Biology 101

Basal Metobolic Rate. It's what is needed to support basic body functions such as breathing, digestion, cell regeneration, blinking, heart beating etc etc.

In order for your body NOT to go into a starvation self preservation mode, one must eat enough calories a day to support these body functions. Eating less will insure your body's break down of lean muscle tissue thus slowing your metobolism even more.

If you go into fitday.com, it shows you just how many calories you burn every single day without doing a single exercise.