From a neuroanatomy perspective if we divide the brain into 3 areas:
1) Cerebral cortex - higher functioning including language, critical thought, etc.
2) Cerebellum - coordination and accuracy of critical thinking, language, and much much more
3) Brainstem - autonomic function
When we think about fuel resources (oxygen and glucose) primarily used by the brain. There is a hierarchy of needs that must be met.
The brainstem gets fuel first as it is necessary for staying alive. The cerebellum although only 10% of brain mass uses a HUGE amount of fuel because it is basically the project manager of the brain. The cerebral cortex demands a lot of fuel as well because any time we go outside of route function such that requires us to stop and think, make decisions, override heuristic tendencies, etc. it needs fuel.
So people who either don't get enough fuel or can't utilize fuel well will get sluggish brains. The symptoms of which can be:
- brain fog
- lack of critical thinking
- old bad habits coming back
- loss of emotional accuracy (too emotional, not empathetic enough, trouble relating to people, etc.)
- muscle tension
- hypothyroid symptoms
- even pain.
Sufficient fuel keeps the entire organism functioning well including intelligence. Also sufficient fuel allows us to critically think and focus which allows intelligence to increase.
So long story short just an anatomical view of what everyone else already said