My goals are not to lose weight, I’m very lean as it is ( around 10% BF) and i hold a fair amount of muscle, i train calisthenics 2-3 times a week and rock climb once a week. My main concern is cognitive function, i have pretty bad ADHD which is extremely food sensitive, My recall ability and just general memory is atrocious.
Ultimately i’d like to be able to eat in such way where i can optimise cognitive function without losing strength or muscle mass. The closest i ever came to this was through Keto but the insomnia was horrific so i stopped, Sleep can be very difficult at the best of times, I’m basically an insomniac. carbs help but they also affect my ADHD negatively.
I’m at a bit of a loss to be quite honest, i don’t know where to turn.
I would advise against prolonged fasting if your bodyfat is that low. If you don’t have the fat reserves necessary to balance out the high calorie deficit, you’re body might prioritize to keep the small amount of fat you have left and break up muscle cells and organ tissue.
Although, I do experience some increased mental capacity after around day 3/4 of a water fast, it’s not really significant and might be a placebo effect as well. Additionally, I could imagine that fasting with so little fat reserves would make you feel really exhausted all the time and therefore even worsen your ability to concentrate.
Definitely consult a doctor about your plans and consider some medication for your ADHD if you’re not doing so already.
Edit: Additionally, your cortisol levels during a water fast increase wich reduces your ability to sleep well quite a lot. During a prolonged fast it’s not unusual for me to get only sleep for 3-5 hours a night and the sleep isn’t high quality either.
Edit 2: The most you could try is OMAD or ADF but I‘m not sure if this will fix your issue.