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The human body has mechanisms for making you Hurt Real Bad when you don't have more calories coming in then going out. Maybe your ancestors had more reliable access to food resources and didn't have to develop these responses. The human population on this planet is incredibly varied. Good for you that you and the people in your social network do well following what you describe as a calorie-restricted diet. The self-…
The science is pretty basic. It's the law of thermodynamics. Your body can't magically create fat or other weight from calories you don't have.
Actually it's not. http://en.wikipedia.org/wiki/Metabolism