Maybe you’ve experienced it. Maybe you’ve ramped up your training and are dreading it: You're three hours into a long run or ride and—out of nowhere—your legs turn to lead, your pace drops, you can’t think clearly, and you’re not sure if you’re about to throw up or fall over. 

That’s bonking.

It’s not just being tired. Bonking (also commonly referred to as “hitting the wall”) is an extreme state of energy depletion—when your body runs dangerously low on carbohydrate-based fuel, especially muscle and liver glycogen, and your brain starts to misfire. Continuing can feel almost impossible.

If you’re an endurance athlete, you’ve likely been there or come dangerously close. But strength athletes aren’t immune either. This article covers:

 

What is Bonking?

Bonking is a sudden onset of severe fatigue during prolonged physical activity, typically brought on when the body runs through both glucose (recently consumed carbohydrates) and a large portion of its stored glycogen—the form of carbohydrate stored in muscles and the liver that fuels sustained effort.

Put simply, glycogen is your body’s fuel source. It drives muscular contractions and helps regulate blood sugar. It’s used in both aerobic and anaerobic efforts, but it becomes the primary fuel—especially during higher-intensity or anaerobic exercise.

Once your body burns through roughly 70–85% of its glycogen—if you don’t refuel—performance starts to fall off a cliff.  Research shows peak power output is impaired when muscle glycogen drops below about 70 mmol/kg wet weight, which translates to having only 35–45% of your fuel tank left. At this point, your legs may feel heavy, neuromuscular coordination suffers, and mental focus starts to slip.

Importantly, glycogen stores are never completely emptied. Your body holds back around 10–20% as a reserve to keep critical systems—like your brain and blood glucose—running. This reserve is why bonking doesn’t make you collapse on the spot—but it does make sustained performance nearly impossible.

While bonking is most often associated with endurance sports like cycling and running, resistance and high-intensity training can also drain glycogen significantly, meaning it’s also a risk during any long or high-intensity effort—especially if your fueling or hydration plan isn’t dialed.

 

Why Does Bonking Happen?

In short, bonking is the result of running out of carbohydrate fuel faster than you can replace it—but several factors can accelerate or increase your risk. 

1. Insufficient Carbohydrate Intake

The most obvious cause. Whether you start under-fueled, under-eat during long training, or just don’t meet daily energy needs, your glycogen stores can get too low to sustain performance. Research shows endurance athletes need between 7–10 g of carbohydrate per kg of body weight per day, depending on training volume and intensity. If you aren’t sure where to start, read more on hydration and fueling strategies for long training.

 

2. Depleted Muscle and Liver Glycogen

Carbohydrate is stored in two main places: your muscles, which use it to contract, and your liver, which regulates blood sugar. Because glycogen stores are limited, depletion can happen in as little as 90 minutes of moderate-to-high intensity effort, with high-intensity workouts using glycogen even faster. 

 

3. Prolonged or Back-to-Back Sessions

Whether you’re doing a long trail run, back-to-back WODs, or a multi-hour ride, time and volume matter. The longer you go without replacing carbs, the more likely you are to hit the wall. Even small carbohydrate deficits add up if you’re training hard across multiple days.

For sessions less than 8 hours apart, carbohydrate recovery becomes time-sensitive. Aim for 1.0–1.2 g/kg/hr in the hours post-workout to help replenish glycogen quickly.

 

4. Inadequate Hydration and Electrolyte Balance

Being dehydrated—even just 2% down—can make a session feel way harder than it should. That’s because fluid loss doesn’t just affect your sweat rate—it impacts nutrient delivery, body temp, muscle contraction, and mental clarity. And if you’re already running low on carbs, this only accelerates how quickly that bonking occurs.

While we’ve focused on hypoglycemia so far (low blood sugar), hyponatremia—a drop in blood sodium from overhydration or inadequate sodium replacement—can also trigger bonk-like symptoms. Think: fatigue, dizziness, confusion, nausea. It’s less common but just as dangerous, and often mistaken for low fuel when really it’s an electrolyte issue.

