Running after several hours of fasting does not mean you have no fuel. Muscle glycogen stores and adipose tissue both provide abundant fuel for energy. By waiting until after your run to eat, you will tap into your fat stores sooner and burn more fat in the course of your run. But before heading out the door with your tank on empty, consider your health and training status.
During endurance exercise like running, your muscles burn carbohydrates, circulating as glucose in your blood stream and stored in the form of muscle glycogen, and lipids, or fats, stored in adipose tissue and within your muscles. Because your blood glucose and glycogen stores are limited, as they become depleted you rely increasingly on fat to fuel your exercise session. After several hours of fasting, your blood glucose is low and you will deplete your muscle glycogen more quickly, causing elevated fat metabolism earlier in your workout.
Fasting and Fat Metabolism
When you consistently exercise in a fasting state, your body undergoes changes that enhance your utilization of fat. A 2012 study published in the "International Journal of Sports Nutrition and Exercise Metabolism" compared the effects of endurance training on fasting and fed subjects during 29 days of Ramadan. They found that the fasting subjects lost both body weight and body fat, while the fed subjects lost only body weight. A 2011 study published in the "Journal of Applied Physiology" found that consistent endurance exercise performed in the fasting state resulted in changes in muscle fibers that enhanced glycogen sparing and increased fat metabolism.
Training Status and Fat Burning
Your training status will influence how much fat you burn on an empty stomach. Trained endurance athletes are more efficient at burning fat because the mechanisms involved in aerobic metabolism are more developed, and because training makes you more proficient at sparing glycogen. A 2008 study published in the "Proceedings of the National Academy of Sciences" compared trained endurance athletes to sedentary subjects and found that the trained subjects were more proficient at recruiting fat for energy. They also found that the trained group had greater insulin sensitivity, meaning that glycogen was more readily replenished after exercise.
A Word of Caution
If you are currently sedentary or if you are diabetic, running on an empty stomach is not advisable because you may become hypoglycemic. Hypoglycemia, or low blood sugar, can lead to dizziness, nausea and unconsciousness in some cases. Beginning exercisers and diabetics should consume a light meal or snack before exercise. As you become more fit, the adaptations from endurance training will enhance your glycogen stores and your capacity to metabolize fat during exercise. After several weeks of consistent training, try running on an empty stomach, but monitor your body's response closely for negative reactions.