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Running Through the Wall – Why You Bonk During Ultramarathon Running and What to Do About It

“You’re not you when you’re hungry.”
- Snickers

The crash comes suddenly.

One moment you’re cruising over the trails, marveling at the scenery and taking in nature’s sounds, feeling at peace with yourself and the world.

And then your legs give out. Your stomach suddenly feels squeezy, and you feel like you can’t run another step. You suddenly lost all sense of humor.

Experiencing this sudden shift from feeling great to feeling like you want to lie down in a fetal position for the first time can be pretty frightening. However, if you’ve been running long distances for a while, you likely know this feeling all-too-well.

It’s the dreaded bonk. The nightmare of many endurance athletes.

So let’s look at why you’re bonking and then at some strategies to avoid it from ruining your next long-run or race. First, we need to understand how your body uses fuel to power you through your exercise session.

How your body uses fuel during running

For endurance exercise, carbs are king. Yes, also for ultra-endurance athletes. While it is true that you can train your body to use more fat as fuel during exercise, you can not change the fundamental laws of physiology.

The truth is that you never burn only carbohydrates or only fat. You always burn both.

How much each contributes to your energy supply depends mainly on your intensity. The higher the exercise intensity, the more your body relies on carbohydrates as fuel.

That’s why study subjects tend to get slower on true ketogenic diets. And that’s why elite ultra-endurance athletes all use carbohydrates during races.

Yes, also those who are often cited as “proof that a ketogenic diet works for athletes.”

They, too, know that to run their best, they need to preserve those precious glycogen stores as much as possible during the run, or else they won’t be performing at their best.

Your carbohydrate fuel tank needs refueling on the run

Glycogen is the storage form of carbohydrates in your muscles and liver. You only have enough to power you through 1.5 – 2 hours of running at a marathon pace.

This is just a rough estimate. Your day-to-day diet and training status influence how much glycogen you have stored and how much sugar versus fat your body utilizes at any given intensity.

Science suggests that the human body can hold around 600g of glycogen. However, your current diet, fitness level, and body size influence the amount of glycogen you have stored in your liver and muscles. Generally, your liver stores only about 100g; the rest is stored in your muscle tissue.

And guess what happens when you keep on running without refueling these stores?

Correct, you deplete your fuel tank.

Your body will have no other choice but to switch from burning primarily carbohydrates to burning fat.

When this switch occurs, you are forced to slow down. A lot.

Everyday nutrition is critical

Your first line of defense against hitting the wall is your everyday diet. When you’re an endurance athlete and eat a high carbohydrate diet, you can increase your glycogen stores significantly.

The benefit is obvious.

You will have more fuel to start with by maximizing your glycogen storage capacity and keeping your fuel tank full. Hence you will last longer before crashing.

In case you might be wondering, what constitutes a high-carbohydrate diet, the current recommendations for endurance athletes point to eating between 8-12g per kilogram of body weight per day.

This is the optimum intake if you want to maximize your glycogen stores.

The second benefit you get from eating a high carbohydrate diet is that your body is used to processing carbohydrates. Your day-to-day eating habits influence your digestion.

If you eat a high-fat diet, you downregulate the enzymes necessary for the breakdown of carbohydrates.

The final days before a competition are your chance to completely refill your glycogen stores by reducing training volume and committing to eating a high carbohydrate diet.

Carbohydrate loading strategies vary. However, the general consensus nowadays is that you don’t need to deplete your stores before refilling them. A few days with a high intake of carbs and low levels of training or none at all are likely sufficient to maximize carbohydrate stores before a race.

Dial-in your nutrition on the run

Your day-to-day nutrition is one part of the bonk-proof-nutrition puzzle. Your nutrition during the run is the other one.

If you want to stave off hitting the wall for as long as possible, you need to ingest some carbohydrates during your runs.

When you eat sugar during exercise, your body will use that to stabilize your blood sugar instead of the glycogen reserves in your liver.

Similarly, your body can use the dietary carbohydrates as fuel for exercising muscles, thereby sparing the glycogen stores in your legs. That means you refill your fuel tank while on the move, significantly reducing your likelihood of bonking.

However, avoid trying to consume the same amount of calories that you burn during your run since your gut won’t be able to process this many calories.

The general recommendation is to consume between 30-and 90g of carbs per hour, depending on your body weight, how hard you exercise, and how long.

Use these numbers as a starting point and experiment.

Whether you should choose sports nutrition products or “real foods” has a lot to do with what your stomach can handle, what you like most, and what you find most practical and affordable.

Both have their pros and con, and you should experiment with various options in training to find out what works best for you.

Sports nutrition products generally have the advantage that they are engineered to provide the optimal combination of various sugars for increased absorption in the gut.

Plus, gels and sports drinks are stripped of fiber, which may be an advantage if you, like me, deal with IBS symptoms.

A great whole food alternative to gels is honey – I have had great success using it as my training and racing fuel.

It is essential to practice your nutrition during training to see what you like best and what your body can handle. Gut training is a hot topic in endurance science circles. It seems an excellent approach to improve your body’s ability to handle carbs during exercise.

The “eat low, train high” approach makes little sense. Your gut will likely not know what to do with the sudden influx of sugars come race day, resulting in GI – issues, which are a leading cause of DNF in ultrarunning races.

Likewise, fasted running is counterproductive. It doesn’t bring any benefits and devoids you from the opportunity to test-drive various approaches to race day fueling.

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