Carbohydrates are compounds that consist of the three elements—carbon, hydrogen, and oxygen—linked together by energy-containing bonds. There are two types of carbohydrates: complex and simple. Complex carbohydrates, such as starch and fiber, are classified as polysaccharides. Simple carbohydrates are known as sugars and they are classified as either monosaccharides (one sugar molecule) or disaccharides (two sugar molecules linked together).
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 there is an excess of glucose, the muscle and liver cells often convert it into 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 (blood 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 type of complex carbohydrate, fiber, is a polysaccharide with bonds holding its component molecules together that cannot be digested by humans. Nevertheless, fiber is an important part of the diet. 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 form healthy stool and increase transit time of waste products through the large intestine, thus helping to keep the bowel healthy and reduce risk of disease. Soluble fiber helps to lower blood cholesterol levels through its ability to bind bile acids, which reduces their reabsorption and decreases the low-density lipoprotein (LDL) cholesterol, also called the “bad” cholesterol. Sources of insoluble fiber include wheat bran, whole grains, and brown rice. Soluble fiber can be found in barley, fruit, legumes, and oats.
Fiber also aids in weight control by displacing calorie-dense fats in the diet. Fiber absorbs water and slows the movement of food from the stomach into the small intestine, promoting a feeling of fullness. Recommended intakes of fiber should be about 25–38 grams daily. The United States Department of Agriculture (USDA) Dietary Guidelines were designed by health professionals to help consumers make nutritious food choices. The guidelines, the newest version of which was released in 2011, 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 at least half of all carbohydrates eaten are whole grains instead of heavily processed grains. For example, one cup of whole-grain brown rice has more nutritional value and fiber than processed white rice.
When carbohydrate intake is low, there is insufficient glucose production, which then causes the body to break down protein for energy. This ultimately prevents the body's protein from performing more important functions, such as maintaining the body's immune system. Without carbohydrates, 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 lead to a state of ketoacidosis that is fatal if uncorrected.
Diabetes is a disease in which the body cannot metabolize carbohydrates and either does not make or does not 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 increase 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 are 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 as the disorder progresses. There is some disagreement in the medical community about the type of diet that the diabetic community, especially type 1 diabetics, should follow. 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.
The recommended daily intake of carbohydrates is generally considered sufficient for individuals who engage in moderate-level recreational athletic and fitness activities. Individuals who engage in endurance events (longer than 90 minutes) requiring vigorous activity or who engage in sports and fitness professionally are likely to require higher amounts of carbohydrates.
Some research has shown that athletes who eat a diet high in complex carbohydrates before vigorous athletic training can continue to workout for longer than athletes who did not eat such a diet. Some experts recommend that for 2 to 3 days before a major athletic event requiring vigorous exercise for 90 or more minutes the athlete eat a diet that is approximately 70% complex carbohydrates. This is commonly called carb loading. Athletes should work with a sports nutritionist to determine the optimal content of their diet giving their specific training and performance needs.
See also Diabetes mellitus ; Fat ; Protein .
Schlenker, Eleanor D., and Sara Long. Williams' Essentials of Nutrition and Diet Therapy. 11th ed. St. Louis: Elsevier/Mosby, 2014.
Smolin, Lori A., and Mary B. Grosvenor. Basic Nutrition. New York: Chelsea House Publishers, 2004.
Warshaw, Hope S., and Karmeem Kulkarni. Complete Guide to Carb Counting: How to Take the Mystery Out of Carb Counting and Improve Your Blood Glucose Control, 3rd ed. Alexandria, VA: American Diabetes Association, 2011.
de alles Painelli, Victor, Humberto Nicastro, and Antonio H. Lancha, Jr. “Carbohydrate Mouth Rinse: Does it Improve Endurance Exercise Performance?” Nutrition Journal 9 (2010): 33.
Jeukendrup, Asker E., and Edward S. Chambers. “Oral Carbohydrate Sensing and Exercise Performance.” Current Opinion in Clinical Nutrition and Metabolic Care 13, no. 4 (July 2010): 447–51.
“Carboyhdrates: How Carbs Fit Into a Healthy Diet.” Mayo Clinic. May 2, 2014. http://www.mayoclinic.org/healthylifestyle/nutrition-and-healthy-eating/in-depth/carbohydrates/art-20045705 (accessed January 15, 2017).
“Understanding Carbohydrates.” American Diabetes Association. http://www.diabetes.org/food-and-fitness/food/what-can-i-eat/understanding-carbohydrates (January 15, 2017).
American Dietetic Association, 120 S Riverside Plaza, Ste. 2190, Chicago, IL, 60606, (312) 899-0040, (800) 877-1600, http://www.eatright.org .
American Society for Nutrition, 9211 Corporate Blvd., Ste. 300, Rockville, MD, 20850, (240) 428-3650, Fax: (240) 404-6797, http://www.nutrition.org .
National Association of Sports Nutrition, 8898 Clairemont Mesa Blvd., Ste. J, San Diego, CA, 92123, (858) 694-0317, http://nasnutrition.com .
Office of Dietary Supplements, National Institutes of Health, 6100 Executive Blvd., Rm. 3B01, MSC 7517, Bethesda, MD, 20892-7517, (301) 435-2920, ods@nih. gov, http://ods.od.nih.gov .
Ken R. Wells
Revised by Tish Davidson, AM