Cycling Nutrition: The Bonk

So . . . you know that muscles burn glucose for fuel, your body stores enough glucose in the form of glycogen to fuel about 90 minutes of moderate exercise, and you need to ingest carbohydrates to provide more glucose for longer or more strenuous rides.  You know about what to eat and when to eat it to get the carbs you need while you’re on the bike.  But you keep hearing about this “bonk” thing.  You keep hearing you’ll bonk if you don’t get the carbs you need.  What’s this bonk everyone keeps talking about?  Believe me, you don’t want to know.  At least you don’t want to know on the basis of firsthand experience.  Reading about the bonk so you know what it is and how to avoid it is good.  Having a bonk is not good.

Eat before you're hungry

“Bonking” is what cyclists call hypoglycemia which is the medical term for abnormally low levels of blood glucose.  You bonk when you have exhausted your glycogen stores, haven’t ingested enough carbs to produce more blood glucose, and are still riding the bike.  Anyone can bonk if they don’t eat properly on the bike.  Lance Armstrong, who probably knows as much about cycling as anyone on the planet, got wrapped up in the race on a stage in the 2000 Tour de France, forgot to eat, and bonked on the climb up the Col de Joux Plane in the French Alps.  The only reason he didn’t lose the Tour de France that day is because he had an iron will and an inhuman capacity to suffer.  Afterwards he called it the worst day on the bike he’d ever had.

Bonking can be especially deadly for cyclists because your muscles aren’t the only things in your body that burn glucose for fuel.  Your brain burns glucose too.  That means that not having enough glucose in your blood to fuel the system has mental and emotional effects in addition to physical effects.  If blood glucose levels drop too low, the body will act to protect the brain and will begin to shut down the muscles first.  However, the muscles won’t completely shut down before the brain begins to be affected and the mental and emotional consequences of bonking can be more dangerous to the cyclist than the physical consequences.

When you bonk, physical exertion becomes extraordinarily difficult.  Your muscles don’t have the fuel they need to operate effectively and forcing them to work becomes more and more difficult.  You feel extremely weak and lethargic.  You may tremble and shake uncontrollably and sweat profusely.  You feel dizzy and light headed.  Your sense of balance is upset.  You may have heart palpitations.  You will probably feel ravenously hungry.  On the mental and emotional side you will probably feel nervous and anxious.  You may become confused and disoriented.  You will have low emotional control and will become hostile, belligerant and easily irritated.  You may experience overwhelming feelings of being defeated, hopeless and unable to go on.  Your awareness of what’s going on around you will shrink and can arrive at an extreme form of tunnel vision in which the only thing you’re aware of is the spot on the road ahead that you’re staring at.  You may have difficulty speaking.  At the extreme, hypoglycemia can produce seizures and coma.  In a word, it sucks.

What do you do if you bonk?  You need to get your blood glucose levels up and you need to do it quickly.  Ingest simple carbohydrates that can be rapidly processed into blood glucose by the digestive system.  The best source for these kinds of carbs that you’re likely to have with you on the bike is a sports drink like Gatorade.  Other sources of simple carbohydrates include energy gels (make sure you drink plenty of water with these), sugar cubes or sweet candy like gumdrops or jellybeans.  Complex carbohydrates like energy bars will take longer to process into blood glucose and will only provide relief in the longer term.  If you catch the bonk early, you can keep riding while you  refuel.  If you let the bonk go too far, get off the bike until you recover.  You don’t want to be riding when your sense of balance is bad, you’re disoriented and you’re unaware of what’s going on around you.

After you’ve bonked and begun to recover, pay careful attention to what you eat for the rest of the ride and make sure you keep your glucose level up by regularly and frequently taking in fast absorbing carbs.  Sports drinks are very good for this.  Perhaps the hardest part of reacting to a bonk is mental.  You need to try and be aware that you’re not thinking clearly and not being as aware of what’s going on around you as you need to be to ride safely.  Forcibly arouse yourself from your lethargy and pay extra attention to what’s happening around you.  Of course this is easier said than done; it’s not easy to be aware that you’re not thinking clearly when you’re not thinking clearly. 

Whether you’ve started to bonk or not, try and get in the habit of monitoring yourself for the early signs of a bonk.  The ride beginning to feel like a chore?  Getting irritable and angry?  Not paying as much attention to what’s happening around you?  If you catch it early and replenish your blood glucose before it gets out of hand, you can usually keep riding safely and effectively.  If you have a regular riding partner whose riding abilities and demeanor on the bike are familiar to you, be aware of their condition as well.  A rider who isn’t thinking clearly is likely to misinterpret or misunderstand what’s happening to them in the early stage of a bonk.  Help them out.  Be aware that you may have to treat them carefully as they may be experiencing increased levels of irritability and hostility combined with decreased emotional control.  If you think you might be bonking, tell your ride partner so they can help you.

