Naturally packed with water and fiber, citrus will help you stay full and satisfied, but grapefruit may have a decided advantage, according to a 2006 Journal of Medicinal Food study. When researchers put volunteers on an exercise plan for 12 weeks and asked them to eat either half a fresh grapefruit or drink apple juice and pop a placebo pill before each meal, the grapefruit group dropped an average of 3½ pounds (compared to just ½ pound for the apple group).