The Science of Fasting

there is a conventional logic that said to eat something small in the a.m. just to get your metabolism kick started. I would always eat a piece of dry toast and half a grapefruit for this. Has this been completely disproven? What I am getting at is would this maybe work if you were otherwise doing the fast with the exception of this small meal? Or is it an all or nothing.

I know how to lose weight, that isn't an issue. I really want to try this for the energy gains and good feeling some describe.