“No matter what I say to myself about my ability, I can’t make myself go any faster. The speed tails off, I can’t keep up, and I feel hopeless.” – Kathryn Kempton, Big Orange Cycling team rider
“You’re stuck inside your mind. You’re willing to eat someone’s peanut butter and jelly energy bar even though you hate peanut butter and jelly. All you want is pizza and ice cream. Possibly a nice Greek omelet.” -Tyler Fradkin, Big Orange Cycling team rider
“Those dreams where you’re trying to outrun a train—it’s the inverse of that. You are chasing a train that is speeding away. The body fails in every way imaginable.” – Bladi Duran, Big Orange Cycling team rider
Any committed exerciser knows the feeling. Your workout is going great; you’re totally kicking butt—then, suddenly, BAM! Every footstep, pedal stroke, paddle stroke, rep, or jump becomes an epic challenge. If this is a race, you’ve lost.
The colloquial term is “bonking.” Sometimes it’s called “blowing up” or “hitting the wall.” The French call it la fringale. Simply defined, it’s when you run out of glycogen—a carbohydrate supply stored in your liver and muscles—in the middle of a hard effort.
Some authorities further define it, claiming that hitting the wall means running out of muscle glycogen while bonking is when you’ve run out of both muscle glycogen and liver glycogen. But, frankly, whatever. From an athlete’s point of view, it all feels pretty rotten and the procedure for dealing with it (or avoiding it in the first place) is the same: you need to get fast-absorbing carbohydrates into your system.
What Is Bonking?
“Vertigo, nausea, extreme fatigue, carelessness.” – Marilyne Deckman, Big Orange Cycling team rider
Your body’s sole energy source is a molecule called adenosine triphosphate, or ATP. Your body creates ATP in a number of ways and, because it’s in short supply, you constantly need to produce it. When you’re exercising, the demand obviously increases.
There are three primary raw materials for creating ATP. First, there’s glucose (blood sugar) and glycogen, which can be converted quickly. If your stores are topped off, the average person has about 90 minutes (or about 1,400 to 1,800 calories worth) of these carbohydrates.
Next is body fat, which converts to ATP much more slowly. You have fat in your muscles as intracellular triglycerides and almost everywhere else as adipose tissue. Even thin people have a huge supply of fat from an exercise perspective.
Finally, there’s muscle protein—as in your body breaks down muscle for fuel. (For the purposes of this article, we don’t need to worry about this one too much, but here’s a quick pro-tip: Muscle protein is your body’s least-preferred fuel source. You can minimalize using it by eating adequate calories, particularly carbs.)
The harder you work, the faster you need ATP, so your body is going to favor glucose and glycogen. When you ease off, your body switches to fat-burning mode because you don’t need ATP as quickly.
When you put in a hard effort (racing, high intensity training, etc.) for longer than 90-minutes, and you’re not able to consume carbohydrates fast enough to refuel your glycogen stores, you run out and your body shifts to burning fat. But because fat can’t change to ATP as quickly, you are also forced to slow down. A lot.
This is bonking.
Burn More Fat? Hooray for Bonking!
Slow down there, weight-loss seekers. It’s not that easy. When you bonk and continue to exercise, your body considers it an emergency, so it also breaks down muscle protein to give you as much fuel as possible. Also, as the Big Orange team riders clearly state, you can’t exercise as hard in a glycogen-depleted state. Put these two factors together and it’s clear that you’re breaking yourself down more than you’re building yourself up, so it’s not doing you any good.
What’s more, bonking causes serious physiological stress, which can put your immune system on high alert and result in inflammation.
Finally, in addition to any physiological damage bonking causes, your brain uses glucose as fuel. This means you can be mentally impaired when you bonk, which is never a good idea when you’re going 30 MPH on two very skinny wheels, wearing nothing but Lycra, a helmet, and a grimace.
Also keep in mind that there’s a difference between fasted-state training, during which glycogen stores are depleted, and training without glycogen (AKA while bonking). Many endurance athletes like to train with low glycogen in hopes of forcing their body to better utilize fat stores. Although a potentially fruitful strategy, it’s different from trying to push through a bonk without feeding because you still have glycogen on reserve.
Bonking: Not Just for Athletes
It’s important to note that the 90-minute glycogen countdown isn’t set in stone. There are times when a body can go way longer on existing stores, and times when you’re lucky to make it past a half hour.
Factors include your diet, how well your body is adapted to burning fat, the state of your glycogen stores, how much glucose you have circulating in your blood, and the size of your muscles. And it’s not like you flip a switch between glycogen and fat. It’s more of a sliding scale. Even when you perceive yourself as exercising full-tilt, your body still taps fat stores to produce ATP, albeit not to the extent that uses glycogen.
