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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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In short: skipping a meal lowers blood glucose levels, which can reduce cognitive performance, for example, memory.

Let's say that between meals you are in fasting state for 8 hours during which you do some desk work and some moderate exercise for 1.5 hours. With such level of physical activity, your last meal and your glycogen stores should provide enough glucose for all your body needs, including the brain. So, during this time, you do not need any ketones or lactate as additional source of energy. According to Medical Hypotheses, 2014:

Estimated amount of glycogen in the human organism is about 200–500 g, and in certain specific conditions could reach up to 1.0 kg, which seems to be optimal for human organism. This amount is definitely sufficient for keeping optimal concentration of glucose in the blood during usual fasting periods not exceeding 24 h (between meals, during sleep, in case of exercise, etc.)

Skipping a meal can lower your blood glucose levels. According to WebMD:

Normal blood sugar levels are less than 100 mg/dL after not eating (fasting) for at least eight hours. And they're less than 140 mg/dL two hours after eating.

During the day, levels tend to be at their lowest just before meals. For most people without diabetes, blood sugar levels before meals hover around 70 to 80 mg/dL. For some people, 60 is normal; for others, 90 is the norm.

Blood glucose fluctuations related to meals:

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Image source: Wikipedia, Creative Commons License

In some studies (AJCN, 1998), skipping breakfast was associated with lower blood glucose levels and worse memory test but not intelligence test results:

This article compares the findings of three studies that explored the role of increased blood glucose in improving memory function for subjects who ate breakfast. An initial improvement in memory function for these subjects was found to correlate with blood glucose concentrations. In subsequent studies, morning fasting was found to adversely affect the ability to recall a word list and a story read aloud, as well as recall items while counting backwards. Failure to eat breakfast did not affect performance on an intelligence test.

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