It's really a simple equation....Calories IN greater than Calories OUT, you gain weight and vice versa. Skylergirl is absolutely right about the starvation, though. Your body shuts down thinking it is in famine times. Even moderate excercise (think walking) can have a tremendous impact on both weight loss and metabolism. Exercise drives up metabolism, but studies have shown that within an hour or so, it has returned to it's pre-exercise rate. A couple other things to consider if you wish to drive up your RESTING metabolism: First, muscle requires more energy to maintain, therefore as muscle mass increases, so does your resting metabolism. Second, 5 to 6 small meals a day, spaced about three or so hours apart also helps to increase your resting metabolism. It basically does the opposite of starvation. It causes to body to NOT slow metabolism since it knows it can use as much energy as it likes because a fresh supply will be avaiable in a few short hours. One last thing to consider is fat intake. The body requires a certain amount to function properly. Body fat surrounds and protects your internal organs. However, you fat intake should be early in the day, and taper off towards evening. Same with the simple carbs. If you want a late night snack, stick to protein and complex carbs. I've found that a diet consisting of 60% carbs (mostly complex), 20 - 25% protein and 15 -20% fat is a good balance. Percentages should be based on calories. Fat has approx. 9 calories per gram while carbs and protein are approx. 4 calories per gram.