HomeNutritionWhy Do We Bonk and How to Avoid It

Why Do We Bonk and How to Avoid It

You’re at mile 23 of your marathon with only 3 miles remaining and you feel pretty good. You feel like you’re going to have a good day. All of a sudden, within the space of 100 meters you go from feeling pretty good to awful. Your legs feel heavy and it becomes impossible to maintain a running pace. You are forced to walk but really feel like you need to lie down. What happened? How can you go from feeling good to feeling terrible in such a short period of time? It is what many refer to as “bonking” or “hitting the wall.” In this article we will explore the physiology of bonking and discuss what you can do to avoid it.

What Energy Sources Does Your Body Use?

Your body stores carbohydrates in muscle and liver in a form that is referred to as glycogen. During exercise, your working muscles can tap these glycogen stores for energy. The working muscles can also use circulation blood sugar, which essentially comes from liver, glycogen, and circulation fat, which comes from our body fat stores.

This gives your working muscles three fuel depots to draw from during exercise. Next we will discuss each depot briefly and then wrap up this article with some recommendations.

1. Burning Circulation Blood Sugar

Although blood sugar can be used by working muscles during exercise, it is a secondary energy source only. This is because glycogen is located inside the muscle cells and therefore is more readily available than blood sugar.

Blood sugar alone cannot provide enough carbohydrates to sustain a high power output during exercise. The liver, the primary source of blood sugar, contains only about 100 grams or so of glycogen compared to the 600 grams of more in skeletal muscle.

2. Burning Body Fat

Your body has enough stored fat to supply your muscles with the energy for a dozen or more marathons.

So why do care about stored carbohydrates? The reason that carbohydrate burning is key is that fats burn more slowly than carbohydrates. Since fat burning is slower than carbohydrate burning, we cannon sustain as high a power output when using fats alone.

If you are going to walk a marathon, you can get away with burning primarily fats because your power output is low. However, as soon as you start running, your power output increases and you need a faster burning fuel – carbohydrates – to sustain the higher energy bur rate requirements.

The optimal situation is to use a combination of fats and carbohydrates while running a marathon, or triathlon. In fact, optimally, you would want to burn as little carbohydrate as possible to sustain your work rate. Remember, you have plenty of fat to use for energy, but carbohydrates can be limiting.

3. Burning Muscle Glycogen

Well trained individuals typically have enough stored muscle glycogen to sustain 2 hours or so of moderate intensity exercise.

Therefore, with a little added fat utilization, trained individuals can usually finish typical marathon without running out of carbohydrates. Running an Ironman Distance even requires considerable fat utilization and carbohydrate replenishing during the event.

Untrained individuals typically store ½ to 1/3 as much carbohydrates as trained athletes.

Muscle glycogen stores are typically twice as high in athletes consuming a high carbohydrate diet than those on a low carbohydrate diet.

The table below is adapted from “The Lore of Running” by Tim Noakes and is based on data obtained by Ahlborg and Felig (Journal of Clinical Investigation 69, 45-54, 1982).

SubjectsType and time of dietIn muscle (g/kg)In liver (g/kg)
Untrained45% carb diet1454
 70% carb diet1870
TrainedTraining daily, low carb diet1430
 Training daily, high carb diet2170
 24-hour fast2110
 3 day low carb diet with exercise710
 Immediate pre-marathon, 3 days carb loading3690
 24 hr post race, high carb1590
 48 hr post race, high carb2790
 1 week post race, high carb30 

You can see by the table above that a high carbohydrate diet (> 70% carbohydrate) has a dramatic effect on stored glycogen.

What happens when you bonk?

So, let’s review the physiology part briefly. When you run out of stored muscle glycogen, you bonk! The reason that you bonk is that the only sources of fuel remaining are stored fat and blood sugar. The stored fast is burned too slowly to process the necessary energy to continue running and circulation blood sugar is inadequate in terms of total amount of delivery rate to sustain a high work rate. As a result, you must revert to a lower work rate (walk) in order to continue.

How do we keep from bonking?

You can see by the table above that a high carbohydrate diet is critical.

In fact, consuming extra carbohydrates during your taper (carb loading) can really pack the muscles with glycogen. Also note, if you consume a high carbohydrate diet while training daily, you can maintain higher muscle and live glycogen levels.

When is the Best Time to Eat Carbs?

The best time to consume carbohydrates is right after training. The muscles soak up carbohydrates like a sponge the first two hours after training. Consuming carbohydrates right after training can speed glycogen repletion and muscle recovery.

Also, you can consume carbohydrates during the event. This can help to keep blood sugar levels from dropping and can help prevent liver glycogen depletion. This is especially critical during the latter stages of the event as your body begins to rely more on stored body fat as a source of energy.

Remember, your body cannot continue at race pace by burning fats alone. Consuming carbohydrates during the event can slow the rate of muscle glycogen depletion and help you sustain a higher work rate while burning stored body fat. This is especially critical during an event lasting longer than 3 hours.

You simply cannot complete an Ironman distance event without replenishing carbohydrates during the event.  It is unlikely that you can consume and store carbohydrates as fast you are burning them. The trick is to slow the rate of glycogen depletion so that you can complete the event before you bonk.

In Conclusion

A good fuel consumption plan is critical. You must start the event with your muscles packed with glycogen and refuel along the way to delay glycogen depletion. In an Ironman event, you should start refueling as soon as you get on the bike and continue to refuel throughout the run. Being proactive is the key. Once you bonk, it is too late.

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Mike Prevost PHD
Mike Prevost PHD
Mike holds a PhD in exercise physiology from Louisiana State University. He specialized in muscle physiology and metabolism. He has trained athletes for many different sports including triathlon, ultra-running, surfing, powerlifting, bodybuilding, mixed martial arts, football, basketball and more. After finishing his PhD, he took a commission in the U.S. Navy as an Aerospace Physiologist in the Navy Medical Service Corps.


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