title pic article carb loading

Carbohydrate Loading for Ultramarathons

Us runners love our carbs. And for a good reason. Carbohydrates are the preferred fuel for endurance exercise that is anything above leisurely pace.

Many of us runners are also familiar with the dreaded bonk – also known as hitting the wall.

Bonking describes the point when your body runs out of available muscle glycogen to keep you going. Your performance drops dramatically, and your mood usually tanks along with your speed. This point usually occurs around the 20-mile mark for most runners in a marathon.

However, with intelligent training and proper nutrition before and during the race, it is possible to avoid hitting the wall during a marathon.

And while you may still bonk during an ultramarathon, you can delay the sufferfest by employing the same strategies. One of those is carbohydrate loading to ensure you start the race with your fuel tank filled to the max.

What is carbohydrate loading?

In the 1960s, researchers were able to do muscle biopsies and found that muscle glycogen stores were low in those runners who hit the wall.

They later discovered that the body tries to restore carbohydrate stores when depleted beforehand. Based on these findings, the first carbohydrate loading strategy was invented.

This strategy is also known as the “Saltin Diet,” named after one of the researchers who proposed this strategy.

British runner Ron Hill used this diet before his 1969 marathon win at the European Athletics Championships. After he finished the race strong and without bonking, interest in the relatively unknown “Saltin Diet” surged.

This strategy involves doing a depletion workout 7 days before your event, then avoiding all carbohydrates for the next 3 days. The goal is to deplete your carbohydrate stores completely before eating lots of carbohydrates for the next 3 days. Because your body has been deprived of carbohydrates, it will now maximally refill your glycogen stores, and you have a full tank going into the race.

While some people still use this approach, I don’t recommend it. Science has shown that reducing your activity levels and increasing your carbohydrate intake simultaneously work equally well to fuel up your glycogen levels.

That means tapering before your big day while at the same time upping your carbohydrate intake is all you need to maximize your glycogen storage.

Is carbohydrate loading useful if you’re running an ultramarathon?

Due to the long duration of ultramarathon events, you will likely exhaust your muscle glycogen stores, and bonking might be inevitable.

So, you might wonder if carb loading makes sense for such long races, where it is almost guaranteed that you will run out of energy.

My answer is a resounding yes. Think about it.

The more glycogen you have on board, the longer you can go without a dip in performance. And the less time you spend feeling completely exhausted, the better.

Also, remember that the pace during an ultra is usually slower than during a marathon, so you will naturally burn a higher percentage of fat for fuel, further delaying the point of carbohydrate depletion.

Properly pacing yourself is vital, as is sufficient training leading up to the race to teach your body to use glycogen more efficiently.

What is the best way to carbohydrate load before an ultramarathon?

What is the best way to carbohydrate load before an ultramarathon? If you have carbohydrate loading nailed for the marathon distance, then congratulations – you have also nailed it for your ultramarathon. They are essentially the same.

However, as already mentioned, I don’t recommend you go through the depletion-deprive-replenish cycle of the “Saltin Diet” protocol. Why? Because depriving yourself of carbohydrates is a stressor that you add to your week before the big event. And adding more stress is the opposite of what you should be doing during race week.

Eating a higher percentage of your calories as carbs and reducing your activity levels at the same time is a good enough stimulus for your body to fill up its glycogen reserves.

How many carbohydrates should you eat? 

Studies suggest that the best way to achieve glycogen super-compensation is by eating between 8-12 g of carbs per kilogram of bodyweight.

Depending on your current diet and personal preference, you eat this amount the last 2-5 days before your event. Some people start to carbohydrate load 7 days out, but I think that’s not necessary for most.

Depending on your current diet, this carbohydrate intake might be easy to eat or feel like a challenge.

For example, I currently weigh 52kg and would need to eat between 364 and 624g of carbs per day.

You have to experiment to see what works for you. If you already eat a high-carb diet and more than 60% of your calories come from carbohydrates, you may find it easy to bump it up during carb-loading days. On the other hand, if you eat a lower-carb diet, you might need to significantly cut back on fat and protein for a few days.

You can track your food intake a few times to develop a feeling for it. And if you are like most athletes, you don’t get the recommended amount of carbohydrates per day necessary for glycogen replenishment.

Nothing new during race week

Just like you should practice your race-day nutrition during training, you should practice your carbohydrate loading regime a few times during your training cycle.

Long training runs are ideal for this, especially if you take one or two rest days beforehand. Test out which foods work well the night before a long run and what pre-race breakfast is optimal for you.

Come race week, you should have a good idea of which foods work well for your system, how much fiber you can handle, and how to structure your eating.

Avoid trying out new routines in the week leading up to your race. If you suddenly start eating large quantities of foods your system hasn’t adapted to, you might need to visit the bathroom more often during the race.

You should absolutely avoid eating a lower amount of carbs until the night before race day and then cram in as many carbs as you can during the pasta party. Overindulging the night before may lead to bloating and severe tummy troubles during your run.

A note on low carb and keto diets and fat adaptation

I am not a big fan of low carb and keto diets from a performance point of view. While it is certainly possible to finish a marathon or ultramarathon on a ketogenic diet, you likely need to be willing to sacrifice top-end speed and power.

Research has repeatedly shown that speed suffers on a ketogenic diet, and my personal experience aligns with those research findings.

If you eat a low-carb diet, you wouldn’t want a carb load in the classical sense. However, many low-carb athletes do increase their carbohydrates in the days before the race or during periods of heavy training.

You also need to figure out a way to fuel yourself during races.

You’d still need carbs to run fast. That’s why elite athletes who are often cited as proof that low carb diets work also eat carbs during competition. 

If you want to reap the benefits of keto and still use carbs on race day, the challenge is to keep your body accustomed to eating carbs while still remaining in a ketogenic state. I found this very hard to do, and my ability to use carbs as fuel greatly diminished on a keto diet.

I won’t go deeper into the science now, but be aware that eating a high-fat diet downregulates the enzymes in your gut that are responsible for carbohydrate uptake. So on race day, it may be that your body doesn’t know what to do with the sudden influx of carbs.

I don’t mean to demonize keto and low-carb diets. Quite the contrary. I have experienced many health improvements by eating a very low-carb diet. However, I had to trade running speed for improved health and endurance.


Carbohydrate loading should be a vital part of your strategy to avoid hitting the wall on race day.

Sufficient aerobic training, avoiding low carb diets, and making sure you fuel yourself properly during the run are critical components too.

You don’t need to deplete your carbohydrate stores before refilling them. Simply rest the 2-3 days before the competition and eat a diet comprised of around 70% carbohydrate. Aim for 8-12g of carbs per kg of bodyweight.

Start with a full fuel tank and crush your next ultramarathon 😎.


Let's Keep In Touch!

I never spam! Read my privacy policy for more info.

Scroll to Top