That's a gross oversimplification of what goes on when you restrict calories. Introducing an energy deficit by increasing exercise or decreasing food signal your body to adapt metabolically. This includes:  Increased mitochondrial efficiency Decreased metabolic rate Increases in hormones that induce catabolism and hunger.  These conspire to create weight loss plateaus, lower test levels, and difficulty in maintaining lean mass. Even after you halt restriction, the adaptations hang around and set you up for body fat overshooting. Weight gained during the presence of said adaptations can also trigger the growth of new adipocytes.  That said, metabolic adaptation does not make weight loss impossible or cause weight gain in a deficit. But, if you fail to keep in mind the need to reverse diet your way back into a less efficient metabolic rate, it's easy to hit your goal and get fatter than you were before.