By Eve Glasergreen
One of the many joys of distance running is the challenge of pushing your body faster and farther. If you’ve experienced bonking, or “hitting the wall,” you’ve likely pushed your body to a new limit. Bonking - when your body and mind run out of readily available energy stores - can happen whether you’re tackling a Sunday morning long run or racing a marathon.
Luckily, bonking isn’t inevitable; it’s preventable. The solution? Bolstering your energy stores. With a deeper awareness of the ways that your body harnesses fuel, you can ride your endorphins for much longer. Even though hitting the wall can feel awful, it’s not something to fear, but rather something to manage.
What Your Body Is Experiencing When You Bonk
When you run out of readily available energy stores during exercise and bonk, it’s the result of several factors including your fueling, exercise duration, and intensity. On a recent phone call, Toni-Lynn Salucci, a registered dietician nutritionist and competitive distance runner, offered a clear step-by-step breakdown of the bonking process. She explained:
- Your body breaks down carbohydrates into glucose, which is a kind of sugar.
- The glucose in your bloodstream is the body’s most available source of energy; it’s already within the body’s nutrient transportation system, and cells can readily absorb, convert, and use it.
- Your body breaks down glycogen when needed during exercise. Glycogen is glucose stored in muscles and in liver cells for later use.
- When you get to a certain duration of exercise, or if you’re exercising at a heightened intensity, you eventually exhaust your easy fuel sources: the glucose in the blood and the stored glycogen in your muscles.
- The body starts shutting down your muscles, and it doesn’t care whether you’re trying to finish a marathon or not.
To be clear, carbohydrates are not the only fuel your body uses or needs. Your body also processes fats and proteins. In fact, your body contains enough fat to run nonstop for about five days. However fat stores are much harder to access, whereas your muscle glycogen stores are easily accessed but are only good for about 100 minutes.
Before Reaching Empty, Find Out How Many Gallons Are In Your Tank
Anyone who runs experiences fatigue or tiredness. That’s a natural part of training—but what makes bonking different? Energy depletion. Bonking happens when you’ve used up your fuel.
How long does that take? Well, that depends. Multiple variables affect any number that gets thrown out there, including:
- Body fat
- Metabolic rate
- Fitness level
For illustrative purposes, on average, the body can store about 625 grams of glycogen and glucose—500 grams of muscle glycogen, 110 grams of liver glycogen and 15 grams of glucose. Each gram converts into roughly 4 kcal of energy, which means you have approximately 2,500 available calories when your tank is full. Assuming you burn 100 calories per mile (another average), you’re going to hit depletion status somewhere between miles 16 and 25.
At that point, you might slow dramatically, feel extreme fatigue, experience brain fog, and/or feel uncoordinated. Remember, the body isn’t the only thing that uses glucose as fuel; the brain does too. Your system will prioritize the brain over the body, so if you start experiencing symptoms that affect your head, such as blurred vision, confusion, headache, or light-headedness, it could mean you’re nearing empty. If you don’t replenish your stores, you might be stopped in your tracks. Literally.
It's Not Just How Far You Run, It’s How Hard
In addition to exercise duration, the effort level, or intensity, of your exercise changes your risks of bonking. Let’s dig into that a little further:
Higher intensity exercise:
Even if the total duration of activity is shorter, high-intensity exercise exhausts more glycogen and blood glucose. As muscles increase the frequency of their contractions, they prioritize breaking down their stored glycogen because it’s local. If your breathing is labored, or if it’s difficult to maintain a conversation, that’s an indicator that you’re exercising at a high intensity and that you’ll burn your glycogen stores more quickly. If you aren’t breathing easily, you’re providing your body less oxygen, which inhibits the breakdown of fatty acids.
