a matter of throughput

To maximize efficiency mechanically, the amount of input must never be higher then the output. To optimize a result, one would want the output to be significantly greater then the input. So here's the thing. What is the minimum amount of food (and what type) can I eat so that I would have enough (or more) energy/concentration to do school work, work out, and work? Somehow I feel a Red Bull can be thrown into the mix, but it probably is not a good idea in the long run. I guess I have to just find some way to keep my matabolism high at all times.