The human body always attempts to achieve a state of equilibrium. This is known as homeostasis. By severely limiting calories you are forcing your body to attempt to function on less fuel. Your metabolism will slow a bit due to your body becoming more efficient, however stalls in weight loss can be negated by incorporating a cheat day once a week that you are dieting. The increase in leptin will act to "reset" your metabolism and allow you to continue on your diet and lose weight effectively.

nope. if that were true, no one would ever lose fat, because to lose weight, you have to burn more calories then you consume. i think what matters is the vitamins, not the calories. :)