[snarf21]

My understanding (and experience) is that this clarity is usually just about the lack of all the crap that normally makes you feel awful. Things like sugar, dairy, and processed grains plus preprocessed foods. I've always found that after 10 days, you start to feel amazing and do so for around two weeks more before your body starts to adjust.

[plutonic]

The mental clarity comes from adrenaline. When you fast your body slowly uses up its glycogen store and releases adrenaline to compensate. Presumably it does so in order to keep you active / motivated enough to go look for food to refill your glycogen stores. It is this adrenaline that gives you the mental clarity and causes you to have trouble sleeping when on a fast: fasting is essentially natural adderall.

On top of this, once you use up all your glycogen, your body enters a period of autophagy which causes disfunctioning cells and protein fragments to become broken down and reused. This causes your body to function more efficiently, including your brain.

[king_panic]

That's true but it's also true that neurogenesis will start at some point during a fast. For me, it's around day 3 or 4. It's an evolutionary mechanism to form new connections in the brain to help find food/survive.