A friend of mine who was into sports physiology and crap like that used to tell me there are 3500 calories in a pound of fat; during exercise, of moderate to intense effort, the body will burn approximately 500 calories per hour. He used to claim that a person who walks vigorously will burn just as many calories as the guy on a bike or a runner if they each do it for the same length of time. The biggest fluctuation in body weight is caused by the loss or retention of water.