Bonk vs Fitness

I was listening to the latest podcast and started to wonder, How can you tell if you bonk due to nutrition or just fatigued due to lack of fitness? I completed a couple of long MTN bike races earlier this year and about mid way felt wiped out. Off the bike pushing climbs-wiped.

Love to hear your thoughts.

To me i differentiate a bonk as glycogen depletion. This results in both the muscle fatigue and inability to generate power but also this zombie mental state from the lack of glycogen being available for brain function. Fitness failure only results in the lack of agility to continue to produce power, and yes your might feel crummy but you still have most of your mental acuity


If you have a power meter, it is easy to triage whether you are in a state of glycogen depletion, which could lead to a bonk.

  1. Have been riding longer than 3–4 hours?
  2. Do you struggle to produce more power than Z2, perhaps middle of Z2?
  3. Are you struggling/riding at speeds, which are unusually low for you?

If you have answered yes to at least the first 2 questions, it is likely you are in a glycogen depleted state.

For me a bonk is a state of severe glycogen depletion. You are forced to stop to take breaks, etc.

Fatigue in my experience presents itself quite differently:

  • Onset of high RPE is much quicker, perhaps immediate.
  • Even with fresh legs you are having trouble producing >Z2 power.
  • Your heart rate is unusual. (Note that unusually high but also unusually low heart rates could point to fatigue.)

If you are having trouble being in long races, I’d really look at your fueling strategy. Fueling can be trained just like other aspects of your fitness can be trained. @Dr_Alex_Harrison’s app might help. Fueling appropriate amounts can and should become a habit where you no longer need to keep track of how much you consume. However, when you plan a fueling strategy for a race, crunch the numbers. If you expect an event to take you 6 hours, make sure you take in 6 hours worth of food and liquids.

In my experience fueling for rides and races longer than, say, 5 hours require a different strategy. For rides/races <= 4 hours, I can primarily rely on liquid calories. Once I cross the 4-hour mark, I need solid, more regular food. You should experiment with different strategies as everyone is different here. Some people are more susceptible to palate fatigue and/or require variety.


Slight derail but related to symptoms of the dreaded bonk:

Last weekend I did a long race, ~5.5 hours of hard effort. I think I did very well managing nutrition and felt great right to the finish line. Had some recovery snacks afterward. About 2h later (with no further aerobic activity) I got the familiar “you’re out of energy” feeling: Light headed, trouble focusing, etc. Because I was doing something else I was not able to eat immediately. Most noticeably, I started sweating like absolute crazy, just pools of flop sweat despite a generally comfortable ambient temperature. Eventually had a snack and felt fine. This is not the first time this has happened to me, but what surprised me was the suddenness of onset of this response, and the weird sweating reaction. Anyone know what’s going on here, physiologically?


Don’t know what was going on, but I’ve had that a couple of times early on when I started doing Sportives. I put it down to a combination of dehydration and glycogen depletion. I haven’t really experienced since I properly paid attention to recovery. The other factor was I was away, so there was walks to restaurants, standing in bars, and genuinely only a couple of beers involved. Just too much for activity, not enough recovery.

I don’t believe I have bonked on the bike, but I have always assumed that’s what I was feeling those couple of times. If it was, I imagine it’s quite scary on the bike (as my wife was very concerned with me the times it happened).

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This sounds like the pretty typical symptoms of low blood sugar, the term is hypoglycemia.


I’m not so sure. One can have all the same with dehydration/electrolyte depletion. I struggled with this for years while consuming 90+ grams per hour. I realized after a PH sweat test I needed 1100mg of sodium an hour while I was consuming 400mg previously. Also, my sweat rate can be as high as 2 liters an hour in hot and humid conditions and I was consuming 24oz an hour.

I couldn’t figure out why after 3-4 hours in the heat I would fall apart in MTB 100s. Some of which I was crawling to the finish line as the hours added up and stuck in low z2. Since ive increased sodium and water I’ve felt so much better and I haven’t changed my carb intake.


That is a good point. I wasn’t thinking of that, because head out with two loaded bottles and drinking enough liquid is the same (for me) as taking in enough carbs. But certainly, that can be an issue, too, especially if you live somewhere hot and humid.

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My n=1 is that bonking means I stop. I have no choice. Full fledged muscle cramps, blurry vision, dizzy… can’t fight that. Body wins. Poor fitness… I have to slow down but there is a gut feeling I can continue, just slower. I’ve ended up bonking due to both too little salt (I now carry salt pills with me for all long rides + my electrolyte drink which is 1000mg sodium) and too little CHO. As someone else said my HR will start doing really weird stuff for bonking too. Like I’ll be riding along at an intensity that feels ok with a HR of 95%. When it’s just an out of shape issue my HR will come back down when I slow down.

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If I’m looking at it, heart rate gives a pretty good indication. For me fatigue results in a higher HR relative to power (or running pace) while bonking results in a suppressed heartrate. During the worst bonk I’ve had I noticed the heartrate being low (~110-120 when I would’ve expected 140-150) I tried to ride harder up a ‘hill’ to raise it. Did get it to 140 and became dizzy and nearly puked.

I’ve had a less severe bonk towards the end of a half ironman. The principal symptom was increasing apathy ‘it’s ok I don’t care about going under x:xx time’. Eventually realized what was happening and raided an aid station and felt better almost immediately and was running probably 1-1.5 min/mile faster.