When exercising, your body uses two main sources of fuel -- carbohydrates and fats. Carbs come from the carbs you eat, which travel around your blood as glucose, as well as carbs stored in your liver and muscles, known as glycogen. Fat comes from free fatty acids and triglycerides circulating in your bloodstream as well as from stored fat. How you eat and how you train can have a big impact on whether you're burning fat or glycogen when training.
Going for Glycogen
Glycogen is your body's preferred energy source for exercise, as it's more readily available, writes Dr. John Berardi in "The Essentials of Sport and Exercise Nutrition." This means that glycogen is a better source of energy when training for athletic performance. According to the University of Massachusetts, decreased levels of glycogen correspond with an increased perception of fatigue; runners consuming 40 percent of their calories from carbs can't fully replenish their glycogen stores every day, whereas athletes with a 70 percent carb-based diet can.
Fat for Fuel
As a high-carb diet increases stores of glycogen, a low-carb diet will decrease them. This starts to shift your body from using carbs as a fuel to burning more fats. The process of breaking down non-carbohydrate substances, such as fats, to use as fuel is called gluconeogenesis. A study published in a 2009 edition of the "American Journal of Clinical Nutrition" found an increase in gluconeogenesis in participants eating a high-fat, high-protein, low-carb diet.
Best of Both
While changing nutritional strategies can lead to a switch in your main fuel source, you will always burn both carbs and fat during exercise. One of the main factors, aside from diet, that influences the ratio of fat burning to carb burning is exercise intensity. At low intensities your body will turn to fat for energy, but as you start to train harder, it will switch to burning more glycogen, notes Dr. Edward Coyle of the Gatorade Sports Science Institute.
Not Worth the Worry
Whether you burn glycogen or fat doesn't matter too much from a fat-loss standpoint, writes Berardi in "The Metabolism Advantage." The main consideration is to burn calories -- it doesn't make much difference whether these are from glycogen or fat. When training for athletic performance, however, you should ideally be looking to use glycogen as your main energy source. Carbohydrate storage is the limiting factor in athletic performance, according to Iowa State University, as when it runs out, you no longer burn glycogen, which can lead to a decrease in performance.
For athletes, or anyone looking to maximize performance, glycogen is the better fuel for training. To ensure you're using more glycogen, consume plenty of carbs in your diet. Between 55 percent and 65 percent of an athlete's calories should come from carbs, notes Bill Campbell and Marie Spano in the "NSCA's Guide to Sport and Exercise Nutrition." A high-carb snack before training can also help you burn more glycogen.