Have you seen that meme that’s taken the internet by storm recently? The one with the tearful blonde pointing her finger at a smug-looking cat who’s sitting at a dinner table in front of a plate of greens?

I think of that meme every time someone asks my thoughts on keto. I’m the tearful blonde imploring the cat, “You need CARBS!” to which the cat smirks, “Keto is life.”

Few topics are more divisive in the nutrition world than the ketogenic (“keto”) diet. Most medical experts aren’t crazy about it — a group of them ranked it the second-worst diet in a January 2020 issue of U.S. News and World Report.

Still, keto has become more common among endurance athletes chasing a performance advantage.

But what is the keto diet, and should endurance athletes try it? This article will take a brief look at keto for sports.

How Does Keto Work?
“Keto is a low-carb diet on steroids,” said Jennifer McDaniel, MS, RDN, CSSD, owner of private practice McDaniel Nutrition Therapy in St. Louis.

The most popular version of the diet calls for 5 percent of daily calories from carbs (just 25 grams per day on a 2,000-calorie diet).

Carb enthusiasts may balk at the idea of eating the carb equivalent of one medium apple a day. Indeed, keto is a far cry from the conventional wisdom on fueling for athletes, which calls for 55 to 65 percent of calories from carbs.

Ketogenic diet food, high healthy fats

To understand the lure, you need to know the physiology of the bonk.

Carbohydrates are our most accessible energy source, but we only store about 500 grams of them. Unless you eat or drink carbs mid-workout, you’ll use up those carbs after about 90 to 120 minutes of moderate intensity exercise.

When this happens, your brain orders your body to shut down in an effort to conserve any remaining glucose. This is the bonk, which causes lots of unpleasant symptoms that can make finishing a workout feel impossible.

This explains why some athletes, including ultrarunning champions Timothy Olsen and Zach Bitter, do keto. You can’t run out of carbs if you don’t eat them in the first place, and even lean athletes have tens of thousands of calories in the form of body fat.

The idea is to avoid bonking by training the body to use fats and ketones (which are chemical byproducts of fat metabolism) instead of carbs. The way to get there? Keto.

Does Keto Work for Endurance Athletes?
Could keto help you chug along forever? It depends on your goals.

A 2019 study of runners suggests that exercise efficiency is comparable between keto and higher carb diets during long, slow workouts. However, stamina and speed decreased as the runners increased intensity.

Pointing to another study with similar findings, McDaniel said, “You might be able to sustain a low to moderate pace, but don’t count on passing your competitor in a sprint to the finish.”

That said, there are a couple of ways in which keto could offer a slight advantage.

First, people who consistently follow keto tend to lose weight. The diet might improve performance in athletes who have a few pounds to spare simply because it takes less energy to move a smaller body. The same could be said for any weight loss diet.

Second, keto may help prevent gastrointestinal upset during workouts in specific athletes. A fat-adapted athlete doesn’t rely on carb gels and drinks for energy. For athletes sensitive to the sugars in these products, keto could be the difference between the finish line and the Porta Potty line.

We need more data to draw definitive conclusions, but there’s no strong evidence that keto is better than higher carb diets as of now.

Is Keto Worth It?
Since keto appears to be no better than higher carb diets, it’s fair to ask whether it’s worth it to slash carbs. Elite fat-adapted athletes sing its praises, but some medical experts are concerned about long-term risks.

Kim Williams, MD, the former president of the American College of Cardiology, said in 2018 that no one should ever do keto. Indeed, keto dieters tend to eat lots of saturated fat and very little fiber — both habits that increase the risk of cardiovascular disease.

What’s more, the transition to keto can be brutal. In addition to several days of a nasty condition called “keto flu,” athletes have reported low motivation, elevated heart rate, electrolyte imbalances, dehydration, and decreased stamina, even during easy workouts.

In a 2019 episode of Sweat Science, elite race walker Evan Dunfee (who participated in a research study on keto) spoke of other volunteers sobbing during workouts that they normally would’ve found easy. And at the end of the study?

“No one was sad to come off the diet,” Dunfee said.

Bottom line: If you do keto and it works well for you, just keep in mind that there may be some health risks. But don’t try to convince me that keto is life.

Author: Kimberly Yawitz is a regular contributor to Terrain Magazine.