Hypoglycemic Versus Dehydration Bonking During Endurance Sports

There are many ways to come to grief on the bike, some of which are called bonking. From the 'Tuned in to Cycling' blog.

There are many ways to come to grief during endurance training, some of which are called bonking. Click image for link to source.

FitOldDog's Paleo Ironman

I suffered from a hypoglycemic bonk when I tried to complete the Lake Placid Ironman, completely Paleo. Needed some carbs during the race. Click image for linked blog post.

The hypoglycemia bonk: I have experienced this kind of bonk during a couple of full Ironman races, about 100 miles into the bike leg, and in the later stages some long training rides when I did not take due care to eat and eat right as I went along. The key symptom was nausea, in association with a feeling of muscle weakness. During the two races mentioned above, I noticed that my wattage dropped for the same perceived effort prior to the onset of nausea. Vertigo was minor or absent. I since learned that the solution is to eat, especially if you don’t feel like it. If you feel nausea coming on deep into your exercise, eat as soon as you can, using moderate glycemic index foods. I eat chocolate-flavored PowerBars. You have to find your own solution by trial and error. Here is a quick piece of information from the Tuned In To Cycling blog about this type of bonking.

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.

The remarkable machinery of the inner ear provides sensory systems for hearing, balance, and acceleration/deceleration.

The remarkable machinery of the inner ear provides sensory systems for hearing, balance, and acceleration/deceleration.

The dehydration bonk: I have experienced this kind of bonk one time, and one time only, and I hope to never go there again. I described my experience in a previous post, bonking out of context. This bonking event occurred during a demanding swim, preceded by five days hard training in the desert around Tucson Arizona, and not following my coach’s orders to take it easy when I got back, after spending many hours in airplanes on my way back. Plane flight is known to induced dehydration. I also didn’t notice that I was more then five pounds under-weight on my return (found that out after my bonk, when my cycling buddy, Rory, suggested I might just jump on a scale). The key symptom was vertigo, which brought on nausea, like a feeling of severe seasickness. Thinking back on it, I was very thirsty during the swim, but too busy swimming to go drink some water! That’s all it takes to bonk, just ignore the subtle symptoms and your body will knock you over the head with clearer message. Ended up in the emergency room on that one, but I never did get the treatment I needed, intravenous fluids, but I did get drugs to stop the nausea that induces vomiting, which wastes even more fluid and critical electrolytes. Dehydration is dangerous, as explained in a post from the LiveStrong.Com website, which I find to be a mine of useful information.

“Fluid imbalances in the inner ear lead to several symptoms and conditions — some more serious than others. Dizziness or vertigo, tinnitus, fullness in the ears or hearing loss can stem from inner-ear fluid imbalances, according to Loyola University. You may also experience a loud noise or “rushing” sound in your ears and loss of coordination or balance. During an Ironman competition, these symptoms are dangerous. Falls from your bike or while running can cause injuries. If you fail to hydrate, serious — and sometimes lethal — health consequences can occur.

When it comes to bonking, an ounce of prevention in the form of an appropriate hydration and nutrition plan is worth a ton of cure. Learn from my mistakes and save yourself a lot of pain and risk.

-k @FitOldDog

Run at the track for as long as feels good, with gentle pickups, as I have to recover from jetlag and of course, travel-induced dehydration – better go check my weight!!!!


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Disclaimer: As a veterinarian, I do not provide medical advice for human animals. If you undertake or modify an exercise program, consult your medical advisors before doing so. Undertaking activities pursued by the author does not mean that he endorses your undertaking such activities, which is clearly your decision and responsibility. Be careful and sensible, please.