Also, 90-minutes assumes your glycogen stores are 100-percent topped off, which is highly unlikely. Given their high volume of training, most endurance athletes live in a chronic state of low glycogen. But many non-athletes regularly short-shrift their glycogen as well. If you’re eating at a caloric deficit or eating a low carb diet, for example, it’s easy to bonk in far less than 90 minutes. So if you’re on a restrictive eating plan, take regular bonking as a signal to increase your daily calorie intake.
How Do I Know I’m Bonking?
“A full bonk is where I lose all power as well as mental willingness to continue. It is generally accompanied with tunnel vision, rapid heart rate, and negativity, hopelessness, and desperation.” – Matt Miller, Big Orange Cycling team rider
Okay, so that’s pretty extreme, but the gist is accurate—when you bonk, you just feel done. Also, given your brain lacks glucose, bonking can leave you feeling anything from foggy to disoriented to completely goofy.
In addition to how it feels, another important indictor is how it hits. Usually, you think you’re okay and then suddenly you crash. On the other hand, if you start your workout feeling meh and it just goes from bad to worse, a lack of glycogen can certainly be a factor, but you’re probably also tired, over-trained, stressed-out, or having a bad day.
Another indicator is when you’re doing something you know that you’re normally quite capable of—and it becomes inexplicably difficult.
How Do I Prevent Bonking?
You eat. Bonking happens due to a lack of fuel, so the solution is to fuel up before you work out. How to do this depends on your situation.
If you’re preparing for a big event like a marathon, triathlon, or gravel grinder, your best bet is to increase food intake, especially carbs, for three days leading up to your event to top off your glycogen. The easy way to do this is to chow down on bread, pasta, and burritos for a few days and then have a solid breakfast two to three hours before your event. (Check out “How to Eat Leading up to Your Big Endurance Event” for a more detailed discussion of carb loading.)
During-event feeding is also a good idea if you’ll be hitting it hard for more than an hour. Unfortunately, the human body was poorly engineered in the sense that it can’t absorb as many carbs hourly as it can burn, so trying to replace what you’re burning as you’re burning it is a tricky business. (Here’s an article outlining the best way to go about it.)
If you seem to be bonking on a regular basis during training, try eating 50 to 100 calories of fast-absorbing carbs (e.g., a banana) about 10 minutes before you work out. That should boost your blood sugar enough to push you a bit further. If bonking is a reoccurring issue, however, take it seriously. Perpetually low glycogen is bad news. In fact, there’s a theory called the “Glycogen Hypothesis” that claims that remaining in this state can lead to overtraining syndrome, which is basically the serious exerciser’s version of chronic fatigue syndrome. But unlike CFS, fixing perpetually low glycogen is easy: Eat more carbs throughout the day (not just before exercise).
What Do I Do Once I’ve Bonked?
“I have to pop something with a ton of sugar – which for me has been gels – real food doesn’t work.” – Kathryn Kempton
At this point, you’re just grinding your gears. So if you’re probably better off calling it a day, if you can.
Unfortunately, it’s rare that one bonks conveniently. You’re typically a couple hours into a race, ride, run, or other activity that requires at least a couple more hours to get to a stopping point. Let’s be real: If you’re 20.5 miles into your 26.2-mile marathon, you’ll be damned if you let 5.7 miles of senseless agony get between you and a finisher’s medal.
In these situations, the solution is simple: Consume fast-absorbing carbs ASAP. Given you’re burning fuel as fast, if not faster, than you’re consuming it, this is one of those rare times when big sugar hits are a good thing. Just keep two things in mind. First, odds are that this bonk is going to hound you for the rest of your event. You might suck down a sports gel and suddenly feel restored, but you’re going to blow through those 100 calories pretty quickly, so don’t be so quick to burn that match. Instead, soldier on at a reduced pace in hopes that you can modulate between fat and carbs before re-bonking.
Also keep in mind that your glucose-deprived brain might throw some serious flavor fatigue your way, meaning you might suddenly find the things you know you should be eating and drinking repugnant. But you need the fuel, so choke them down anyway, unless you have the luxury of choice, in which case smart nutritional choices can take a backseat to surviving the next few hours. Sure, an electrolyte-balanced sports drink might be ideal, but many a bonk-savvy cyclist acknowledges the sugar, caffeine, and water trifecta of an ice-cold Coke as “survival in a can.”
Like it or not, bonking happens. And when it does, it’s awful. But as long as you manage it correctly, you’ll get through it. Just think of it as good lesson on your body’s relationship with carbohydrates—and a great story to tell over linguine arrabbiata the next time you’re carbing up with teammates before for your next big event.