(this isn't a discussion on the attractiveness of large cyclists and likelihood of sexual congress...)

In my cycling career I have varied between 22 stone and 15 stone. At 6'2" the leanest I have ever been is still over 20% body fat.

I am not sure I've ever bonked...

My normal cycling day is an 18 mile ride into work before I have eaten or drunk anything. I have a yogurt granola thing for breakfast and salad for lunch. Some fig rolls mid afternoon then cycle 18 miles home.

I have done 80 mile training rides and really not eaten anything much.

I do sometimes feel tired and a bit weak but I thought that bonking was worse than that.

I assume that as I carry quite large energy reserves around with me that I don't bonk.

  • 7
    Bonking has more to do with blood sugars and glycogen stores than fat stores. When you run out of blood sugars you can no longer metabolize fat (glucose is a primer in the reaction). Most, even low body fat athletes, have more tissue fat stores than they can use in an event, even extreme events. See the "Hitting the wall" article.
    – Rider_X
    Sep 1, 2016 at 20:48
  • 3
    Rats @Rider_X, your comment is better than the answer I was planning to write. You should have submitted that as an answer.
    – rclocher3
    Sep 1, 2016 at 21:17
  • @rclocher3 - Feel free to borrow and extend the comment as an answer. It could use some refinements in the explanation (e.g., glycolysis vs fat metabolism as an engergy source for muscle activity)
    – Rider_X
    Sep 1, 2016 at 22:00
  • It might be of use to future readers to explain in the question what is meant by bonking. Took me a while to find out, and urban dictionary didn't help... :)
    – fgysin
    Sep 2, 2016 at 5:49
  • There is a way to combine "sexual" and "congress"? Ewww ...
    – Kaz
    Sep 26, 2016 at 3:12

2 Answers 2


Bonking occurs when your body cannot metabolise stored fat (and muscle protein) fast enough to replace the glycogen reserves, you deplete the reserves in your muscles and liver, and eventually you run out of glycogen. Fat conversion requires high levels of oxygen, and is slow, so once those reserves are gone, your blood sugar plummets.

I suspect your body has adapted to your regular routine of 18 miles, after an overnight fast, on an empty stomach. You will not only have large glycogen reserves, you will have also adapted to convert fat efficiently while exercising. Therefore, your observation of what happens for you is based on how you body responds to exercise rather than the amount of fat you have in reserve. Additionally, 80 miles is only twice your daily commute distance (presuming you ride home), so its not a big increase over your normal weekly workload.

Even the slimmest person has more than enough fat to keep them going provided they are exercising at a level that fat conversion can keep up with. Therefore "bonking" or not is less about how much fat you carry and more about how adapted you are to endurance sport.

Another aspect of this is how fast you are riding. If you are riding at your aerobic threshold, as people who are racing do, you will almost certain bonk on an 80 mile ride unless you consume food, however, if your 80 mile ride is a leisurely pace and you can hold a conversation (implying plenty of blood oxygenation), you probably won't Bonk.

Don't give up hope of ever experiencing it. One day, it will happen. :)

  • Ah, you had the time zone advantage. This is everything I would have wanted to say, especially Another aspect of this is how fast you are riding. +1 :-)
    – andy256
    Sep 1, 2016 at 22:21
  • @Roaders Sounds like you're doing exactly what you need to do to burn fat.
    – andy256
    Sep 1, 2016 at 22:23
  • In cycling terms, bonking is unpleasant. You do have to experience it to understand how bad it feels. Heat makes it a little faster, but dehydration has little effect on the time to bonk. I remember stopping at a servo and contemplating eating before getting to the till.
    – Criggie
    Sep 2, 2016 at 4:31
  • 2
    Thanks for the great answer, although I am slightly offended at you referring to me cycling at a leisurely pace!!! ;-) (only joking). I had a particularly good ride in this week, 18 miles through central London in 1 hour 9 seconds (this time includes waiting at red lights). Average power output 250W for that hour.
    – Roaders
    Sep 2, 2016 at 7:28
  • 1
    Hit some long sustained climb (7k at 5℅ just before returning home is my bonker) to finish your 80 miler and you will probably see how it becomes mind blowing slow, kinda like being high on thc and going up some flights of stairs, wondering about the illusion of motion, and wishing for bigger and bigger rings in your cassette, and doubling your usual time on the hill
    – gaurwraith
    Sep 2, 2016 at 18:13

In my experience, you know when you bonk (or hit the wall). I'm not sure if it varies for different people, but I know it's coming for a good while and it happens in this kind of order:

  1. Legs start to burn up on efforts which I would normally be able to push through without any issue.
  2. Legs struggle to push a harder gear on any kind of gradient (even though generally I prefer to push a bigger gear at a slow cadence).
  3. I have to sit down and spin up hills (usually an out of the saddle climber - as above).
  4. Struggle to put down any power whatsoever.
  5. End up in the smallest gear I have willing my front door to sprout legs and walk to me rather than having to continue cycling to it - I think this is the full bonk.

For me this usually happens in that order over the course of an hour or so at the end of a hard ride and once it's coming it's too late. For reference I'm 5ft 6inches, weigh circa 55kgs and have very little body fat.

I'd like to hear how it affects other people though...

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