Fasting is useful for seeing how your mental state is while limiting external food resources (you're using internal fat reserves otherwise), otherwise intermittent fasting is the best option.
This measuring alone ensures that they get enough fats, which seem to be the main reason for brain fog in most people (combined with too much artifical sweets).
Also because of this extreme diet they completely throw all all processed food & artifical sugar/sweets.
Buddhist monks are not supposed to eat after noon, although some cheat by adding a lot of sugar to their evening tea or coffee. This is supposed to keep their minds sharp and clear and aid in meditation. It appears to be a well-tested rule.
I use my own experience with Igg food sensitivity testing and removing those foods to see how I felt after. After allowing my system to calm from the foods then reintroducing just one of the foods that I was conditioned to the discomfort of, it let me see how much discomfort/pain different foods that showed up on the Igg test that make me feel horrible.