The phrase “hitting the wall” during a race refers to the point in which you’ve used up all your glycogen stores and have started to burn fat. At this time, fatigue sets in, and the end of your competition is in sight whether or not you want it to be. The time it takes you to use up your glycogen stores is dependent upon how physically fit you are and how much energy you have stored. Modifications in your diet to consume more carbohydrates can increase your glycogen stores, enabling you to exercise longer.
Fuel for Aerobic Exercise
An aerobic workout consists of exercise performed at a moderate level of intensity with an increased heart rate for an extended period of time. This is opposed to anaerobic exercises, which are performed at a vigorous intensity for quick bursts, such as sprinting and weightlifting. During aerobic exercise, you need oxygen to get energy from stored carbohydrates. Once your carbohydrates, or glycogen stores, are gone, your body relies on fat. Fat metabolism is a less efficient process, so your performance declines and fatigue takes over.
Your body breaks down the carbohydrates you eat into glucose, which gives you the fuel you need for whatever your day has in store. The glucose you don’t use is converted to glycogen and stored in your muscles and liver. Your body utilizes liver glycogen to keep your blood sugar levels up and muscle glycogen during high-intensity workouts. An untrained person who takes in 45 percent of her calories from carbohydrates is capable of storing 100 grams of glycogen in the liver and 280 grams of glycogen in muscles.
Duration of Glycogen Stores
During an aerobic workout, you depend on liver glycogen to keep your blood sugar levels steady. At a moderately steady pace of exercise, you metabolize blood glucose at 1 gram per minute or 60 grams per hour according to exercise physiologist David Peterson. With full liver glycogen stores, a less fit athlete will run out of glycogen after one hour and 45 minutes of exercise. Once liver glycogen is gone, your blood sugar levels drop and your workout or event is negatively affected.
The better conditioned you are and the more carbohydrates you eat, the more efficient your muscles are at storing glycogen. With more glycogen stores, you can exercise longer. If you haven’t been in training but raise your carbohydrate intake to 75 percent of total calories, you can increase your liver and muscle glycogen stores to 490 grams total. If you are well-trained and start a 75 percent carbohydrate diet, you can store up to 880 grams of total glycogen. According to registered dietician Sara Haas, carb loading can start anywhere from one to seven days before a race. She recommends eating 3 ½ to 5 ½ grams of carbohydrates per pound of body weight.