>The two compliment each other, but exercise is not strictly necessary to weight loss.
After you reach a certain age with a certain weight your body will deliberately keep itself within that weight range as much as possible, you need to compensate the loss of fat with muscle otherwise your body is just going to drive you to get fat again. Another thing is the feeling of starvation after you lose a certain amount of weight without eating, that requires willpower to resist that a mere diet is not capable of accomplishing alone.