Lower intensity exercise:
When you stay within comfortable paces and when you can carry a conversation while running, you’ll burn more fat in addition to glucose. Essentially, your body isn’t rushing to break down muscle glycogen to keep up with you. Therefore, your glycogen stores will last longer at lower intensities. If you can talk comfortably, your body likely has the oxygen it needs to tap into the fat sources of fuel. Interestingly, studies have shown that women are less reliant on carbohydrates for fuel, potentially giving them endurance benefits.
There are multiple strategies you can learn to not only prevent bonking, but to help you enjoy your runs and races all the way through to the finish. A well-fueled runner is a happy runner.
How To Fuel The Tank
By addressing your fueling before, during, and after your hard training efforts, you can prevent bonking, and build healthy fueling habits for your long-term wellbeing.
Start fueling before the race or the long run. Toni recommends that you “top-off” your glycogen stores in the days leading up to your race. The longer the intervals are between your meals and snacks, the more you’ll dip into the energy stored in your body. To maximize your stores, you’ll need to eat frequently enough. Frequently enough means every 3-4 hours at a minimum. It also means listening to your hunger cues. If you’re hungry 2 hours after you ate lunch, trust your body, and have a snack! That way, you don’t tap into the glycogen stored in your muscles.
Make fueling part of your training. When you know how you respond to a specific fuel source, you’ll feel confident about using it on race day.
Another great way to empower your running and to prevent bonking is to make a race plan. Note the pace that keeps you within your wheelhouse and pay attention to what that level of intensity feels like, so that you’ll be able to recognize it on race day. Plan the times when you’ll take your nutrition during the race and pack accordingly.
When it comes to fueling during exercise, Toni says that you should fuel if you’re moving for over an hour. Some other tips for fueling during running include:
- Fuel early in the run so your body has time to absorb the energy. You might get a mental refresh just from tasting the sugar in your mouth, but you still need time to digest.
- Let your tolerance guide the frequency of fueling. If you’re able to consume sports drinks every three miles in a marathon, take advantage of the fuel.
- Don’t wait until you feel hungry. Your stomach might not feel like it needs fuel, but your muscles do. They just don’t have a convenient way to alert you, unlike your grumbling stomach.
- Don’t wait until you feel fatigued. Waiting for signs of bonking means that you’re already exhausting your energy stores. Be proactive with fueling.
The longer the race is, staying conservative early and building into the effort is a general guideline for maximizing your energy. Pacing yourself is key.
If you hit the wall during a race, it’s time to listen to your body. Respect it and make good choices. You wouldn’t continue to try driving your car without any gas (or electricity), so offer yourself the same consideration. Whatever you choose, you can come away with a learning experience and improve your fueling for next time.
Anytime that you finish hard exercise, whether you bonk or not, it’s vital to start recovering. Toni advises that you eat thirty minutes to an hour after exercise, saying it’s ideal for rebuilding glycogen stores as quickly as possible. You might even crave a particular food. In that case, go with your gut.
Otherwise, focus on foods high in carbohydrates and lower in fiber to replenish your glycogen stores, such as:
- Chocolate milk
- Starchy vegetables
Overall, you want to consume foods that are easy to digest and high in available energy, i.e. high in calories.
Ask An Expert
You can find nutrition solutions that are right for you by experimenting on your own. However, it’s important to note that sports dieticians and nutritionists are for runners of all performance levels, not just elite athletes. Dieticians can write specific fueling plans for race day or spot fueling gaps in your day-to-day consumption.
Whether you train exclusively on the weekends or throughout the work week, you can benefit from a deeper understanding of how to fuel properly. The next time you go on a long run with your slightly faster friend, you might find that a mid-long-run snack keeps you going. Not only will you perform better by harnessing nutrition, but you’ll enjoy the process of training and racing even more.
This article was reviewed by Dr. Linda Alvarez, MD, Levelle's Co-founder and CEO.
Eve Glasergreen is a writer and poet living in the Finger Lakes. If she’s not at her desk typing and drinking coffee, you can find her running mileage, cuddling her cats, and enjoying nature.