Most of the time bonking happens when you’ve ridden for a long time and haven’t been eating properly.  However, a bonk can also happen in unexpected circumstances.  Laura and I once rode in a week-long cycling tour through the Rocky Mountains in the Glacier National Park area in Montana and Canada.  We approached this tour as a cycling vacation rather than a training opportunity and rode at a much slower pace than we usually do.  We also stopped and had lunch mid-ride with other riders on the tour which was something we had never done before.  The lunches were great with good company and good food but we had no experience in how to incorporate eating a meal like this into a long ride.  The first time we had lunch mid-ride I completely screwed it up.  We continued riding after lunch and when we were about 20 miles from finishing I began to experience the early stages of a bonk.  What had happened?  I hadn’t pigged out at lunch but even a small lunch was much more than I would typically eat during a ride.  My stomach was full and the idea of eating either didn’t occur to me or, if it did, it wasn’t attractive.  Although my stomach was full, it was full of mostly protein and fat which could not be broken down fast enough to provide the energy I needed to finish the ride.  I was getting very little glucose from the food I’d eaten and my glycogen stores were exhausted so I started to bonk and didn’t recognize it for what it was.  Those last 20 miles were miserable; the temperature had dropped, we were riding directly into a strong headwind, and I was completely demoralized and shivering uncontrollably by the time we reached the lodge where we were scheduled to stay the night.  Fortunately, the lodge had an enormous fire roaring in the center of the main area with a bench-like hearth running around it on all four sides.  While we waited for the van to arrive with our luggage, I sat huddled by the fire in a private little world of misery slowly recovering and getting warm.  The lesson I learned?  You can bonk on a full stomach if your stomach’s full of the wrong things.

As unpleasant and dangerous as bonking can be, the good news is that it’s easily avoided.  Start your clock as soon as you get on the bike and eat regularly and properly throughout the ride and you’ll never have to experience a bonk.  Eat before you’re hungry, eat before you bonk.

48 thoughts on “Cycling Nutrition: The Bonk

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  3. I did bonk once. It was a great ride, but had to stop. Never happened again. Had some bananas and other complex & simple carbs on me. Did the trick. It’s like your brain skipping like a record does. I continued for another hour at a lower pace, to get back to my car, and kept an eye on how I felt.

  4. I just bonked this weekend on mile 42 of a 56 mile bike portion of my half-ironman triathlon…. just plain old misery.

  5. I bonk all the time and I have experimented with different ways of starting the ride. Some times eggs, soimetimes PB and J, sometimes smoothies. Thanks for writing this article. I thought I was the only one who experienced this and you described it perfectly. I´guess I have always thought that it had to do more with my physichal condition at that particular time.
    I had heard about eating and drinking more during the ride but was just ignoring so I didnt have to carry more weight. Now I know a lot more and will begin t be very carful with my eating during the week and during my rides.
    Thanks

    • I bonked yesterday 3hrs into a planned 50mile ride. I’d ridden for about three miles up a testing seated climb segment and suddenly felt completely drained & unable to continue. I diverted course and headed downhill towards home which even became quite difficult I stopped and headed for a convenience store & drank an energy drink & chocolate bar and could feel the relief flowing through my body. within a few short minutes I felt well and able enough to continue. I could have probably carried on to complete the 50 miles but thought I’d save it for another day.
      I’ve been on a low carb diet for a considerable length of time but can see how necessary they are for endurance events. I shall have to incorporate them into my training schedule for a 50 mile event coming up in the next few weeks.

  6. Thanks for the article. I’m new to riding and bonked on mile 27 (2 hours) into a 30 mile ride. Now I know how to keep my levels up during the ride. I’m heading out on a 36 mile ride now. We’ll see how it goes.

  7. I bonked 2 weeks ago when I did a 100 km not enough carbs during the ride .
    I am so grateful for this story.
    I will be aware of bonking and will be on top of it before it gets me.

  8. I dont think this has ever happened to me, fortunately. I must be doing something right! I usually carry a 3L camelbak with 1/3 gartorade 2/3 water and it usually lasts me all day in the saddle. I also make sure I carry a couple of bars of energy and a nice lunch too. Maybe I’m eating too much?!

  9. Great post on bonking, cycling bonking that is… Bonking really sucks!!
    I put a link on my recent post, I hope you don’t mind. Looking forward to read more on Tuned in to Cycling.
    AMR

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  15. Thanks for the info on bonking.. I had also faces similar issues, Will try to maintain the glucose levels to avoid bonking

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  31. I am new to cycling and bonked after 75 miles. I have bonked several times from running but never as severely as through cycling (I was sweating like mad, shivering, shaking and completely lost it .) I have experimented with running in a keto adapted state to great effect, you can measure this with a special meter that measures b ketone levels (B-hydroxybutyrate) I think I might try keto adapted cycling from now on.

