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.
Purpose
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.
Precautions
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.
Description
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.
Fiber
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.
Complications
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.
Insulin
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.
KEY TERMS
Diabetes—A condition characterized by inadequate use of insulin preventing a person from controlling blood sugar levels.
Fructose—Monosaccharide known as fruit sugar.
Galactose—Monosaccharide known as milk sugar.
Glucose—Monosaccharide used for energy; also known as blood sugar.
Marlett, Judith A., and Joanne L. Slavin. "Health Implications of Dietary Fiber—Position of ADA." Journal of the American Dietetic Association 97 (1997):1157-1159. <http://www.eatright.org/adap1097.html>. (April 18,2001).
"Pasta Power! Debunking the Myths about Pasta." Journal of the American Dietetic Association Online 1997. <http://www.eatright.org/nfs/nfs73.html> (April 18,2001).
