Running on an empty stomach may not burn more calories, but exercising when you are hungry can burn more fat. Your muscles burn carbohydrates for energy, but when a sufficient quantity of carbohydrates is not immediately available, your body turns to stored energy, or fat, to fuel your workout.
Because running burns a high amount of calories and can be done almost anywhere without any exercise equipment other than a good pair of shoes, this aerobic activity offers a number of health benefits while being relatively safe. A 200-pound person can burn between 760 and more than 2,000 calories per hour while running, depending on his speed. The number of calories burned depends on your current weight, how fast and how long you run. While your body may have to dip into reserved stores for energy, the amount of calories burned remains unchanged.
Burning Calories When Hungry
One pound of body fat contains 3500 calories and one pound of muscle mass contains 2500 calories. So, on average one pound of body weight falls somewhere between the two. To lose a pound of weight, you must expend or cut between 2500 and 3500 calories. All forms of exercise require calories to fuel your muscles to move your body weight through space. The amount of exercise you get accounts for approximately 15 to 30 percent of the amount of calories you burn each day. Your basal metabolic rate, the amount of calories you require to fuel the basic functions of your body, such as breathing, digestion and circulation, accounts for about 60 to 75 percent of the amount you burn. Digestion consumes another 10 percent. Exercising when you are hungry simply accesses different forms of energy in your body.
Many people concerned about burning calories really want to burn more fat. If you burn more calories than you eat, your body dips into its fat stores for energy and you begin to lose weight. A 2007 study conducted at the Research Center for Exercise and Health in Leuven, Belgium, found that people who exercised before eating burned more fat than those who exercised after eating.
As you exercise, your body burns sugar from your blood stream, muscles and liver. When you are hungry, you have less sugar available in your blood and liver. When blood and liver sugar stores are depleted and you've tapped out your muscle glycogen stores your body turns to your fat cells for fuel.
Although you can burn more fat exercising when you are hungry, overeating after exercise simply replaces the calories you burned. If you burn fat during exercise but don’t expend the calories you consumed the rest of the day, your body will store the excess as fat. Exercising while fasting may increase your risk of injury. Talk to your doctor before beginning an exercise program, especially if you plan to exercise on an empty stomach.
"Running on empty" may affect your performance. According to a 1999 study published in the Journal of Medicine and Science in Sports and Exercise, people who ate a meal three hours before they exercised were able to bike for approximately 30 minutes longer than when they did not eat beforehand. These results may correspond to your ability to run longer distances after eating a meal.