This is what I do for a living,,let me make it simple..A calorie is a unit of energy, we take energy in and give energy back,,as long as we give back more than we take in we will burn fat...As for Atkins,,carbs hold water each gm of carbs holds 3.2 gms of water,,drop the carbs and the water disappears,,also carbs convert to glycogen ,,glycogen is our preferred energy source,,in absence of glycogen your boday will burn muscle for energy (or it will die)...So ehat do you lose on the Atkins diet,,muscle and water...not good