I teach/proctor the SAT’s and other standardized tests. (I only proctored one MCAT…that is a marathon, man.)
The advice I give to students is just to eat healthy: stay away from anything that gives you a “burst” of energy because you always pay for it with a crash.
If you drink coffee, drink the same amount as usual.
For the lunch break, bring a good sandwich and some fruit; a banana would be great. Not only is it yummy and good for you, but it will help with the not-going-to-the-bathroom thing.