Brain Bonk versus Muscle Bonk (N-Zone Topic)

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  • #52273

    I would like to discuss two popular methods of fasted training and the rationale/physiology behind them.

    Method 1: One eats a 50/50 carb/fat (by calorie) dinner, skips breakfast, and does an endurance training session the next morning. Using this approach he/she has fasted for 12 hours prior to training.

    Method 2: One performs an endurance training session mid-morning, eats a non-carb lunch, then performs another training session in the late afternoon.

    Which method causes a brain bonk and which causes a muscle bonk? Which causes both? Which cause neither? Are they, in fact, different? Which is more important to train through?

    Furthermore, does fat metabolism play a role in when (and if) these bonks occur? Should a fasted training session below AeT be qualified as more intense zone of training, say falling in the 20% of an 80/20 easy/hard schedule? How low does muscle glycogen actually get, even at the end of an ultramarathon?

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    Thomas Summer, MD on #52283

    Hey Steve!

    Good topic. Some quick first thoughts:

    Method 1: likely to bonk because of low blood sugar. muscle glycogen is still full the next morning, but liver glycogen will be more depleted because it is used to stabilize blood sugar during the night. So it’s likely that low blood sugar (brain bonk) occurs.

    Method 2: is this with breakfast? ether way this training regime will be probably more beneficial for training adaptation. As it has been shown, that the longer you wait with replenishing the glycogen stores in your muscles the more mitochondria and other adaptations for aerobic fat metabolism will occur. Bonking in the second training session is likely. Probably both ways.

    BUT: isn’t it always the brain that senses fatigue and causes you to slow down or even bonk?

    Good fat metabolism plays a huge role. And also the ability to produce ketones.

    The 80/20 “rule” is only defined by intensity. If you train below AeT it doesn’t matter if you fasted before or not. Fasted maybe feel harder, but it’s not higher intensity (probably even lower). That doesn’t mean that the stress on the body isn’t higher. You don’t count it as higher intensity, but you have to be careful about the training load.

    I don’t have any exact numbers in mind, but muscle glycogen is never totally depleted (even with a ketogenic diet). It’s always the brain that senses if something is getting low or out of balance. As it senses this as a risk for the body, it shuts down before serious damage occurs (in most cases)


    russes011 on #52287


    Well said–I concur.

    Method 2: the athlete eats breakfast so only training session #2 for that day is glycogen depleted.

    I would maybe go a step further and propose that Method #1 may not be very useful at all–not sure. As you mentioned, this athlete does not use much, if any, of their muscle glycogen sleeping and shows up to training with full muscle glycogen stores the next morning. I believe their liver glycogen stores are depleted perhaps 1/2 or so overnight. So a brain bonk occurs, but this ‘should’ have no direct effect on muscle glycogen stores, which is the true stimulus for mitochondrial creation from fasted training.

    Method #2, as long as the first workout truly exhausted the available glycogen stores (brought them down to a low enough level to be sensed as ‘low’ or ’empty’), then the second workout that day will be a true muscle bonk with the potential benefits of training in those circumstances–specifically mitochondrial production, which seems to be quite sensitive to training in a muscle bonked state.

    I understand the 80/20 is regarding intensity and traditionally intensity is defined by training zones. Nevertheless, training fasted like method #2, I have read, can provoke a a training adaption in the muscle that is much more than that expected for an easier intensity workout. For this reason I speculate that perhaps method #2 fasted workouts should be upgraded to something like Z2+, or something similar, and stuck into the 20% category. Once again, just speculating.

    Also, I think I read that the average amount of glycogen in the calf muscles is about 60% full at the end of a 100K race, but that 60% full registers as ’empty’ to the brain and calf muscles. Makes sinceDon’t quote me on this number–I’ll try and find a better reference. (as an aside, I think heart muscle paradoxically increases its glycogen store when you fast)

    — Steve

    Thomas Summer, MD on #52304

    What is the point of hidden replies? Especially if I can read them anyway? That makes no sense;-)

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