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Why You Still Need Carbs If You Want To Run Well

Us runners love our carbs. Pasta parties, carbohydrate loading, and aid stations stocked with fruit, cakes, and coke illustrate the importance of carbohydrates for running performance.

But are carbohydrates the preferred fuel for your athletic pursuits, or is fat a better alternative?

Should you give up on sugary gels and sports drinks and opt for nut butter packets, olives, and cheese instead?

A growing crowd of low-carb and ketogenic diet proponents argues you should. “Carbs are not essential,” they say, and “fat is the better fuel.” “Bonk – proof yourself,” they promise.

So is the time for high-carb diets for athletes over, and should you instead focus on fat as fuel?

All evidence – scientific and real-life, suggest you shouldn’t. And in this article, we will explore why.

How Your Body Uses Fuel

Since the beginning of the 20th-century, scientists have known that carbohydrates are the preferred fuel for high-performing endurance athletes.

While a mix of carbohydrates and fats fuels all exercise intensities, the general rule is that your body uses a higher proportion of fat at lower intensities. On the other hand, a higher percentage of carbohydrates is used as fuel at higher exercise intensities.

In other words: For light activities, such as walking or a light jog, your body relies primarily on fat to power your efforts. When you start to run faster and increase the intensity, your body increasingly uses carbohydrates as fuel. When you exercise at very high intensities, your body almost exclusively uses carbohydrates, regardless of diet.

This relationship between exercise intensity and fuel use is described as the “crossover” concept by Brooks and Mercier in 1995.

The intriguing argument many low-carb and ketogenic dieters make is that you can shift this crossover point and teach your body to burn a higher percentage of fats at higher exercise intensities.

But is this a valid argument?

What About "Fat – Adaptation" And Shifting How Your Body Uses Fuel Through A Ketogenic Diet?

Most of the studies to date focused on the impact of carbohydrates on performance. However, some also studied the effects of ketogenic diets on endurance exercise. Dr. Steven Phinney – a ketogenic diet advocate – presented the results of one such study in 1983.

Dr. Phinney and colleagues studied a group of cyclists who ate a ketogenic diet that comprised <20g of carbs/day for several weeks. And he did indeed show that the cyclists’ bodies shifted to using a higher percentage of fat at lower intensities. However, the cyclists experienced a significant drop in anaerobic performance. The lack of carbohydrates in their diet negatively impacted their ability to sprint, as they struggled to generate enough quick energy.

These results notwithstanding, interest in low-carb and ketogenic diets, have been rising in the running and ultra-running communities, and more research has been conducted.

However, no scientific studies so far could show any performance benefit of low-carb or ketogenic diets for endurance athletes.

On the contrary, two recent studies by Louise Burk and her colleagues at the Australian Institute of Sports showed that being a “fat-burning-machine” leads to slower race times for elite race walkers.

Yes, these athletes improved their body’s ability to use fat as fuel, but this came at the cost of reduced energy efficiency. Like the cyclists in Phinney’s 1983 study, the race-walkers in Burke’s study got slower.

To win races, however, you have to be able to go fast – both in competition and in training. A low-carb, high-fat diet reduces your ability to do that.

What About Training Low And Racing High?

There has been an idea floating around in the running community that you can “train low and race high” to have the best of both worlds.

Eat a very low carb, high-fat diet during training to teach your body to rely primarily on fast as fuel and use carbohydrates “strategically” during racing as some “rocket fuel.” But does that strategy work?

Probably not because there are two issues with this strategy.

The first has to do with the adaptation processes of your body.

When you eat a low-carb, high-fat diet for an extended period, your body loses its ability to use carbohydrates as fuel. Your body loses the ability to transport glucose through the membrane of your small intestine to your bloodstream.

This is is an important trade-off to be aware of since you might not be able to use the carbs you want to ingest on race day as “rocket fuel” if you have been following a strict ketogenic diet for several months before that.

The second has to do with practicality.

Let’s assume you want to go all-in and decide to follow a ketogenic diet, even when racing. As you might be aware, boiled eggs and cheese are not the main fuel options at aid stations. Not in a marathon and not in longer ultra-marathon races.

Carrying your own food is also somewhat of a hassle. When I ate a ketogenic diet, I had serious problems finding suitable fuel options on the run. Boiled eggs use a lot of space, nuts are hard to digest, and I don’t do well on them. Cheese goes bad quickly and is not a practical option for warmer temperatures unless you are willing to eat it all the same day or don’t care about wasting food.

Also, what happens when you run out of your packed food or lose some, and you suddenly need to rely on aid station food or on your fellow competitors to tide you over?

It would be better if your body would tolerate a wider variety of foods instead of going into a mild panic when confronted with a banana.

What Do The Elites Do?

“Success leaves clues” is a quote by motivational speaker and coach Tony Robbins. He means that you can look to others who came before you and achieved what you want to achieve and learn from them.

Want to be a successful writer? Look at other successful writers, their habits, their learning curve, their failures, and setbacks. Then do what they found out to work.

I think we can take this thinking and also apply it to our athletic pursuits. It helps to step away from science a bit and look at real-life examples of the world’s best endurance athletes to discern what has been working for training and racing at an elite level.

And when you look at the world stage, you will see that the top-performers in endurance sports eat a high-carbohydrate diet. I have yet to come across an example of a world-class athlete who wins sprint events, half-marathons, or marathons on a ketogenic diet.

If you know of one, feel free to contact me.

Even ultra-runners, for whom a ketogenic diet would make the most sense – due to the relatively low speeds compared with shorter events – eat mostly carbohydrates during their events.

Yes, even those who are touted as examples of “elite ultra-runners who eat a ketogenic diet.” Zach Bitter, for example, uses carbohydrate sources in periods of high training. During races he uses gels.

Similarly, you can see Mike McNight eating all kinds of carbohydrates during his FKT on the Colorado Trail. I am not telling this with mean intentions; to the contrary.

I want you to be aware that trying to race a fast marathon or successfully finish an ultra in a respectable time on coconut oil, eggs, and butter might be an exercise in futility.

When Does A Ketogenic Diet Make Sense?

Does that mean you should never eat a low-carb or ketogenic diet? Not necessarily.

If you have a health condition that requires such a dietary approach for therapeutic reasons, it would be silly to change it in the hopes of better performance.

However, if you don’t need to follow a ketogenic diet for health reasons, I would argue against it to maximize your potential as an endurance athlete.

If you don’t care for placings or follow a ketogenic diet for other reasons not touched upon here, and you are happy with it, more power to you.

Be aware of the trade-offs, though, and maybe more importantly – be willing to accept the sacrifices in anaerobic capacity, food choices, and lifestyle you have to make when following such a diet.

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