Classification of Carbohydrates

CLASSIFICATION

Carbohydrates are divided into three groups: monosaccharides, disaccharides, and polysaccharides (Table 4-1).

Monosaccharides

Monosaccharides are the simplest form of carbohydrates. They are sweet,require no digestion, and can be absorbed directly into the bloodstream from the small intestine. They include glucose, fructose, and galactose.

Glucose, also called dextrose, is the form of carbohydrate to which allother forms are converted for eventual metabolism. It is found naturally in corn syrup and some fruits and vegetables. The central nervous system, the red blood cells, and the brain use only glucose as fuel; therefore, a continuous source is needed.

Fructose, also called levulose or fruit sugar, is found with glucose inmany fruits and in honey. It is the sweetest of all the monosaccharides.

Galactose is a product of the digestion of milk. It is not found naturally.

Disaccharides

Disaccharides are pairs of the three sugars just discussed. They are sweet andmust be changed to simple sugars by hydrolysis before they can be absorbed. Disaccharides include sucrose, maltose, and lactose.

Sucrose is composed of glucose and fructose. It is the form of carbohy-drate present in granulated, powdered, and brown sugar and in molasses. It is one of the sweetest and least expensive sugars. Its sources are sugar cane, sugar beets, and the sap from maple trees.

Maltose is a disaccharide that is an intermediary product in the hydro-lysis of starch. It is produced by enzyme action during the digestion of starch in the body. It also is created during the fermentation process that produces alcohol. It can be found in some infant formulas, malt beverage products, and beer. It is considerably less sweet than glucose or sucrose.

Lactose is the sugar found in milk. It is distinct from most other sugarsbecause it is not found in plants. It helps the body absorb calcium. Lactose is less sweet than monosaccharides or other disaccharides.

Many adults are unable to digest lactose and suffer from bloating, ab-dominal cramps, and diarrhea after drinking milk or consuming a milk-based food such as processed cheese. 

This reaction is called lactose intolerance. It is caused by insufficient lactase, the enzyme required for digestion of lactose. There are special low-lactose milk products that can be used instead of regular milk. Lactase-containing products are also available.

During the process of making hard cheese, milk separates into curd (solid part from which hard cheese is made) and whey (liquid part). Lactose becomes part of the whey and not the curd. Therefore, lactose is not a component of natural cheese. However, manufacturers can add milk or milk solids to pro-cessed cheese, so it is important that persons who are lactose intolerant check the labels on cheese products.

There is no test for lactose intolerance. If eating dairy products consis-tently produces symptoms of flatulence, diarrhea, and abdominal pain, the doctor may recommend eliminating dairy products from the diet and adding them back after a period of time to ascertain the client’s reaction. If the symp-toms persist, the client is lactose intolerant.

Polysaccharides

Polysaccharides are commonly calledcomplex carbohydratesbecause they arecompounds of many monosaccharides (simple sugars). Three polysaccharides are important in nutrition: starch, glycogen, and fiber.

Starch is a polysaccharide found in grains and vegetables. It is the stor-age form of glucose in plants. Vegetables contain less starch than grains be-cause vegetables have a higher moisture content. Legumes (dried beans and peas) are another important source of starch as well as of dietary fiber and protein. Starches are more complex than monosaccharides or disaccharides, and it takes the body longer to digest them. Thus, they supply energy over a longer period of time. The starch in grain is found mainly in the endosperm (center part of the grain). This is the part from which white flour is made. The tough outer covering of grain kernels is called the bran (Figure 4-3). The bran is used in coarse cereals and whole wheat flour. The germ is the small-est part of the grain and is a rich source of B vitamins, vitamin E, minerals, and protein. Wheat germ is included in products made of whole wheat. It also can be purchased and used in baked products or as an addition to breakfast cereals.

Before the starch in grain can be used for food, the bran must be broken down. The heat and moisture of cooking break this outer covering, making the food more flavorful and more easily digested. Although bran itself is indi-gestible, it is important that some be included in the diet because of the fiber it provides.

Glycogen is sometimes calledanimal starchbecause it is the storageform of glucose in the body. In the healthy adult, approximately one-half day’s supply of energy is stored as glycogen in the liver and muscles. The hormone glucagon helps the liver convert glycogen to glucose as needed for energy.

The Fibers.Dietary fiber,also called roughage, is indigestible becauseit cannot be broken down by digestive enzymes. Some fiber is insoluble (it does not readily dissolve in water), and some is soluble (it partially dissolves in water) (Figure 4-4). Insoluble fibers include cellulose, some hemicellu-lose, and lignins. Soluble fibers are gums, pectins, some hemicellulose, and mucilages. 

See Table 4-2 for food sources. Cellulose is a primary source of di-etary fiber. It is found in the skins of fruits, the leaves and stems of vegetables, and legumes. Highly processed foods such as white bread, pasta (other than whole wheat), and pastries contain little if any cellulose because it is removed during processing. Because humans cannot digest cellulose, it has no energy value. It is useful because it provides bulk for the stool.

Hemicellulose is found mainly in whole-grain cereal. Some hemicel-lulose is soluble; some is not. Lignins are the woody part of vegetables such as carrots and asparagus or the small seeds of strawberries; they are not a carbohydrate.

Pectin, some hemicellulose, gums, and mucilage are soluble in waterand form a gel that helps provide bulk for the intestines. They are useful also be-cause they bind cholesterol, thus reducing the amount the blood can absorb.

Fiber is considered helpful to clients with diabetes mellitus because it lowers blood glucose levels. It may prevent some colon cancers by moving waste materials through the colon faster than would normally be the case, thereby reducing the colon’s exposure time to potential carcinogens. Fiber helps prevent constipation, hemorrhoids, and diverticular disease by softening and increasing the size of the stool.

The optimal recommendation for fiber intake is 20 to 35 g/day. The normal U.S. diet is thought to contain approximately 11 grams. In general, Americans do not consume sufficient amounts of fruits and vegetables. They should eat no fewer than five servings of fruits and vegetables each day. Fi-ber intake should be increased gradually and should be accompanied by an increased intake of water. Eating too much fiber in a short time can produce discomfort, flatulence (abdominal gas), and diarrhea. It also could obstruct the GI tract if intake exceeds 50 grams. Insoluble fiber has binders (phytic acid or phytate), which are found in the outer covering of grains and vegetables. These can prevent the absorption of minerals such as calcium, iron, zinc, and magnesium, so excess intake should be avoided. The type of fiber consumed should be from natural food sources rather than from commercially prepared fiber products because the foods contain vitamins, minerals, and phytochemi-cals as well as fiber. Table 4-3 lists the dietary fiber content of selected foods.