The Brain & Bonking

This might be obvious if I did some research but thought I’d throw it onto the forum.

A few questions I have concerning the brain & bonking:

Does the brain use more carbs during exercise?

Is there any link between fatigue and the brain not having enough fuel? Or is it strictly an “empty” muscle issue?

Can muscles still contain glycogen if the brain “bonks”?

Guess that’s it for now… Thanks!

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When I bonk hard, I get a crazy field of vision problem: the effect is like when you rub your eyes too hard and you see bright neon donuts. That’s when I know, for sure, that the fuel light is on.

I’d be interested to know the same thing. My guess is that the Brain uses a similar amount of glycogen during exercise, but it’s being pumped to the muscles first… hence my donut hallucinations are a result of a deprived brain.

Go watch the second episode of the science of getting faster podcast. I think this is where I recently picked up that muscles still have quite a bit of glycogen in them even during a hard bonk. Basically no such thing as fully empty.

Correct me where required.

I know I have had 2 legit bonks… they were in my keto/ low carb days. Best way i can relate is almost being drunk. I was dizzy, and the nausea for me was the worse. And my recollection is a bit fuzzy honestly. I knew i needed to eat but i physically couldn’t. When i finally got some food down me it was almost like having the flue for the rest of the day Just felt awful. The muscle cramps came later. A true legit bonk is horrible and it took days to recover.

Considering thinking only varies brain energy-consumption by about 5%, I would be surprised if it consumed more during exercise.

Yeah, I can see that. I guess it makes sense, too. From an evolutionary stand point, if you were chasing or being chased, you’d want your brain to be functioning as stable as possible and also not sucking up valuable carbs your muscles could use.

I’ve also read that (from animal studies) and it too is an evolutionary trait, just in case you get attacked again, your body holds back a booster pack of carbs.

Interesting that there is actually a threshold (although incredibly high; near death?) where the body classifies muscles to be of greater importance than a brain starving of glucose.


Yes, many years a good I tested my capabilities doing Soduko while working out easy, moderate and hard. My time to complete puzzles extended as the workout got harder.

As you know, the brain uses sugars in the blood so it makes sense that the less fuel available the foggier thinking becomes.

Here’s a handy research paper:

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I wonder if oxygen demand is another factor.

I would think that reduced blood flow would be the greater factor, if we were talking about such a draining effort before blood oxygenation became an issue.

Well, since I bonked Saturday doing the Uber Pretzel, I can’t definitively say for sure, but in latter fourth and fifth hour, the show I was watching (Vikings) started getting weirder and weirder and I couldn’t really comprehend it. It was a really weird part of the show, so it could have been the show, but I swear I felt like I was dreaming during it. I don’t really have good memory of what happened.

And as was said above, felt like I was sick with the flu after. Good news was it must not have been too bad because I nailed Bashful+6 this morning after 2 days off the bike.