Not scientific but i think some people bonk from lack of nutrition and others from exhaustion.
I haven’t done the wobbly marathon runner thing or bonked from lack of nutrition but definitely hit the barely able to turn the pedals on the flats from sheer exhaustion.
Good mental lesson in getting your mind to say yes when your body says no…I also never want to repeat it again
There’s a climb in Gran Canaria called the Valley Of The Tears. It’s remorselessly steep and it’s often very hot.
First time I tried it, my heart rate and breathing was rising alarmingly on the steep section so I stopped to let them subside, then carried on. I had to do this twice. That’s not a bonk, that’s just reaching your limit.
Second time it was 10 degrees hotter and I came to a grinding halt along a section that wasn’t even the steepest, and had to walk the bike for a while, leaning over the handlebars for support and taking frequent breaks, before finally re-gaining enough energy to carry on. THAT’S a bonk.
Bonked many times over the years but the worst resulted in loss of colour vision (which I only noticed when it returned), loss of feeling in face and lips, confusion and irrational thoughts. That particular episode was brought about by not preparing my nutrition properly for 100 mile MTB race (Kielder 100) and assuming I would be ok with the event provided feed stations. This was a very very bad idea. 68 miles in and I was in a whole world of hurt. Fortunately I made it to a big feed station and just sat there eating non stop for about 90 minutes. Did not finish that race but it was a big lesson learned.
That would be really wierd
This video may have been heat stroke/hyperthermia, as opposed to bonk (running out of fuel).
They obviously can occur together, but the former is more dangerous vs the latter.
Hyponatremia (excessively low sodium) can also show some similar signs to these.
The best treatment for each is different - so if you find yourself in a situation where you feel like $hit, good to have some idea as to the cause.
Super low blood sugar. This is the worse when you’re somewhere unfamiliar, like a long race course or in the middle of a national forest. Once you bonk, you’re done. You can recover from it, but the race is done. I never leave home without food, even the easiest/shortest rides. A gel can make or break a ride.
@DaveWh Has she said that? Or does the triathlon community just know that now because of her reaction/“symptoms”? Reason I ask is that besides being an amazing champion (“Queen of Kona”), that clip has become synonymous with “bonking”. When ppl use the term, it literally flashes in my mind. I can even hear Tesh’s voiceover. But I’m more than happy to be corrected on this one. 
As you and others have alluded to, the term has become somewhat of a catch-all for hydration and nutritionally related degradation in performance. Not quite as vague as “Hitting the Wall” for marathoners back in the day (could be several causes, not always fuel).
I couldnt remember her name but this is the video clip I was thinking of when I was trying to describe bonking.
You have nothing left literally
@John_Hallas We’re aging ourselves! hahaha
Only once for me about 10 years ago, it was ugly. Moved my TT saddle forward nearly 1cm before a big interval workout. Only made it through 75% of the intervals before I realized something wasn’t right. Turned for home and had about 35 miles to go with a couple of climbs. Second climb was seeing stars and feeling like blacking out. Sat on the side of the Pacific Coast Highway for close to an hour, completely disoriented as dusk settled in. Walked about a half mile to the top of the climb, coasted down but almost crashed twice. Pulled into a gas station and called my fiance to pick me up, not a good day.
That’s it for me too. Basically out of energy to push, I can basically go for 50% of FTP at best.
I recall seeing this video used as an example of heat stroke a while back.
“heat exhaustion is what Paula Newby Fraser famously exhibited in the finishing miles of the ’95 ironman Hawaii”
Agree, “bonking” is often used as a catch all phrase that could encompass a variety of conditions.
Well, there ya go! I edited my response.
I had thought I had bonked a few times until I really bonked the first time I did the Tahoe Trail 100. Thought I was doing fine with food and hydration until about 10 miles from the finish starting the last climb back to Northstar. Was climbing along and then out of nowhere I had nothing…my hr spiked, my arms and hands went numb and I could hardly move. I had to get off and sit in the shade…I had a gel and some electrolyte drink and tried to ride…all I could do was crawl along. If somebody would had offered me a ride back I would have taken it. I ended up finishing but felt like crap for a few days.
“worst resulted in loss of colour vision (which I only noticed when it returned),”
I had that twice last year (2nd time in my SM100 DNF). I ended up crashing in a rock garden and splitting my knee open pretty good. After medical bandaged me up trailside, I decided to ride the 8 road miles back to the aid station where my wife was volunteering. After significant hike a bike on the normally easy road section back, I arrived at the aid station and was informed by my wife that we were going to the Dr to get my knee cleaned/stitched up. Once at Prompt care, before they did anything, the room turned white and the next thing I knew, they were offering me crackers and a Coke. Then they informed me that I had had a seizure of some sort, passed out and my blood pressure had dropped to the point that they couldn’t feel a pulse. After a couple cokes, fries and a burger, I was “ok” although I didn’t want to look at my bike for about a week…
I bonked at about mile 18 of a marathon. Legs felt like lead, you feel like you have no energy and have been transferred to a planet with twice the gravity of Earth.
I had a bad nutrition plan or lack of one.
Have your FTP be 345. Bonk. Think you’re setting a nice tempo pace in a race, look down and it says 75 watts.
bonk
To be serious there are threedifferent things that I think happen to people:
The last bonk I had, I ended up in a cold shower, lying in the tub with all of my cycling kit still on to cool off while drinking a tallboy of CocaCola and it probably took me 45 minutes to feel human again.
Never again.
Like warm apple pie
This. The bridge signals, and the engine room just doesn’t respond. Seriously, the one time I experienced it, I couldn’t go any faster than 12 kmh/8 mph on level ground. It took me an hour to crawl to the nearest town, where I downed a liter of “fat” coke. After 20 minutes, it felt like someone had turned the lights back on. But I kept it pretty much to zone 1 for the 60 km ride home.
It’s different to fatigue, where the muscles hurt and say “please stop”. This is more a case of “I’ve got nothing”. (Although in practice, if you have ridden enough to bonk, you are probably fatigued as well - assuming you haven’t tried to ride Alpe d’Huez fasted).