Another interesting factor is how your body naturally adapts to your diet in order to do whats "best" for itself. Even if it goes against what you want to achieve with your body. when you diet (lets say, a low carb diet), your body needs about 2 weeks to notice and adapt accordingly. You will stop loosing weight at such a rapid rate unless you change your diet again. In a way, you need to throw your body off, in order to keep a high metabolism going. so if your doing good on your diet, and you pig out or eat unhealthy for 1 day out of the week, you may not be doing as much bad as you think. If anything, it might "throw off" the body a little. and keep fat burning at a good speed. fitness experts say its very important to follow a routine of: 2 weeks of high calorie eating, 2 weeks of low calorie and 1 week of low carb eating. (atleast for guys trying to achieve a "body building" type of shape.)