What are carbohydrates?
The carbohydrate group includes sugar, starch, and fiber. Most forms of carbohydrates are composed of carbon, hydrogen, and oxygen. Plants are the main source of carbohydrates. During photosynthesis, plants produce glucose by using carbon and oxygen from carbon dioxide in the air, hydrogen from the water, and energy from the sun. Plant either store the glucose or transform it into starch, fiber, fat, or protein.
The simpler forms of carbohydrates are called monosaccharides and disaccharides. Monosaccharides are single sugars with the general formula of (CH2O)6. Disaccharides are double sugars, made from 2 monosaccharide sugars, and have the general formula of (CH2O)12. The more complex types of carbohydrates are called polysaccharides and contain many glucose molecules linked together (such as glycogen, starch, and fiber. For more information about of monosaccarides, disaccharides, oligosaccharides, and polysaccharides, check the end of the article.
Carbohydrates in food
Carbohydrates are found in a wide variety of foods, such as table sugar, jam, jelly, fruit, fruit juices, soft drinks, potatoes, rice, pasta, cereals, and breads. Other foods, such as dried beans, lentils, corn, peas, and dairy products are also good sources of carbohydrate, although they provide protein and in some cases fat to our diets as well. Foods with little or no carbohydrate include meat, fish, poultry, eggs, vegetable oils, butter, and margarine.
Starches contribute much of the carbohydrate in our diets. A diet rich in grains, legumes, and tubers also can provide significant amount of dietary fiber. Substances that impart sweetness to foods fall into 2 broad classes: nutritive sweeteners, which can be metabolized to produce energy, and alternative sweeteners, which provide no food energy. The sugar alcohols, sorbitol, mannitol, and xylitol, are nutritive sweeteners in sugarless gum and candies.
Digestion of carbohydrates
During digestion, starch and sugars are broken into monosaccharide units that are small enough to be absorbed. The enzymatic digestion of some carbohydrates begins in the mouth with the action of an enzyme called salivary amylase. Salivary enzyme is inactivated by the acidity of the stomach.
In the small intestine, polysaccharides are digested further by pancreatic amylase and specialized enzymes in the absorptive cells of the small intestine. Glucose and galactose are absorbed by an active absorption process. Fructose is taken up by the absorptive cells via facilitated diffusion. Monosaccharides are transported via the portal vein to the liver. Within the liver, fructose and galactose are converted to glucose. Glucose is transported through the bloodstream for use by the cells of the body.
Recommended intake of carbohydrates
Adults need about 130 grams per day of digestible carbohydrate to supply adequate glucose for the brain and the central nervous system, without having to rely on partial replacement of glucose by ketone bodies as an energy source. The Dietary Guidelines for Americans recommend limiting added sugars to approximately 6% of total energy intake.
The Adequate Intake for fiber is based on a goal of 14 grams/1000 kcal consumed. For adults up to the age of 50 years, the Adequate Intake is set at 25 grams for women and 38 grams for men. After the age of 50, the Adequate Intake drops to 21 grams/ day and 30 grams/ day, respectively. In North America, carbohydrates supply about 50% of the energy intakes of adults. Sugar intake tends to be higher than recommended and fiber intake lower than recommended.
Very high intake of carbohydrates
Adequate carbohydrate intake is important for maintaining health and decreasing the risk of chronic disease. Very high intakes of fiber combined with low fluid can result in digestive problems and may decrease the absorption of minerals and increase the risk of deficiencies.
High intakes of sugars can displace more nutritious foods and increase the risk of weight gain, obesity, and dental caries. The “supersizing” trend noted in food and beverage promotions is contributing to this concern. For example, in the 1950s, a typical soft drink serving was a 192 ml (6.5 ounce) bottle. Today, a 592 ml (20 ounce) bottle is a typical serving. This change alone contributes 170 extra calories of sugar to the diet. Drinking one bottle per day for a year would amount to 62.050 extra calories.
Carbohydrates in detail
The common monosaccharides are glucose, fructose, and galactose.
Glucose is the most abundant monosaccharide, even though we eat very little of it as a monosaccharide. Much of the glucose in our diets is linked together with additional sugars to form disaccharides or polysaccharides. Sometimes, we refer to glucose as “blood sugar”.
Fructose is found in fruits, vegetables, honey (which is about 50% fructose and 50% fructose), and high-fructose corn syrup. The presence of fructose in these products makes fructose a common sugar in our diets.
Galactose is the third major monosaccharide of nutritional importance. Usually, galactose is not found free in nature, or at least not in major quantities. Instead, it combined with glucose to form a disaccharide called lactose, which can be found in milk, cheese, and other dairy products.
