The grapefruit is considered to be one of the best diet fruits and metabolism foods available.  Not only is it full of essential vitamins and minerals, but it is also a great source of fiber, which the body needs more energy to digest. That increases the natural metabolic rate to provide the extra energy to digest the fiber of the grapefruit. In addition to these brilliant benefits the grapefruit also block insulin production to a certain degree. Lower insulin levels mean that the body is forced to metabolise sugar rather than store it, which helps keep the weight off your body.