Carb loading can be defined as the act of consuming large amounts of carbohydrates to increase glycogen storage in your body to provide extra energy during a physically demanding exercise, such as a marathon.
In the case of a marathon, carb loading is a must during the week of the marathon, particularly in the final three days before race day. Our bodies have the capability of storing large amounts of glycogen which is converted from consumed carbohydrates. By consuming more and exercising less (your taper period), your body is able to store more glycogen.
As you run during the marathon, your body will burn glycogen. When glycogen runs out, your body will transition to burn fat as the primary source of fuel. The object is to store and replenish enough glycogen so that you do not run out before you reach the finish line.
Carb loading is a method that will lessen the likelihood that your tank will run empty. During your training period, carbohydrates should account for 60% to 70% of your total daily calorie consumption (Caloric Needs Calculator). The week of the marathon, carbohydrates should account for 75% to 80% of your overall calorie consumption.
Complex carbohydrates are the most beneficial for your body and include such foods as potatoes, yams, beans, peas, wheat bread, bananas, macaroni, spaghetti, cereal, raisins, apples, bagels, syrup, brown rice, corn, apples, carrots, and root vegetables. Mix a variety of carbs into your diet to create a nice balance.
Caution: The week of the marathon, only consume foods that you have consumed throughout the training period. Introducing a new type of food to your body, particularly within 48 hours of the marathon start, may lead to an upset stomach and require unwanted stops at the portable restrooms during the marathon.