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A man wearing earbuds, leaning forward mid-workout
A man wearing earbuds, leaning forward mid-workout

Have you ever hit a wall during an intense exercise session and felt like you couldn’t go on? If you’re an endurance athlete or someone who participates in high intensity exercise, you might be familiar with a dreadful sensation called bonking.

Bonking is a state in which your body runs out of its glycogen stores during intense exercise. High-intensity athletes may be at a greater risk of bonking, or hitting a wall, if they don’t properly fuel, rest, and recover.In this article, we break down what bonking is, why it happens, and how you can better support your body to prevent this phenomenon while exercising.

What Does Bonking Feel Like?

Bonking occurs when your body runs out of its stored glucose, or glycogen. It most commonly affects endurance athletes such as long distance runners or cyclists who are training for long periods of time.

You may be familiar with the experience of “hitting a wall” whether it be at the end of a long day or when you get tired on a long hike. Bonking, however, is a much more intense sensation, and can leave you feeling as if your body’s ‘engine’ is failing.

A bonk can cause a sudden shortage of energy. This is followed by symptoms like leg cramps, dizziness, extreme weakness, and even the inability to continue moving.

How Glucose Is Connected to Bonking

Dinner plate with salmon, greens, and rice plus bowl of tomatoes on dinner table

Glucose enters your body in the form of carbohydrates that you consume from foods (like whole grains, legumes, or fruits) and acts as your body’s main fuel source. After you eat, glucose travels through the digestive system straight into the bloodstream.

Insulin is then released by your pancreas to help glucose enter your cells so that it can be used for a variety of different functions. When we consume more glucose than we might need, the excess is stored as glycogen in liver and muscle cells as well as converted to fatty acids when glycogen stores are filled.

Your body uses these glycogen stores as fuel when the body runs out of glucose. If you’re on a keto diet and limiting your carb intake, your body will use ketones as its primary fuel source.

What Causes Your Body to Bonk?

Glycogen is stored in several locations throughout the body, such as your liver, muscles, heart cells, brain cells, and kidney cells. As you engage in exercise, your body will first use glucose that is available in your bloodstream. Once that runs out, it’ll turn to the liver and muscle glycogen stores.

When this source becomes depleted, your body recognizes that it has reached its physiological limit. The brain then starts sending signals to the muscles to limit their function and avoid muscle damage.

Bonking happens when all the energy stores are used, leading to glycogen depletion. Typically, because there is so much glucose and glycogen that is available for your body to utilize, bonking doesn’t happen right away.

The sensation of bonking occurs slowly when you engage in continuous high-intensity efforts. Running a marathon or ultramarathon, participating in a triathlon, or cycling for an extended period of time are common activities that can lead to a bonk in certain people.

If you participate in these kinds of endurance efforts, using a CGM for running or swimming can be helpful for gauging your fuel stores.

Warning Signs of Bonking

Woman sitting with her chin in her hand

Some of the symptoms of bonking overlap with symptoms of hypoglycemia, so it’s important to listen to your body, especially if you are prone to abnormal glucose levels. Whether you’re worried about experiencing bonking, want to prevent it, or be more informed, here are some symptoms to watch out for.

  • Nausea
  • Extreme fatigue
  • Feeling light-headed
  • Physical weakness
  • Lack of coordination

What are the Dangers of Bonking?

Bonking can have a negative impact not only on your training, but also on your overall health. Here are five potential dangers of bonking.

1) Loss of Muscle

If you are participating in a lengthy training session or race, it’s important to properly fuel your body. This means consuming plenty of carbs, sports drinks, and other sources of glucose before, during, and after your training.

Over time, if you are not consuming enough carbohydrates, the body will start sourcing energy from fats and proteins, which can lead to muscle loss.

 2) Impaired Immune Function

Woman in blue workout gear sitting on steps

The immune system helps the body to adapt during training, and physical exercise can help strengthen it. However, bonking and overtraining can actually have negative effects on your immune system. Research suggests that prolonged, intense exercise can suppress your body’s natural anti-inflammatory response. Your immune system fights off pathogens to keep you free of infection. So if the immune system is suppressed by overtraining, your risk of sickness or infection becomes higher.

3) Physical Pain Caused by Exhaustion

When your body reaches the exhaustion point, it can cause a disruption in your training rhythm, which can lead to fatigue and stress. Stress from long bouts of intense physical exercise triggers the secretion of cortisol. Stress can aggravate existing health issues, and it can even increase physical pain within the body.

