onking is a bit like losing your virginity: it’s a right of passage that’s disorienting, physiologically complex, and ultimately a learning experience to be improved upon next time. If you’ve never done it, you’re just not trying hard enough. My first time was a well-documented 70.3 distance triathlon in New Hampshire in which I alternately was cold, could barely move one foot in front of the other, felt dizzy and lightheaded, and experienced Darkenfloxx-level symptoms of psychological despair. A coach I’d been corresponding with ultimately concluded that I just hadn’t eaten enough during the triathlon — 700 calories instead of three times that.

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In my case, that may have done the trick, but there’s a lot happening in the body that’s implied by the catch-all word “bonk” (a.k.a. “hitting the wall”). While the resulting symptoms can occur at once as a symphony of pain and delirium, it’d be a mistake to think they all have the same cause and the same treatment. In my opinion, the the most valuable distinction for beginner long-distance runners is between dehydration and a glycogen bonk, or, generally speaking, running out of stored carbs to burn. I’ve run with a lot of people training for their first marathon who don’t start drinking water until they’re starving for it eight miles in, or who generally just train without water. This is a good recipe for poor performance and bonk-like symptoms.

In the months leading up to the Vermont 50, I tested a handful of nutrition products while trying to come up with an optimal nutrition plan for the race. One product that addresses the hydration problem specifically is Osmo Nutrition, a powdered drink mix designed to optimize fluid absorption rather than replace calories. It was developed by Dr. Stacy Sims, an exercise physiologist and nutrition scientist — not to mention endurance athlete — who has worked with the Garmin-Slipstream crew, Lance Armstrong, and Xterra champion Dan Hugo.

This dehydration leads to the dead legs that people perceive as a bonk caused by insufficient calories, and it takes hours to feel better.

As Sims explains it, exercise causes blood flow to shift from non-essential organs to the working muscles and to the skin, where it offloads heat. When we sweat and lose body water, our blood volume goes down (blood being largely water), which means there’s less total blood going to working muscles and skin. “The other part is the competition between the blood and the skin — and the skin always wins out”, Sims says. “As blood volume drops and you have less effective circulation, the blood flow to the skin isn’t going to be compromised as much as muscle blood flow because the heat needs to be offloaded, and the blood isn’t going to the muscle because that’s what causes heat production.” This dehydration leads to the dead legs that people perceive as a bonk caused by insufficient calories, and it takes hours to feel better.

This is just the tip of the iceberg in the bonking conversation: there’s muscle glycogen, liver glycogen and blood sugar, all of which have unfortunate physiological consequences when they get low. There’s even a guy at Harvard who has a formula to predict when you’ll exhaust your glycogen stores, which he can use to prescribe an appropriate marathon pace and mid-race fueling strategy. For now, though, while we’re thinking about pushing our limits with ultramarathons of 50k or more, it’s a good start to make sure we understand the most fundamental challenges we’ll have to overcome. It’d be nice to say it’s as simple as drinking more water; it’s more like decoupling fuel calories from hydration and making sure hydration includes the appropriate salts and sugars to maximize fluid absorption. Still, that’s less complicated than sex.