An interesting question would be whether stimulants, like caffeine, can be used to increase brain function without calories. I wouldn't suspect this to be a healthy option though...
Personal experience indicates that they can. In fact, I find that is one of the main problems I have with stimulants - they tend to suppress my appetite, but since I am still able to function well on them I don't notice how hungry I am until they wear off and I crash becasue I haven't eaten in 3 hours.
Don't you notice the depletion of neurotransmitters? It's been a awhile since I played with nootropics, but my understanding of how many cognitive enhancers work is that they stimulate the production of underlying neurotransmitters (norepinephrine, for example).
If you are not providing the body with sufficient raw material (feeding your head, as it were), these transmitters get depleted and no amount of stimulants will help.
