ACSM'S Health & Fitness Journal:
Departments: Fitness Focus Copy-and-Share
Thompson, Dixie L. Ph.D., FACSM
Dixie L. Thompson, Ph.D., FACSM, is the director of the Center for Physical Activity and Health and professor in the Department of Exercise, Sport, and Leisure Studies at the University of Tennessee, Knoxville.
Carbohydrates have been the center of attention for many recent diet plans. Although some dietary programs encourage a diet high in carbohydrates, others point to carbohydrates as the culprit in the rising rates of obesity and type 2 diabetes. What is the real answer?
Symbol What are Carbohydrates?
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Carbohydrates are essential nutrients critical for supplying the body's need for energy. Many types of carbohydrates exist, from simple sugars found in fruit juices to complex carbohydrates found in whole-grain bread. The body converts carbohydrates to glucose to use for fuel. When consumption of carbohydrates is higher than the immediate need for energy, they are stored as glycogen. The liver and muscles store glycogen, and the body depends on the liver reserves during periods of fasting between meals.
When carbohydrates are eaten, the blood glucose (i.e., blood sugar) goes up. This normal response allows glucose to be carried to tissues needing energy or to storage sites. The glycemic index describes the rate at which blood sugar goes up after a food is eaten. Foods such as white bread cause a rapid spike in blood glucose (high glycemic index), whereas foods such as kidney beans that have a combination of proteins and carbohydrates cause a less rapid rise in blood sugar (lower glycemic index). Debate rages about the value of using glycemic index in dietary planning, but clearly, a diet high in nutrient density and complex carbohydrates promotes good health.
Symbol What is the Recommended Level?
For good health, it is recommended that 45% to 65% of a person's daily calories come from carbohydrates. This range allows a person to fit his or her dietary preferences while still ensuring adequate nutrition. The choice of which carbohydrates to eat is critical. Carbohydrates high in nutritional content, such as fruits, vegetables, and whole grains, should make up the majority of carbohydrates consumed. Foods low in nutritional value and high in simple sugars (candy, sodas, etc.) should be consumed in moderation to limit excess weight gain.
Symbol What is the Role in Athletic Performance?
Carbohydrates provide a rapid source of energy for muscular contraction. Athletes who participate in high-intensity sports must consume a diet high in carbohydrates to meet the body's energy demands. A diet consisting of 60% to 70% carbohydrates is not uncommon for an athlete. In general, elite, high-intensity athletes should consume 6 to 10 g of carbohydrate for each kilogram of body weight. Athletes also may need to engage in carbohydrate loading (i.e., tapering exercise and eating higher than normal levels of carbohydrates) to fuel performances such as marathon running.
During events lasting an hour or more, it is wise to consume carbohydrates while exercising. Glucose-containing beverages and easily digested foods are most effective for keeping the body's glucose levels up. More information on eating to enhance athletic performance can be found in ACSM's Position Stand on eating for athletic performance (www.acsm.org).
To summarize, diets should be balanced and varied and contain carbohydrates high in nutritional value. Fruits, vegetables, and whole grains should be eaten on a daily basis. To learn more about planning a healthy diet to meet your nutritional needs, see the U.S. Department of Agriculture's My Pyramid Web site (www.mypyramid.gov).