You can&#x27;t extract more calories from food than their are.  Calories are measured by actually incinerating the food and measuring how much heat it gives off.  That is literally how much energy there is.  So if you eat 2000 calories of food energy, you can not (if you are obeying the laws of thermodynamics) gain more energy than that.  You can only absorb less.<p>Similarly, as I said in a message above, there is a lower limit for how much energy your body must burn in order to maintain body temperature.  You will find if you look at the rate at which corpses lose heat to ambient temperature that it is very, very close to what is listed on those base metabolic rate charts.  Again, I have to stress this: you can not burn less than that without decreasing your temperature and dying.  It is a physical impossibility.<p>There are obviously variations, of course, but they must (by physical necessity) be on the up side.  So you can easily eat 3000 calories and not gain any fat, even though you don&#x27;t exercise, if you have a thyroid problem, or if you have a digestive problem that causes food to shoot through you before you can process it.<p>On the other hand, you can not (due to the laws of physics) eat less than the number of calories required to maintain body temperature and not lose weight.  This is conservation of energy.  And as I said, the minimum amount of energy required to maintain your temperature is very close to those base metabolic rate charts.<p>Sure you can be off by 100 calories a day.  You can&#x27;t be off by 1000.  It just isn&#x27;t possible.  So if you are in deficit for several hundred calories a day and you aren&#x27;t losing weight it is necessarily because you have measured your exercise or diet incorrectly.  As I stated several times, this usually occurs because companies outright lie about how many calories you can burn through exercise (and are often off by 1 or 2 binary orders of magnitude).  It is also because people completely overlook high calorie foods, even if they are otherwise very careful.