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Fitness Focus Copy-and-Share: Carbohydrate Loading

Thompson, Dixie L. Ph.D., FACSM

ACSM's Health & Fitness Journal: March-April 2008 - Volume 12 - Issue 2 - p 5
doi: 10.1249/01.FIT.0000312408.09637.f9
Departments: Fitness Focus Copy-and-Share

This copy-and-share column provides information about carbohydrate loading.

Dixie L. Thompson, Ph.D., FACSM, is the director of the Center for Physical Activity and Health and a professor in the Department of Exercise, Sport, and Leisure Studies at the University of Tennessee, Knoxville.

Carbohydrate loading is a common practice used by athletes to enhance performance during endurance events. In the following paragraphs, you will learn why carbohydrates are an important energy source during exercise and the general techniques used in carbohydrate loading.

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The body can break down carbohydrates, fats, and proteins for energy during exercise. Although there is a great deal of potential energy stored in the body as fat, the rate at which fats can be broken down is inadequate to fuel intense exercise. Likewise, there is a lot of protein in the body, but the breakdown of proteins for energy is rather inefficient and supplies only a minor fraction of energy needs during exercise. On the other hand, carbohydrates are the primary fuel needed during strenuous exercise because these molecules can be broken down quickly for energy. Additionally, the brain is reliant on carbohydrates for its energy needs during rest and exercise, so a steady supply is needed for normal central nervous system function. Although some of the energy stored in carbohydrates can be harnessed without the need for oxygen (i.e., anaerobic), most energy stored in carbohydrates can be released only when oxygen is available. It also is important to remember that the fast-twitch muscle fibers recruited during strenuous exercise are highly dependent on carbohydrates for energy because their ability to use fat for energy is limited.

Carbohydrates circulate in the blood as glucose and are stored in muscles and the liver as glycogen. During exercise, glycogen breakdown is triggered by hormones such as adrenaline (or epinephrine). Muscle glycogen is used as a rapid source of energy within the contracting muscles. To help meet energy needs during exercise, the liver releases glucose into the blood, which travels to the active muscle where it is taken up and used. Sustained exercise will gradually deplete glycogen levels in muscle and liver.

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Classic studies in exercise physiology demonstrated that one of the factors important in sustained endurance exercise performance is the amount of stored glycogen. Individuals who begin an exercise event with low levels of stored glycogen will be unable to sustain high workloads and will fatigue faster. It was this recognition that led to the practice of carbohydrate loading.

Carbohydrate loading is most important in events that require sustained activity for longer than an hour. Examples include marathons and most triathlons. To maximize glycogen storage before competition, individuals should manipulate two variables: diet and precompetition training. It is generally recommended that in the 3 to 4 days leading up to an event, an athlete should consume 65% to 70% of their calories from carbohydrates. These should be mainly complex carbohydrates (e.g., pasta) and not merely simple refined sugars (e.g., soft drinks). At the same time, the athlete should taper workouts during the week before competition. This tapering should include decreasing the length and the intensity of workouts, particularly on the day before the event. This type of diet and training strategy will allow the body to maximize its stored glycogen.

Carbohydrate loading can aid competitive athletes looking for performance enhancement. Specific questions regarding dietary manipulation for performance should be directed to registered dietitians with training and experience in helping athletes.

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