Since ancient times, Greek and Roman athletes prepared for competition by consuming complex carbohydrates like grains and legumes with the goal of optimizing performance. However, the carb-loading technique wasn’t developed until the 1960s when Swedish physiologist Gunvar Ahlborg discovered a positive relationship between the amount of glycogen stored in the body and athletic performance in endurance sports.

Over the years, the carb-loading strategy has evolved, but does carb-loading actually impact performance? The short answer is yes, but not for every sport, and not as a long-term strategy.

The Science of Carbs

The body’s primary energy source comes from breaking down carbohydrates, which are the sugars, starches, and fibers found in food, and turning them into sugar that is stored in the muscles as glycogen. When the body’s glycogen is depleted, it hits a wall and must work harder to turn fat (its secondary food source) into energy. Without its primary fuel source, muscles slow down.

Who Should Carb-load?

Typically, it takes 90 to 120 minutes to run out of glycogen, so that’s why carb loading is only effective for marathoners, triathletes, and others who participate in endurance events that require non-stop, moderate to high-intensity levels of exertion that last more than 90 minutes. In fact, it can be potentially harmful for shorter activities, potentially causing digestive issues, water retention, blood sugar spiking, and placing added stress on the body. Additionally, carb-loading isn’t necessary if you can replenish your body’s glycogen by eating carbs during breaks in the competition.

How to Correctly Carb-load

Even if you are participating in an endurance event, it’s important to carb-load correctly. The traditional practice of enjoying a few full plates of pasta the night before a race will not really improve performance. In fact, many experienced marathoners avoid eating a big meal the night before a race. One large meal will not provide all of the glycogen you need, so carb-loading should begin three to six days before an endurance event. In addition, it’s critical to eat the right type of carbs, including low-glycemic foods like sweet potatoes, oatmeal, whole-wheat pasta, and quinoa, and avoid refined, white carbs and sweets. It is generally recommended that 70 percent of your calories come from carbs (as opposed to the usual 50 percent) when carb loading.

Maintain a Balanced Diet

While carb-loading can improve performance for endurance events, prolonged periods of excessive carb consumption can result in obesity. So, to maintain a healthy body composition, focus on a balanced diet of carbohydrates plus sufficient protein and fiber intake.  

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Authored by Zach Meeker, Research Assistant for Dr. Brian Cole at Midwest Orthopaedics at Rush University Medical Center

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