It takes too long for this type of fat to get to the muscles. Instead, the muscles get their fuel from storage tanks located in the muscle fibers themselves. The fat jiggling around on our hips and thighs does little good in a marathon. The fuel tank for the marathon is in the muscles themselves. Running out of Matches In order to ignite the lighter fluid in a barbecue you need a match.
Carbohydrates provide the match that ignites the body's fats.
The body burns a mix of fat and carbohydrates, and even if the muscles contain enough fat to fuel the run, without carbohydrates the body has a very hard time making use of this fat. Running fast, the body uses up many more matches to light smaller and smaller puddles of lighter fuel, and the body uses up its carbohydrates quickly. At a marathoner's speed, the supply of carbohydrates stored in muscles begin to run out after about three hours of running. And once this happens it's very hard for the runner to keep up his or her pace. With the spark plugs extinguished the engine can't burn the gas.