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  34. Love this article and site, so much great information. I have prebonked I guess you would say. Knowing the signs can prevent full on bonking . Best advice is to know and listen to your body.

  35. That was eerily accurate, including the bit about your brain stopping working. When I was bonking and I finally reached a station where I could refuel, I wanted a sports drink. I said ‘do you have any…..’ And I couldn’t think of ‘sports drink’ so I just stared at the guy for about a minute.

  36. wow…what an excellent and accurate article….my question is : can you ride through a Bonk …and then carry on cycling afterwards (without taking on simple carbohydrates at the time when you are experiencing the Bonk. ) .
    As I understand it , at the time of the Bonk your body is using up the last of the glycogen stores from your liver to produce the energy your muscles need to power them . But then your body turns to your FAT storage to get the energy from there to power your muscles . This process is not as efficient , but none the less , once your body breaks through the barrier of using up the glycogen and proceeds to use your FAT , would I be correct in assuming you can carry on cycling ?
    The reason I ask this question is that I remember “hitting the wall” (which is a similar experience to the Bonk) whilst RUNNING many years ago ….however , I kept on running through this pain , and then I got my “second wind” and with this new lease of life I was able to keep on running easily . (I did not take on any simple carbohydrates whilst this was happening).

    • Thank you for the kind words. Unfortunately deriving glucose from fats, glycogen or ingested carbs isn’t quite this simple. First of all, liver glycogen is a glycogen source for the entire body and is highly volatile. It’s almost certainly long gone well before you start to get into bonk territory. The body doesn’t exhaust all glycogen sources and then start breaking down fats. Fat metabolism gets underway as soon as the system’s blood glucose levels are stressed. If your exercise period goes on long enough, you will be making use of glucose derived from stored fat. The problem with relying on fat metabolism as the primary source for glucose is that it is inefficient (glucose that would be better used to fuel the muscles is used for fat break down) and very slow. An increased rate of fat metabolism can be measured the day following an intense exercise period. You just can’t get enough glucose fast enough from fat to efficiently fuel ongoing athletic activity.

      “Bonking” is an old cycling term for hypoglycemia and it’s not an all-or-none event. As your blood glucose levels drop, performance declines, emotional well-being deteriorates, emotional control loosens, and your awareness of the world around you shrinks and may become distorted. You can keep going through all of this but you’re mental and emotional state becomes increasingly intolerable and you’re putting yourself at increasing risk of accident as conditions worsen. At the extremes, you can experience hallucinations and your body can shut down. I’ve seen some world-class pro riders at the end of summit finishes who looked like they might have pushed themselves into this territory. The effects are not going to go away until blood glucose levels increase and that is unlikely to happen based on fat (and protein) metabolism while the exercise is ongoing.

      The phenomenon runners call the second wind is not well understood with theories ranging from it’s related to fat and protein metabolism as you suggested, to it’s an endorphin effect in the brain, to it has to do with finding an optimal oxygen balance, to it’s a psychological phenomenon. While I’ve experienced it, I have no idea what caused it.

      • Thank you for your reply…this will really help my cycling… I have a 50 mile ride planned for the British Heart Foundation on 27th April and then a 100 mile ride for MacMillan Cancer on 6th July.

  37. Great article! For me, what I thought was the bonk was actually a lack of electrolytes (sodium). Years ago I experienced my legs being locked and had to stop and could barely just keep standing while my legs recovered. This was about 2 hours and 30 miles into a ride and had 35 more miles to go. The muscles in my quads were quite stiff and was similar to a cramp but didn’t cause that much pain. I prayed and slowly got back and pedaled real slow and easy to finish the excruciating ride. I now use S-caps, which endurance runners use, and it’s been great ever since. I also found eating a big breakfast (dinner food and portion) helps keep my energy levels up through century rides. Another VERY important thing I learned is not to start a long ride fast and ease into it, keeping my heart rate at 75-80%. Always leave some ‘legs’ to the end of the ride for some unexpected hills.

    • Hi Alex…I have had bad cramping in my muscles all my life when performing strenuous exercise…I totally stopped this pain by using SIS Electrolyte Powder… I can recommend this 100%

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  39. I undertook a 60 miler the other week having bonked the previous week after 2 hrs. I had a hearty porridge breakfast with pineapple & bananna, took a couple of Granola energy bars with along with three sports gels and 2 bottles of energy sports drinks.
    I rehydrated & had a nibble on an energy bar every 20 minutes then consumed a sports gel every hour.

    At the end of the ride I was as good as at the beginning and had enough energy to really push on during the last ten miles. I’d taken 20 minutes off my previous best over the same distance.

    Can say that I now understand how to beat the Bonk although not much use for my weight loss programme. For that I realise I’ll need to keep my level 3 excercise periods to no more than 90 minutes.

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