the body needs less energy after losing weight on a diet. people have less fatty tissue and often less muscle after the diet. muscle tissue needs more energy than fatty tissue. reducing the amount of muscle tissue therefore reduces the need for energy. also, limit the number of calories people take in. at some point, a dieter's body will change the way it uses energy if there are less calories to burn. the body does this to make sure it survives when less food is available. these changes in the body cause people to gain weight rapidly when they stop dieting and return to their old eating habits.