People are commenting that some people eat 500/1200/etc. calories and still not loosing weight. Can someone explain this to me? Your body needs a certain amount of calories for basic functions and this is around 2000 calories. How can you eat less than 2000 calories and not lose weight. A calorie is the amount of energy in the food that is measured through burning in a bomb calorimeter. Your body can't extract more calories from a 500 calorie Big Mac than 500 calories. If all you eat in one day is 500/1200 calories, where are the extra calories that are needed for basic body functions coming from? Are certain people more efficient at using calories than others? By a factor of 2, only needing 1000 calories? Or by a factor of 4, only needing 500 calories?
The reason is that the body needing 2000 calories is a very very rough approximation (very rough multiplicative factors to BMR and linear approximations from BMR data that is never shown to be linear). You can easily survive on 1000 calories a day if you don't exert yourself.
It is also believed that the body has a mechanism for decreasing calorie expenditure. Feeling sluggish and tired are probably symptoms of the body trying to minimize calorie expenditure.
So, assuming we need 2000 calories every day and that this value is not affected by any other factors is the error here.