The Science of “Bonking” and Glycogen Depletion
“Bonking” in a marathon is a miserable experience.
Bonking, or “hitting the wall” as it is also known, is a well-known phenomena among marathon and ultramarathon runners which occurs when your body runs out of sugar to burn.
Sugar, stored in your liver and muscles bound into large chains called glycogen, is the prime fuel for a distance runner. While your body can burn fat directly for energy, it tends to prefer glycogen, as it is easier to burn. Much of the lore about the marathon being “half over at twenty miles” has to do with the fact that this is about as long as the average person’s muscle glycogen stores will last.
In this article, we’re going to look at the science of “bonking” so you better understand what might be going on and develop a a specific nutrition strategy to help you run to your absolute best on race day.
The science of glycogen depletion
An intriguing computational approach to glycogen depletion, published by Benjamin Rapoport of Harvard University in 2010 illustrated that your probability of hitting the wall during a marathon is contingent on the amount of glycogen you store in your muscles, how fast you run, your pacing strategy, your body size, and your muscle mass.
There’s a lot of interplay between these variables—for example, a faster pace burns a higher percentage of glycogen vs. fat, but also allows you to cover ground faster—but the upside of all of it is that most people don’t have the glycogen reserves to make it all 26.2 miles of a marathon at a fast pace without running out of fuel.
Additionally, there’s a factor that Rapoport didn’t take into account: new research suggests that the brain anticipates glycogen depletion and slows the body down gradually to conserve energy.
While it’s well documented that actually ingesting carbohydrates improves performance in long (2+ hour) competitions,2 simply rinsing your mouth with a carbohydrate drink, then spitting it out, seems to help too. Obviously, no significant amount of sugar could make its way into your bloodstream through rinsing your mouth; the performance boost must be coming from the brain sensing that more carbohydrates are on the way.3 So it’s reasonable to propose that performance will be inhibited significantly before your body actually runs out of glycogen.
It’s a given that on race day, you’ll want to be consuming some kind of carbohydrate drink during competition. But what about during training?
Training in a glycogen depleted state
A 2011 study by Karen Van Proeyen et al. investigated the effects of training in a glycogen depleted and glycogen-replenished state using twenty young male cyclists.4 The men were split into two groups, both of which had equivalent diets and training regimens. The first group did all of their training after an overnight fast, while the second group took a carbohydrate-rich breakfast about 90min before their daily training session (a 60-90min bike ride at a fairly hard pace in the morning).
After six weeks of training, both groups had improved a similar amount on a 60-minute time trial. However, there were several changes in the “fasted” group that indicated that their bodies had adapted to more efficiently burn fat as fuel.
- First, levels of enzymes associated with fat metabolism increased significantly in the group which trained after the overnight fast, but not in the group which had a large breakfast before training.
- Additionally, their fat utilization increased throughout a range of intensities. That is, they could maintain a given pace with less reliance on glycogen, enabling them to last longer in a race without hitting the wall.
Another study (albeit of lower quality, as it used only a handful of untrained men as subjects) by Nybo et al. confirmed the findings of Van Proeyen et al.5 The subjects who trained in a fasted state in the Nybo et al. paper also increased their fat burning abilities more than the carbo-loaded subjects; furthermore, they also increased their stores of muscular glycogen.
While you might suspect that these changes would have enabled these untrained men to lose more weigh, there was no difference in the weight lost between the men who trained after an overnight fast and the men who did not.
Other factors related to training and glycogen depletion
So, it appears that doing some training in a glycogen-depleted state can be of benefit to marathon runners. But there are some important considerations to be made.
First, keep in mind that the fasted-training subjects in the Van Proeyen and Nybo studies did not get in better shape than the carbo-loaded subjects! Their improvement from training was identical; the only differences were related to fat metabolism. If you were training for a relatively short race, like a 10k or a 5k, there would be no advantage to doing long runs after an overnight fast—the race isn’t far enough for glycogen storage to become a factor.
Second, the ability to run a given pace with less reliance on glycogen does not necessarily mean your aerobic endurance has improved (i.e. your oxygen consumption at the given pace).
Finally, there is a very practical reason to practice consuming carbs before and during a long run: you have to be able to do it when you race! It definitely takes training to get used to eight or twelve ounces of sports drink sloshing around in your stomach after ten miles of running, and you definitely want to make sure you can tolerate your pre-race meal.
Setting aside some non-critical runs to be done right away in the morning (i.e. without breakfast) appears to be a good way to increase your ability to directly burn fat while running.
Recommendations and best practices
If you’ve had issues hitting the wall at 18 or 20 miles into a marathon, you should definitely consider including some fasted training in your schedule. But you should also practice eating a big breakfast and consuming a sports drink during your run at least a few times during your marathon build-up, since you’ll have to do it on race day.
If you have “dress rehearsal” workouts (e.g. a 15-miler at marathon pace, starting at the same time as the marathon and on a similar course), you’ll definitely want to be consuming carbohydrates before and during your run.
And if you’re training for something shorter, don’t sweat it—your fitness will be the most important factor.
For more information about the marathon and energy/glycogen, consider these other helpful articles we’ve published previously: