Growth hormone increases as a result of exercise. This is thought to influence protein synthesis and fat metabolism. A study from the U.S. Army Research Institute of Environmental Medicine led by Bradley Nindl showed that growth hormone increases were greater during a two-hour aerobic workout than during a one-hour aerobic workout, one-hour weight workout or two-hour weight workout. The results might help explain the amount of exercise necessary for significant weight loss. Most studies show that people must run at least 20 miles per week to trigger significant weight loss through exercise alone. This finding might have something to do with the amount of exercise necessary to increase growth hormone substantially.
Source: Medicine Science Sports Exercise, 46: 1917-1927, 2014