[toast0]

> When I was a kid, I recall the teachers always telling us to have a good meal before a test. Even then I felt that was silly, as a big lunch always gave me brain fog.

Insufficient direction from the teachers there, I think. A good meal in this context doesn't mean a big meal, usually. You'd want to eat like you were planning to do some strenuous exercise shortly afterward. If you eat big and then go running, that's no good; if you have a roughly typical diet, you want a mix of carbs, fat, and protein, as those provide energy on different timescales. If you keep a diet which avoids one of those things, you do you, but you still want the same goal of energy available over time, not eating too much that digestion needs overwhelm everything else, nor not little that hunger distracts.