The more intense your running, the more heavily your body relies on glycogen for fuel.
If you're familiar with distance running, you've probably heard terms such as "bonking," hitting the wall" and "carbo-loading." Each of these relates to glycogen, your body's storage form of carbohydrate and ultimately the chief source of energy in endurance sports such as running, cycling and cross-country skiing. How running affects your muscle glycogen levels depends both on your diet and your training.
Glycogen is a polymer -- a chain of many identical glucose molecules linked together by chemical bonds. It is stored in your liver and muscles, and along with fat is burned for fuel during exercise. According to exercise physiologist and former Olympic marathon runner Pete Pfitzinger, M.S., the fitter you are, the more glycogen you can store, and more intense running burns a higher proportion of glycogen and less fat than does easier running. When you run low on glycogen, you're left with fat as fuel, and your performance suffers because your body doesn't burn fat as efficiently as it does glycogen.
Your body can normally store about 15 grams of glycogen per kilogram of body weight. (Ref. 3) For a 70-kilogram person, this amounts to a little over 1,000 grams, about half of which is stored in liver. (Ref. 2) With prolonged training and by modifying your food intake to increase the proportion and total amount of carbohydrates your diet contains -- that is, carbo-loading -- you can increase this by about 500 grams. While there is some controversy in the running world about the effectiveness of intentionally depleting your body's glycogen stores through exercise and a few days of carbohydrate reduction in order to increase the effectiveness of the subsequent loading phase, it remains a common practice and has been borne out by experimentation.
Exercise Intensity and Glycogen Use
Unlike races lasting under an hour, the 26.2-mile marathon and longer competitions have glycogen depletion as a chief limiting factor in performance. According to Pfitzinger, you use up your glycogen stores after about 90 minutes of running, well short of the two-plus hours it takes even the most accomplished athletes to complete a marathon. Since fatty acids are only about 85 percent as efficient a fuel source as glucose during all-out marathon running -- which, although not run at nearly the same intensity as a 5K or 10K, still requires you to run at about 79 to 88 percent of maximum heart rate, where the body favors glucose over fats -- you can stave off glycogen depletion by taking in a carbohydrate drink, gel or other food along the way.
You may not feel like eating in the immediate math of a marathon or a long, hard training run. As Pfitzinger points out, though, the sooner after exercising you take in carbohydrates, the more readily those carbs are stored as glucose in your muscles. So if you can literally stomach it, eat and drink up as soon as you come to a stop. Also, since you need 2.6 grams of carbohydrate to store one gram of glycogen, you should not be alarmed if you gain several pounds before a marathon or other competition; in fact, it's a reliable sign of carbo-loading well done.