[crpatino]

Human metabolism is a (set of) complex process(es). You are not just burning the plate of food in a furnace.

Of course if you eat less calories than the bare minimum needed to sustain life, you will loose weight no matter what, but that is an unhealthy way of doing it and no sane nutritionist will risk advising you to do so.

On the other hand, it is actually possible to gain weight by restricting calories just a little. Humans did not evolve in an environment where superabundance of food required voluntary abstinence to keep in good shape. When you just cut calorie intake your body enters "starvation mode": metabolism slows down, non essential processes get suspended, you tire out more easily, etc. The result is that if you do not overcompensate with will force and do exercise, you will end up undercutting your calorie usage below your reduced calorie intake, and grow fast.

The body is wise: it knows when things begin to go bad, they always can get worse.