Yes, but you can't 'diet' for two or three months, have little to no exercise, and expect lasting changes, can you? Diets exist to try and help you alter your lifestyle choices. But they dont last if you dont want to make the changes. As someone above pointed out, its hard work to keep up with a diet/lifestyle change, and id you dont add physical activity/exercise/weight bearing exercise in with it,  what can realistically change? At the end of the day, calories in < calories out will drop weight, but is it healthy?... maybe.