Table of Contents
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
(Illustration by GGS Information Services/Thomson Gale)
polysaccharides. Simple carbohydrates are known as sugars and they are classified as either monosacchar-ides (one sugar molecule) or disaccharides (two sugar molecules).
In the digestive tract, carbohydrates are broken down into glucose, which provides energy for the body’s cells and tissues. Glucose is the body’s primary source of fuel.
When food is eaten, 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 (sugar) 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 pol-ysaccharide 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 lipo-protein (LDL) cholesterol, also called the “bad” 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 United States Department of Agriculture (USDA) Dietary Guidelines were designed by health professionals to help consumers make nutritious food choices. The guidelines, released in 2005, replace the food pyramid that the USDA used for many years. Instead of recommending a certain number of servings per food group, as the food pyramid did, the new guidelines advise consumers to eat a diet that emphasizes fruits, vegetables, whole grains, and fat-free or low-fat milk and milk products; includes lean meats, poultry, fish, beans, eggs, and nuts; and is low in saturated fats, trans fats, cholesterol, salt, and added sugars. The guidelines recommend that 45–65% of total calories come from carbohydrates and that foods containing complex carbohydrates (such as whole-grains) are preferred over simple carbohydrates (such as table sugar and white flour.) As an example, one cup of whole-grain brown rice has more nutritional value and fiber that processed white rice.
There are no known adverse dietary interactions associated with carbohydrates.
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, including endocrinologists (specialists that tread diseases of the endocrine (glands) system, including diabetes) 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.
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. There is some disagreement in the medical community about the type of diet diabetics, especially type 1 diabetics, should be on. The conventional diet is one of low-fat, high-carbohydrate food, which is recommended by the American Diabetes Association. Some doctors, particularly endocrinologists, recommend the Bernstein diet, which is low in carbohydrates and high in fat, to maintain constant, normal blood sugar levels throughout the day.
Carbohydrate intolerance is the inability of the small intestine to completely process the nutrient carbohydrate (a classification that includes sugars and starches) into a source of energy for the body. This is usually due to deficiency of an enzyme needed for digestion. Lactose intolerance is the inability to digest the sugar found in milk.
Parents should consult their child’s pediatrician, physician, or endocrinologist if they are unsure the child’s diet has a nutritional balance of carbohydrates. A doctor also should be consulted before a child or adolescent goes on a low-carbohydrate diet (such as the Atkins, Zone, and Sugar Busters diets) for weight loss.
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American Academy of Family Physicians. 11400 Tomahawk Creek Parkway, Leawood, KS 66207. Telephone: (800) 274-2237. Website: http://www.aafp.org
American College of Nutrition. 300 South Duncan Ave., Suite 225, Clearwater, FL 33755. Telephone: (727) 446-6086. Website: http://www.amcollnutr.org
American Diabetes Association. 1701 N. Beauregard St., Alexandria, VA 22311. Telephone: (800) 342-2383. Website: http://www.diabetes.org
American Dietetic Association. 120 South Riverside Plaza, Suite 2000, Chicago, IL 60606-6995. Telephone: (800) 877-1600. Website: http://www.eatright.org
American Society for Nutrition. 9650 Rockville Pike, Bethesda, MD 20814. Telephone: (301) 634-7050. Website: http://www.nutrition.org
United States Department of Agriculture; Food, Nutrition, and Consumer Services. 3101 Park Center Drive, Alexandria, VA 22302. Telephone: (703) 305-2281. Website: http://www.fns.usda.gov
Ken R. Wells