Metabolism doesn't just speed up and slow down like that.  What happens is just that the body shifts to a different energy source, going from energy that was recently consumed to energy that was stored.  A calorie is still a calorie and it will always produce the same amount of energy.  Chemistry is fairly predictable. The change in "metabolism" that people see is the difference in movement.  What's really funny is that no matter what you do, fats will claim that it slows down metabolism.  Eat less?  Fats claim it slows the metabolism.  Exercise like a maniac on the Biggest Loser?  Again, fats claim it slows the metabolism.  When you don't eat, you don't have energy to move around as much, so you remain sedentary.  Once eating again after a restrictive diet, these fat slobs remain sedentary and don't burn off the calories they were before.  Conversely, after exercising to lose weight, they reach whatever goal they decided on and then stop exercising, but don't alter their diets to compensate.  Both ways, it's the same thing.  They're moving less and eating more.  Metabolism has jack all to do with it, other than remaining steady and burning calories exactly as chemistry dictates.