4) Dehydration 

During some types of training, athletes may be unable to drink adequate water during their performance, which can lead to severe dehydration. Being dehydrated can lead to bonking, as well as fatigue and cramping.

Chronic dehydration is dangerous and can cause a slow decline in cognitive functions. If you want to stay properly hydrated, here are some tips you can follow.

5) Cognitive Issues

Woman with head slumped on table holding her phone above her head

The brain uses glycogen stores to get energy. If you hit the point of bonking, your body’s glycogen deposits will be emptied, and you may temporarily lose the ability to focus, to be motivated, and to make any decision. While these effects are temporary, they can be dangerous depending on your surroundings.

How to Prevent Bonking

Bonking is not ideal for any athlete, and learning how to avoid this state is important. Let’s discuss some of the most effective ways to do so.

1) Consume Adequate Carbohydrates 

Fueling your body adequately is one of the main ways that you can prevent bonking. This is generally around 45-65 percent of total daily calorie intake and it’s essential to make sure you are fueling your body with glucose.

Athletes have different carbohydrate requirements based on training intensity, type of workout, and timing during their season. In general, athletes may need to consume three to five grams of carbs per kilogram of body weight daily for light activity and upward of eight to 12 grams per kilogram body weight per day for more intense training.

Studies have shown that athletes who consume carbs during training sessions can train for a longer time and at a higher level. If you are practicing strenuous exercise for an extended period of time, make sure you are adequately fueling your body before and during your workout. Energy gels or energy bars are a great, portable option for long rides or runs.

If you’re in need of guidance on the right amount of carbohydrates for you, you can start by learning how to count and track your macros. A sports nutritionist can also help you determine your carbohydrate needs based on your activity level. You may even consider using a CGM for fitness training to better monitor your energy stores.

2) Drink Plenty of Fluids 

Redhead woman drinking water from bottle

During exercise, your body loses a lot of water. That’s why it’s important to drink fluids like water or electrolyte drinks during a training session.

Apart from drinking plenty of water, there are many hydrating foods you can include in your diet. Try to include hydrating fruits and vegetables in your diet, such as watermelon, melons, cucumbers, oranges, grapefruits, tomatoes.

Smoothies, milkshakes, and soups are also a great choice for hydration. There are also lots of electrolyte drinks available on the market, but make sure to opt for an option with low added sugar content.

3) Create a Plan Before Strenuous Exercise 

The best way to prevent the body from experiencing bonking is by creating a plan that takes meal timing, rest, and training sessions into account. Timing your meals and deciding when to fuel your body according to your workout sessions can benefit your performance.

Undereating or eating infrequently can deplete the glycogen deposits and cause low energy levels during training. It’s important to consume carbs around the time of your training to make sure your body is prepared for strenuous activity.

You can replenish your glycogen stores regularly with high-quality carbs such as whole grains, fruits, and starchy vegetables. For best performance, getting plenty of rest and taking time to recover properly can also be beneficial.

What if I’m Already Experiencing Bonking?

Two people sitting on sidewalk

If you bonk, here are three effective ways you can support your body for a speedy recovery.

1) Refuel with Carbs and Protein 

To recover from a bonk, it’s important to refuel your body adequately with each essential macronutrient as well as electrolytes. Including lots of carbohydrates, healthy fats and protein in your recovery meal plan.

To combat DOMS symptoms, make sure to refuel with high quality protein sources like lean chicken breast, pasture-raised eggs, Greek yogurt, or tofu. Carbs such as whole grains and healthy fats from olive oil or avocado are also great options.

2) Drink Adequate Amounts of Water and Electrolytes 

Hydration is very important to keep the muscles in good shape. Water with electrolytes is great for preventing dehydration, by maintaining the osmotic pressure in the cells. Some electrolyte drinks can also supply you with glucose, which is great for replenishing your blood-sugar levels after physical training or a bonk.

3) Get Some Well-Deserved R & R 

Man laying and stretching on gray bed with tufted headboard

Rest and recovery are important not only for relieving pain, but also to allow your body to replenish its glycogen stores. It can take up to 24 hours for your glycogen stores to be filled back up. Ensure you are refueling your body with plenty of nutritious carbohydrate, fat, and protein sources while you recover.

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Heather Davis, MS, RDN, LDN

Reviewed by: Heather Davis, MS, RDN, LDN

Heather is a Registered and Licensed Dietitian Nutritionist (RDN, LDN), subject matter expert, and technical writer, with a master's degree in nutrition science from Bastyr University. She has a specialty in neuroendocrinology and has been working in the field of nutrition—including nutrition research, education, medical writing, and clinical integrative and functional nutrition—for over 15 years.

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