When I was a young climber, I was making my way up the Roman Wall near the top of Mt. Baker. And that's when I felt it. All of my energy was just gone. I felt like I wanted to sit down and go to sleep. But instead, I ate a bar, and suddenly felt good again.
What I experienced is often referred to as "bonking," or "hitting the wall." Wikipedia has a pretty good definition of what happened to me:
In endurance sports such as cycling and running, hitting the wall or the bonk is a condition of sudden fatigue and loss of energy which is caused by the depletion of glycogen stores in the liver and muscles. Milder instances can be remedied by brief rest and the ingestion of food or drinks containing carbohydrates. The condition can usually be avoided by ensuring that glycogen levels are high when the exercise begins, maintaining glucose levels during exercise by eating or drinking carbohydrate-rich substances, or by reducing exercise intensity.
Here's the thing. Your body will store between 1,500 and 2,000 calories of glycogen. When you're walking up hill with a pack, you rip through those calories. It's estimated that a mountaineer on 30-to-40-degree terrain will burn 700 to 1000 calories an hour. That is a massive amount when it comes to glycogen in your system.
Climbers on Steep Terrain on Mt. Baker
When I'm guiding, I like to tell people that they should eat something, drink something and use the bathroom (to go #1) at every break. If they don't feel like eating or drinking, and they don't have to pee, it's all the more important that they eat and drink.
It's not a bad idea to "carbo-load" the night before the climb either. These carbohydrates will help decrease the likelihood of the bonk.
In my case, eating a bar on the Roman Wall was pretty terrible. The route is a mountaineering route, but the Roman Wall is the crux. It's not a good place to take your pack off for a break. If I'd been diligent I wouldn't have had to deal with that.
In addition to all this, training helps. A lot.
Part of the reason you bonk is because your body is switching from the use of glycogen to fat stores. The more you train, the more your body is able to make this transition without "hitting the wall," at least without hitting it as hard as you do when you don't train...
--Jason D. Martin