"If I were to grab you by the throat, and choke off your air supply, it would immediately become apparent to you that oxygen is absolutely essential for life. If I were to lock you in a room with no water, after several hours, the degree of thirst you would experience would indicate to you that water is a requirement for life. If I were to lock you in that room with water, but no food, it would take a little longer, a matter of a couple of days, before you would be ravenously hungry, and there would be no question in your mind that food was absolutely essential for life. However, it often takes years before ones body begins to show the harm done by a lack of proper exercise."