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I know from many studies that have been done, that resting versus active use of the brain has relatively similar overall energy expenditure levels.

On the other hand, however, we know that the brain along with our eyes require glucose in order to function properly. Ketone bodies, lactate, etcetera may act as glucose sparing agents, but certain parts can definitively only use glucose as an energy source.

Anecdotally, I’ve noticed that when performing low intensity mental tasks like jogging or hiking for example, if I haven’t eaten for a while and am in a fasted state, I might get hungry for a bit, but if I just keep on going, after a while I feel fine and can pretty much go on forever after that. With high intensity mental tasks like programming though, the feeling of hunger doesn’t really seem to go away, and if I try to push through it, it just results in consistent brain fog and an inability to perform at any meaningful level.

My theory is that in cases where high mental performance is not required, alternative sources of metabolism provide the needed energy to function, and where intensive cognition is required, the rate of glucose production is unable to keep up with demand from the parts of the brain that depend on it which effectively creates a bottleneck in the cognition process causing the aforementioned brain fog and low performance.

Is this what is actually happening though? I’d like to think that it makes sense, but I’d prefer to have a biologically accurate understanding first and foremost.

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