The additional monosaccharides found in nature are ribose and deoxyribose. They are classified as pentoses because they contain 5 carbons. Although these types of sugars are only needed in small quantities, they are important in the body, as they are an essential part of the cell’s genetic material. Ribose is part of ribonucleic acid (RNA), and deoxyribose is part of deoxynucleic acid (DNA).
Carbohydrates containing 2 monosaccharides are called disaccharides. The linking of 2 monosaccharides occurs in a condensation reaction. During this reaction, 1 molecule of water is formed and released by taking a hydroxyl group from 1 sugar and a hydrogen from the other group. One carbon on each monosaccharide participating in the condensation reaction chemically conds with a single oxygen. There are two forms of C-O-C in nature: alpha (α) bonds and beta (β) bonds. Beta bonds differ from alpha bonds in that they cannot be easily broken down by digestive enzymes for absorption in the small intestine. Thus, foods that contain saccharide molecules linked together by beta bonds (ex., milk, dietary fiber) are often difficult or impossible to digest by individuals because they lack the enzymes necessary for breaking beta bonds apart.
Maltose contains 2 glucose molecules joined by an alpha bond. When seeds sprout, they produce enzymes that break down polysaccharides stored in the seed to sugars such as maltose and glucose. Malting, the first step in the production of alcoholic beverages lets grain seeds sprout. Few other products contain maltose. Most of the maltose which we digest in the small intestine is produces when we break down longer-chain polysaccharides.
Sucrose, common table sugar, is composed of glucose and fructose linked by an alpha bond. Large quantities of sucrose are found naturally in plants, such as sugar cane, sugar beets, or maple tree sap. The sucrose from these sources may be refined to various degrees.
Lactose, the primary sugar in milk and milk products, consist of glucose joined to galactose by a beta bond. Many people have difficulties in digesting lactose due to the lack of the enzyme lactase, which is needed to break this beta bond. The unabsorbed lactose is metabolized into acids and gases by bacteria in the large intestine and therefore, it produces bloating and cramps.
Oligosaccharides are complex carbohydrates that contain 3 to 10 single sugar units. Two important oligosaccharides are raffinose and stachyose, which can be found in onions, broccoli, cabbage, whole wheat, and legumes (kidney beans, soy beans, etc.). Oligosaccharides can’t be broken down by our digestive enzymes. Thus, when we eat foods with raffinose and stachyose, these oligosaccharides pass undigested into the large intestine, where bacteria metabolize them and produces gas and other by-products. Although many people have no symptoms after eating legumes, other experience unpleasant side-effects.
Polysaccharides are complex carbohydrates that often contain hundreds to thousands of glucose molecules. The polysaccharides include some which are digestible (ex. starch), and some which are indigestible (ex. fiber). The digestibility is mainly determined by whether the glucose units are linked together by alpha or beta bonds.
The major digestible polysaccharide in our diets, starch, is the storage form of glucose in plants. There are 2 types of plant starch: amylose and amylopectin. Both are a source of energy for plants and for animals that eat plants. Amylose and amylipectin are found in potatoes, beans, pasta, rice, and other starchy products, typically in a ratio of 1:4.
Amylose and amilopectine contain many glucose units linked by alpha bonds. The difference between the two is that amylose is a straight-chain molecule, whereas amylopectin is highly branched. Cooking increases the digestibility of these starches by making them more soluble in water and helping the digestive enzymes attack more easily.
Glycogen is the storage form of carbohydrate in humans and animals, and contains many glucose units linked together with alpha bonds. The structure of glycogen is similar to the structure of amylopectin, but it’s even more branched. Liver and muscles cells are the major storage sites for glycogen. The amount stored in these cells is influenced by the amount of carbohydrate in the diet. Although the amount of glycogen that can be stored is limited, glycogen storage is extremely important. The approximately 400 kcal of glycogen stored in the liver can be converted into blood glucose to supply the body with energy, whereas the 1400 kcal of glycogen stored in muscles supply glucose for muscle use, especially during high-intensity exercise.
Fiber refers to the dietary fiber that occurs naturally in foods, as well as the functional fiber that can be added to foods. Fibers are composed primarily of the non-starch polysaccharide cellulose, hemicelulose, pectins, gums, and mucilages. The lignings are the only non-carbohydrate components of dietary fibers. The monosaccharide units in fibers are linked by beta bonds, unlike digestible polysaccharide. Monosaccharide molecules joined by beta bonds are not broken down by human digestive systems. Therefore, these undigested fibers pass through the small intestine into the large intestine, where bacteria metabolize some and form short-chain fatty acids and gas.
Cellulose, hemicelluloses, and lignins form the structural part of the plant cell wall in vegetables and whole grains. Because of their chemical structure, these fibers cannot be dissolved in water, and are often referred to as insoluble fibers. Pectins, gums, mucilages, and some hemicelluloses dissolve easily in water and are classified as soluble fibers.