What Happens When You Bonk?

April 29, 2021 by
Credit: Annie Thorisdottir

Exercise for long enough, and you are bound to experience a bonk. 

It’s particularly common in endurance events where calorie intake leading into a race is vital. If you are lucky enough to experience this in a gym and not on a run, you can typically just stop your workout and grab a bite to eat. Either way, understanding the cause of this unfortunate physiological phenomenon.

You cannot talk about bonking without talking about glucose. Specifically glycogen, which is the stored version of glucose in the body. Glycogen comes in two forms, proglycogen and macroglycogen. Proglycogen is fast acting and depends highly on the carbohydrates we eat. Macroglycogen is formed in the body using glucose units from other substrates and is made and used only after proglycogen is depleted. This is why “bonking” tends to be a phased experience. There is an initial bonk followed by a recovery and then the second and more potent bonk.

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