As a runner, your aerobic capacity, also referred to as "VO2max," can limit your performance. Aerobic capacity is a measurement of how much oxygen your body is able to deliver to your muscles. Your performance is also affected by the amount of glycogen stored in your liver and muscles. Carbohydrates are a good fuel source when training for a race, as they supply glycogen and require less oxygen than fat does in order to be metabolized and used as an energy source, according to Professor Jennifer Anderson, a food and nutrition specialist at Colorado State University Extension.
Exercise Depletes Stored Carbohydrates
When you first begin a session of moderate exercise, your body obtains up to 50 percent of its energy by burning stored carbohydrates. Your body breaks down the carbohydrates that you eat and stores them in your muscles as glycogen. When you exercise, this glycogen converts to glucose, which your muscles burn for fuel. Your ability to continue exercising is linked to the amount of glycogen stored in your muscles. This supply is depleted after about 90 minutes of running, when your body switches to burning fat, a far less energy efficient source. Fat is burned much more slowly than carbohydrates. When your body switches to fat burning, you slow down.
Why Carbohydrates?
You need plenty of oxygen in order to run, which is why your rate of respiration increases as you exercise. Carbohydrates are desirable as an energy source due to their efficient use of oxygen. They use less oxygen for every calorie of energy produced than fats or proteins do, making them the endurance athlete's optimal choice as an energy source.
Benefits of Carb Loading
If you're preparing for a race that is longer than 90 minutes in duration, you may benefit from eating a high-carbohydrate diet for up to three days prior to the event. This type of diet can help fill up the glycogen storage spaces in your muscles and liver and ensure that you're carrying the maximum capacity of stored glycogen that your body can hold.
How to Carb Load
If you want to load up your glycogen stores before a big race, you should start eating a diet that derives 70 percent of its calories from carbohydrates, two to three days before your race. Resist the urge to start earlier than that, as carb loading for a longer period may increase water retention and make you stiff and sluggish during the race. Professor Anderson suggests a breakfast of oatmeal with whole-wheat toast, jelly and a banana washed down with orange juice and a glass of milk. A typical high-carb lunch would include a ham and Swiss cheese sandwich on whole wheat with lettuce and tomato, two cookies, some apple juice and milk. A high-carb dinner could include spaghetti with tomato sauce, mushrooms and Parmesan cheese, French bread and angel food cake with strawberries and ice cream for desert.
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