A majority of people gain weight after THR/TKR. There have been 3 studies done on this question. One study and 106 people, and 66% gain weight. The average weight gain was 14lbs. In another study, they took 450 people, 225 gained weight and 140 lost weight and the rest stayed about the same. Even though the patient gains greater mobility, for some reason there is a weight gain. Each study suggested that there should be more research in this area.