Personal ADHD anecdote - being on low carb or keto was really helping with energy and focus.

We were not exactly counting all the nutrients in details. Since we normally eat a lot of protein and fat (think almost mediterranean diet), we basically focused on eating as little carbs as possible - no bread/pasta/potatoes/etc (even whole banana would be too much carbs in a day).

On the other hand - back as undiagnosed kid, I've used "white coffee" (milk, sugar and 10%~20% of normal coffee/caffeine) and even bits of chocolate throughout study/tests/exams.