It really depends what you mean by low calorie, if you mean something very low calorie like Cambridge Diet which is basically meal replacement shakes, then eventually you do need to go back to eating normally eventually. If you just mean low cal like 1200 cals per day then I suppose you could sustain that forever really... calorie counting is one of the few diets that can be carried on forever. I combined cal counting & weight watchers with great results.
Good luck