During exercise, energy is provided primarily from the breakdown of glycogen and fat, depending on the intensity. Low intensity exercises rely mostly on fat while higher intensity exercises rely more on glycogen. Exercising in the fasted state can increase the reliance on fat as a fuel source resulting in greater rates of fat burn during exercise.
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Exercising Before Breakfast and Fat Metabolism
Exercising after an overnight fast increases the rate of fat metabolism compared with exercising after a meal as shown by a study published in the January 2011 issue of the "Journal of Applied Physiology." This study also showed that consistently training in the fasted state increases fat metabolism at higher intensities. Fasting lowers the levels of insulin, a hormone that normalizes blood glucose levels and inhibits the breakdown of fat. Exercising while insulin levels are low relieves this inhibition and increases the rate of fat breakdown from the body's stores.
Training repeatedly in the fasted state has also been shown to positively impact body composition. A study published in the February 2012 issue of the "International Journal of Sport Nutrition and Exercise Metabolism" showed reductions in body fat percentage in men who trained in the fasted state with no changes in body fat in the group that trained after meals. While both groups lost weight, only the group that exercised in the fasted state reduced their body fat percentage.
Currently, there is no evidence to suggest that exercising before breakfast in the fasted state causes a loss of muscle. In fact, the study above from the "International Journal of Sports Nutrition and Exercise Metabolism" showed that men exercising during Ramadan maintained their muscle mass despite cycling, running, and rowing for 40 to 60 minutes three times per week while fasting. Thus, a workout before breakfast appears to be a great way to maximize fat burn while preserving muscle mass.