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When I'm out cycling I know that once I feel hungry, I only have around half an hour to refuel, otherwise I will bonk in a way that all performance is lost and it feels as if the only way to get me home is by pure will. This seems to be a rather common phenomenon since among German speaking riders there is even a name ("Hungerast") for this state. On the other hand I can go out for a 1-hour-plus run at similar intensity while already being hungry without bonking and even am able to push it hard at the one or other uphill during the run. Even if I push this quite far it does not feel as bad as in cycling but only gets more or less an "annoying" feeling that my thoughts start to revolve more around being hungry.

Does anyone have an explanation why this does feel so different in these two activities?

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  • I suspect that in cycling you are running at a higher metabolic rate, and more consistently so. But I've never been a runner. – Daniel R Hicks Jan 14 '16 at 13:02
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    Try and find out how many calories a runner will burn per hour. For a cyclist it can be up to around 1000, and that could be for 5-6 hours maybe. So if you can get data from runners, that'll either disprove your theory or answer your question. – PeteH Jan 14 '16 at 13:11
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    Boring but if you are more than 1/2 hour out and start to feel hungry then bring your pace down to like 2/3 immediately. Let your body start to metabolize fat. It takes energy to metabolize fat so you need to give your body some cushion. – paparazzo Jan 14 '16 at 19:23
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    I wonder if some small part of this could be related to the greater cognitive demands of cycling? Something like the brain protecting its own supply of glucose from being depleted, because it can tell it's doing a lot of important processing to keep you alive (constant stream of data about the road ahead of you, etc) whether or not there's an "awareness" of this fact... – junkyardsparkle Jan 14 '16 at 19:57
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    @junkyardsparkle That was one of my theories as well but I was too unsure of it to mention it. – Benedikt Bauer Jan 15 '16 at 7:46
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The Science behind bonking

This is a really interesting article applied to running but no doubt the same applies to cycling. The interesting quotes being "It is impossible to prove that muscle glycogen depletion alone limits prolonged exercise performance," and "the inseparable relationship between our head and our legs"

And this highlights that glycogen depletion is only a theory of bonking. Iirc, glycogen stores are relatively small anyway and my experience of hitting the wall was (time & effort wise) way beyond the glycogen storage capacities of muscle & my in-ride refuelling.

The other interesting bit was the psychology of hitting the wall and how the brain could be playing a role also.

So maybe the drastic effect and differences between cycling and running are due to - different exercise - different psychology?

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  • That article has some very interesting insights. Should be worth to read it several times to extract all the implications it poses. – Benedikt Bauer Jan 15 '16 at 10:05
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The short answer is that it is purely down to your individual training level and mechanical efficiency in each of these disciplines.

Bonking is a result of fully depleting your glycogen stores. Once this occurs, your body's only source of fuel to power your exercise is to metabolise fat. This does not change regardless of what activity you are performing.

However, your state of training and efficiency can impact how long your remaining glycogen stores can last.

As an example, consider the case where an athlete is significantly less trained at running, than cycling. In this case the perceived effort level may be the same, but in fact, when running, the muscles are generating less power, hence requiring less fuel, and allowing them to last longer before bonking.

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    Your last sentence is unclear. It's not clear that less trained individuals can last longer before bonking while running than they do when cycling. – R. Chung Jan 14 '16 at 15:38
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    It is about the comparative level of training in each sport rather than the overall training level. When the cardiovascular system is trained, it will perform for a number of sports, however the muscular demands can be quite different. Using myself as an example, i'm a well trained cyclist, but if I tried to run my muscular system would not allow me to perform at the same level in terms of kcal/hour. This leads to the case that my glycogen stores will last longer as they are not being drained so quickly. – Andy P Jan 14 '16 at 15:48
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    When you exercise at aerobic threshold, your body will not metabolise fat due lack of oxygen. A well trained in cyclist who is has poor running fitness can cycle at Aerobic threshold, but cannot run hard enough to reach aerobic threshold, so has oxygen to metabolise fat and can replace the glycogen - therefore not Bonk. – mattnz Jan 14 '16 at 21:50

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