and Lipids." In Krause's Food, Nutrition, and Diet Therapy, edited Kathleen L. Mahan and Sylvia Escott-Stump. 10th ed. Philadelphia, Pa.: W. B. Saunders, 2000.
FAO/WHO. Carbohydrates in Human Nutrition: Report of a Joint FAO/WHO Expert Consultation, Rome, 148 April 1997. Rome: World Health Organization, Food and Agriculture Organization of the United Nations, 1998.
Guthrie, Joanne, and Joan Morton. "Food Sources of Added Sweetners in the Diets of Americans." Journal of the American Dietetic Association 100 (2000): 438, 51.
Kiens, B., and E. A. Richter. "Types of Carbohydrates in an Ordinary Diet Affect Insulin Action and Muscle Substrates in Humans." American Journal of Clinical Nutrition. 63 (1996): 473.
Macdonald, I. A. "Carbohydrate as a Nutrient in Adults: Range of Acceptable Intakes." European Journal of Clinical Nutrition. 53 (1999): S101106.
Debra Coward McKenzie Rachel K. Johnson