11.7: Carbohydrates

Digestible Carbohydrates

Common dietary carbohydrates come in several forms. Some of them may be monosaccharides such as glucose and fructose, which are abundant in many packaged foods and fruits. However, a major portion of dietary carbohydrate consists of disaccharides such as sucrose, which is found in table sugar and in most sweet foods, and lactose, which is found in milk and milk products. Sucrose consists of glucose and fructose, and lactose consists of glucose and galactose. The other major dietary carbohydrate molecules are polysaccharides. The most common digestible polysaccharides are starch, which is found in plant foods, and glycogen, which is found in meats. Both are made of glucose molecules.

The specific disaccharides and polysaccharides mentioned thus far are broken down into individual monosaccharides before being absorbed. Once absorbed, galactose and much fructose are converted into glucose by the liver. The remaining fructose is broken down by a process similar to glycolysis.

Indigestible Carbohydrates: Fiber

Many polysaccharides in foods from plants cannot be broken down by digestive enzymes. These indigestible polysaccharides are called fiber. Fiber that does not dissolve in water is called insoluble fiber and includes cellulose, hemicellulose, and some noncarbohydrate material (lignin). Fiber that dissolves in water is called soluble fiber and is abundant in most fruits and vegetables, especially oats and beans.

Uses

Sugars

The body uses glucose and fructose as sources of energy. Some cell types, such as brain cells and red blood cells, rely almost exclusively on glucose for energy, and active muscles and the heart consume large quantities of sugars for energy. Since glucose is an excellent energy source, the liver makes it from fragments of amino acid molecules and lactic acid. Only a small amount of glucose can be made from fat because only the glycerol portion in fat can be converted into glucose.

Some glucose helps supply energy by transferring many fragments from fat and amino acid molecules into the Krebs cycle. Without glucose, these fragments can yield no energy and are converted into ketoacids. Fragments from sugar molecules are also used to manufacture fat, other lipids, parts of amino acids, and sugar molecules in DNA and RNA.

When the blood contains more sugar than the cells require, much of the sugar is stored as glycogen or is converted into fat for storage. The level of blood sugar is controlled by several hormones.

Fiber

Since fiber cannot be digested, the sugar molecules of which it is made cannot be absorbed and used in the body. However, dietary fiber is essential for good health because it stimulates intestinal motility by providing bulk. In this way, it promotes proper intestinal functioning and helps prevent several disorders of the large intestine. Furthermore, soluble fiber reduces the risk of atherosclerosis by decreasing the absorption of cholesterol. It can also help diabetics by slowing the absorption of glucose.

Recommended Dietary Intakes

Ample energy from digestible carbohydrates can be obtained by eating 50 to 100 grams of digestible carbohydrate a day. Individuals who are quite active should consume more digestible carbohydrate. Another guideline is to consume 55 to 60 percent of the total energy intake as digestible carbohydrates. Most of this intake should be in the form of starch or glycogen.

Daily fiber intake can range between 20 and 35 grams per day and probably should be close to the higher value. Individuals with a low fiber intake probably should increase it gradually to allow the GI tract to adjust to the change in diet. Individuals with special problems such as constipation, diverticulosis, atherosclerosis, and diabetes mellitus may benefit from higher fiber intake.

Carbohydrate Deficiencies

Deficiencies in digestible carbohydrates are rare because many foods have large quantities of these nutrients. However, if such a deficiency develops, body cells lack adequate energy. The consequences of a low energy supply include weakness, lethargy, and reduced mental functioning. In addition, fat and amino acid fragments that cannot be channeled into the Krebs cycle are converted into excess ketoacids, which disturb homeostasis by altering acid/base balance and causing excess sodium and potassium loss in the urine.

Inadequate fiber intake is a common problem. Several undesirable results, such as problems with the large intestine, were mentioned in Chapter 10.

Carbohydrate Excesses

Consuming excess digestible carbohydrates is common because foods high in carbohydrates are relatively inexpensive and because sweet foods are pleasant to eat. Eating excess sugars promotes tooth decay and, by suppressing the appetite, reduces the intake of other important nutrients. For example, some elderly people suffer from protein-carbohydrate malnutrition (PCM) because the carbohydrate-rich foods they eat in abundance contain little protein. A high carbohydrate consumption is also often accompanied by undesirable weight gain and hampers maintenance of blood glucose homeostasis.

A high fiber intake can lead to deficiencies in calcium, zinc, and iron because fiber inhibits the absorption of these nutrients. Excess fiber consumption also hampers maintenance of water homeostasis because fiber binds water in the large intestine and therefore reduces water absorption. For this reason, individuals on high-fiber diets should drink large quantities of water.

Certain types of indigestible carbohydrates result in gas production in the large intestine. Therefore, foods such as beans and peas, which contain high amounts of these substances, can cause discomfort and embarrassment.