That’s where hydration drinks like Blonyx Hydra+ come in—designed to support hydration and sodium replacement without excess sugar, it’s an easy way to help reduce the risk of both dehydration and hyponatremia.

 

How to Prevent Bonking

The key to avoiding the wall is keeping your glycogen stores topped up before, during, and after workouts. Whether you're training for endurance or strength, carbohydrate availability directly impacts performance, especially as training volume increases. Here’s how to stay ahead of the crash:

1. Start Fueled: Begin long or intense sessions with your glycogen stores full.

We suggest:

• A carb-rich meal (~3–4 g/kg body weight) 3–4 hours before your session

• A smaller snack (~1 g/kg) 30–60 minutes out if needed
  
  

This is especially important for athletes doing multiple sessions in a day or long-duration efforts.

 

2. Fuel During: Don’t skimp on, carbs especially in training sessions longer than 60–90 minutes.

As a guideline:

• 30–60 g/hour for most endurance workouts

• Up to 90 g/hour for ultra events or double sessions, especially when combining glucose and fructose sources.

For strength athletes, intra-workout carbs may be less common—but 20–30 g of carbs during training can support intensity and reduce perceived exertion when glycogen is low.

 

3. Refuel After: Post-training carbs help replenish glycogen—especially when sessions are long, intense, or less than 8 hours apart.

Aim for:

• 1.0–1.2 g/kg of carbs per hour for the first 4–6 hours post-exercise

• Pair with 0.3 g/kg of protein to support muscle repair without slowing glycogen recovery.

If you have over 24 hours before your next session, regular balanced meals will usually do the job.

 

4. Use Low-Glycogen Training Sparingly

Training with low glycogen may help increase fat oxidation and mitochondrial adaptation—but it also increases your risk of under-fueling and bonking. Caution is advised and this should not be used on key intensity or performance days. Doing it too often or without adequate recovery may compromise immune function and training quality.

 

5. Listen to Your Body and Know the Signs

Experiencing fatigue, dizziness, irritability, or a drop in coordination mid-workout? Don't ignore these signs. A quick carb source like a gel or some Hydra+ and a banana can turn things around before a full-on bonk hits. 

 

What to Do If You’re Bonking

Stop or slow down drastically: Your body needs time—around 10 minutes—for fat oxidation to ramp up and provide usable fuel when glycogen is gone.

Refuel immediately with 20–40 g of fast-digesting carbs: Look for high-GI sources like gels, bananas, or sports drinks to spike blood glucose and restore brain and muscle energy.

Hydrate with electrolytes: Sodium helps restore plasma volume and enhances glucose uptake during and after exercise.

Eat a carb and protein meal within 30–60 minutes: Aim for ~1–1.2 g/kg of carbs + ~0.3 g/kg of protein to optimize glycogen resynthesis. Blonyx Egg White Protein Isolate offers a whole-food protein source that’s easy to digest and ideal for getting your recovery started.

Rest and recover for 24–48 hours: Avoid intense sessions until your glycogen stores are replenished. Prioritize sleep and nutrition to speed up recovery.

 

Can Strength Athletes “Bonk”?

Absolutely. While we usually associate bonking with long endurance efforts, strength and high-intensity athletes are also at risk—especially during high-volume sessions, double days, or when training fasted. Lifting heavy or doing repeated high-intensity intervals depletes glycogen, and when carb stores drop too low, you’ll feel weaker, slower, and less coordinated. If you find yourself slowing down more than usual mid-session and haven’t eaten enough, glycogen depletion might be at play.

 

Key Takaways

Bonking shouldn’t be considered a rite of passage for endurance atheltes—it’s a sign that your fueling or hydration strategy needs work. Fortunately, by keeping glycogen stores full, staying hydrated with the right balance of electrolytes, and learning to spot the early warning signs, you can avoid the dreaded wall.

If you learned something new from this article and are curious to know more, if you learned something new from this article and are curious to know more, head to the Blonyx Blog or our growing list of weekly research summaries where we help you further improve your athletic performance by keeping you up to date on the latest findings from the world of sports nutrition.

— That’s all for now, train hard! 

 

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