The Ketogenic Diet Sucks for High-Intensity Exercise

Suzie Glassman

Critical advice for athletes looking to improve performance

I can’t imagine there’s anyone left who hasn’t heard of the ketogenic (keto) diet. The low to no-carb approach remains incredibly popular despite ranking as the worst out of 35 diets by U.S. News and World Reports.

Why so popular? Because most people lose weight — and fast. Each gram of glycogen — the stored form of carbohydrates in the muscles and liver — is accompanied by three to four grams of water. Drive your muscle glycogen super low without replacing it, and you’ll drop water weight.

I’ve seen clients lose up to eight pounds in a week. That’s motivating! Forget that giving up carbs (keto diets require 15–30g of net carbs per day) long-term is incredibly hard and that it rules out healthy foods like fruits, whole grains, and starchy vegetables.

A large part of why it ranks so low is that few people stick to it long-term.

The keto diet has also gained a massive following in the endurance world, as well as with high-intensity sports like Crossfit and Obstacle Course Racing.

Why? The Journal of the International Society of Sports Nutrition reports,

In theory, if endurance athletes tolerate the ketogenic diet, they could achieve longer training periods with sustained energy levels and reduced need for refueling, allowing them to maximize the aerobic benefits from training and competing.

As for high-intensity athletes, keto diets help drop bodyweight and improve body composition. If you can drop fat, you can move faster and, in theory, improve performance.

Yet, there’s a reason theories don’t always play out on a practical level. Here a few explanations about why the keto diet keeps you from reaching the high end of speed and power.

Carbohydrates and Performance

A concept known as the crossover effect, shown in the graph below, demonstrates at a certain level of intensity, our bodies switch from burning a higher percentage of fat to carbs. Following a keto diet and training at low to moderate intensity can help push the crossover point further to the right — meaning you’re able to burn fat longer before your body switches energy sources.

Notice in the graph that once you start pushing aerobic power to its max, your body prefers carbohydrates for fuel. Without carbohydrates, your body uses stored body fat. The liver then turns this fat into ketones, which get used for energy.

In general, fat is the body’s preferred source of energy for long and slow-to-moderate efforts. Also, we have far more stored body fat to pull from than we could ever have in stored glycogen. The longer your body uses fats, the more energy you have to push through the long efforts of an endurance event.

Dr. Stephen Phinney, co-author of The Art and Science of Low Carbohydrate Performance, explains the rationale for high-end athletes to try a keto diet:

Dip into the ketogenic diet, consuming less than 10 percent of calories from carbs and 80 percent from fat (whatever is left comes from protein), and the liver cranks out ketones, compounds proved to help fight inflammation and cellular damage. Because of this, recovery after exercise is dramatically enhanced.

And we all know professional athletes will do anything to get an edge.

Men’s Journal goes on to reference a study in Phinney’s book,

Trained runners were given a compound that prevented their bodies from burning fat and were then asked to hop on a treadmill and run a half-marathon. They ran just as fast as they did when their bodies could access fat freely for fuel.
The takeaway: It doesn’t matter how good you are at using fat as a fuel source, says study co-author Jill Leckey. If you are exercising at high intensity, your body is heavily reliant on carbs.

There’s more. Another new study also found some shocking conclusions.

The authors took 16 young men and women and tested them over four days under:

1) a low carb condition of fewer than 50 grams per day and

2) a high carb diet at 6 -10 grams/kg/day (for a 180-pound male, this is at least 490g carbs)

They used an exercise method known in the research community as “Wingates” on a bike and the yo-yo test. For Wingates, think of pedaling really fast, and then all of a sudden, a super evil research student ups the resistance by a massive amount. Within 10 -20 seconds, it feels like your quads were replaced with concrete. The yo-yo test includes intermittent sprinting.

The low carb keto group saw their power drop by seven percent on the Wingate test. They also ran a 15 percent shorter distance on the yo-yo test.

If you are exercising at high intensity, your body is heavily reliant on carbs.

It turns out the body can’t create ketones fast enough to compete with carbohydrates.

Also keep in mind that while keto athletes are working their way through keto flu and adapting to the keto diet, they are losing speed and power.

What about a carb re-feed

You may then ask the question, “What if I train in a keto-adapted state but then eat carbohydrates before my event?”

Good question, but the answer likely isn’t one keto advocates want to hear.

The catch is that your body’s ability to access the carbs from keto fully is compromised (e.g., downregulated).

To get technical for a minute, we have an enzyme called pyruvate dehydrogenase (PDH). PDH serves as the gate-keeper to glycolysis (carb-based metabolism).

High fat, low carbohydrate diets cause changes to PDH, making us less able to use the carbohydrates we’re ingesting pre-race to help with performance. You can reverse the changes to PDH, but this process doesn’t happen overnight.

And the longer you’ve been on a keto diet, the longer it takes for your body to adapt to using them efficiently again.

While adding in carbs before an event is a great idea, in practice, it falls short.


Removing carbohydrates, the primary fuel for anaerobic exercise (high intensity), is not good for performance. You may also find you feel like dog poo any time you try to push hard.

It may not matter to you, as a keto diet can be beneficial for athletes who are overweight or who are not concerned with finish times.

Planning to introduce a bunch of carbohydrates pre-event also doesn’t work well if you’ve fully adapted to a keto diet.

The good news is that if you’re an athlete looking to improve your intensity, healthy carbohydrates are your best friends. Go ahead and eat them!

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I write about health and fitness with the goal to help you live a healthier, happier life.

Denver, CO

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