No one is sure exactly how the grapefruit diet helps people lose weight, but according to one study, it can work. People who ate half a grapefruit with each meal lost an average of more than three pounds. Some of the dieters lost up to ten pounds. However, the people in the study followed the diet for twelve weeks, not twelve days. The study also stated that the dieters had increased their physical activity as well as eating more grapefruit.