Ketogenic diets limit carbohydrate intake and prompt the body to produce starvation-fueled chemicals called ketones, breaking down more fat for fuel.

The "keto" diet has quickly become popular in Silicon Valley, where people credit the high-fat regime with helping them stay lean, feel good, and, hopefully, live longer, too.

But a new small study of athletic adults suggests that the diet could inhibit athletic performance.



The ketogenic diet is all the rage in Silicon Valley, where people credit the high-fat, low-carb, butter-, egg-, and cheese-filled diet with helping them lose weight and curb sugar cravings. But a recent study suggests that the popular "keto" diet may hamper athletes' abilities to perform because the diet is rooted in high-acid foods.

In a small-scale study reported in The Journal of Sports Medicine and Physical Fitness, 16 people who tried the keto diet over a four-day period performed worse on fitness tests than they did when they were on a high-carb diet. The study was the first to pit the high-fat keto diet against a high-carb one to study athletes' performance on short, high-intensity bursts of exercise.

There are several caveats. It's hard to say anything for sure about the overarching effects of a diet based on such a small study, but the performance of people in the program was affected enough by the keto diet that it gave the researchers pause.

It's also possible that the study wasn't long enough to show real results. Keto dieters often complain of a mild but draining "keto flu" that can kick in during the first week of a ketogenic diet. This is part of an adjustment period when the body is switching from burning carbohydrates to burning fats. Dieters often complain it can make them irritable, sore, dizzy, and nauseous.

Participants in this study were asked to first spend four days on a keto diet and then pedal for 40-second cycling tests, ramping up to high speeds. They were also timed while running short 40-meter sprints. Then they tried the performance tests again after four days on a high-carb diet, in line with recommendations from the American College of Sports Medicine for endurance athletes.

Athletes in the study performed 6% to 7% worse on their high-speed tests when they were on keto diets than they did when they consumed protein-rich, high-carb performance diets.

"The key problem with a keto diet is it produces acid in the body," Edward Weiss, a kinesiologist from St. Louis University and the study's lead author, told Business Insider.

Weiss said high-intensity exercise becomes harder to do when we have more acid in our bodies, much in the same way that lactic-acid buildup affects a runner's ability to perform. Those ill effects aren't just limited to long, aerobic workouts; they can come into play during shorter stints of high-intensity interval training too.

"It predisposes the person to early fatigue during high-intensity exercise," Weiss said.

"What's sort of scary is that [the keto diet] is an experiment that the population is doing on itself," he said. "No drug company would get away with introducing a drug in the population without thorough research."

This is your body on ketones

Ketogenic diets strictly limit carbohydrates and emphasize fats from foods like fish, meat, eggs, avocados, and coconuts. Shutterstock

The ketogenic diet, is, by its nature, a high-acidity plan.

Keto diets limit carbohydrate intake to less than 50 grams daily. That means if you're a strict follower, you can't have more than one apple or eight carrots in a day without setting yourself over the top, carbohydrate-wise.

Popular foods in the ketogenic diet include avocados, fatty fish, like salmon and tuna, eggs, cheese, and butter. Those foods all raise the level of acid in the body, triggering what Weiss calls a "subclinical" state of acidosis.

Most people in the Western world are probably already getting a little too much acid in their diets, Weiss says. But keto followers may be hampering themselves even more.

Or maybe not. Some elite record-holding athletes swear by the keto diet and credit it with enabling stellar performances.

Ultra-marathoner Zach Bitter, who holds the world record for longest distance run in 12 hours, says that after he switched to a high-fat keto diet - including staple foods like duck fat and eggs - and cut more carbs out of his training meals, he started seeing all kinds of benefits. He was sleeping better, he felt less hungry while running, and performed better generally. He still fuels up with carbs on race days, though, since they're quicker to digest.

"The 4-Hour Body" author Tim Ferriss also says that going into a monthly fasting state of ketosis helps boost his energy levels.

In spite of the recent small study, the science of elite athletic performance and keto diets is far from settled. One 2012 study of gymnasts found the keto diet had no effect on their performance, while a 2017 study of elite race walkers found that those who followed a keto diet needed more oxygen during their races and their performance suffered.

Off the field, keto diets have been more consistently linked with big benefits for people struggling with diabetes and seizure disorders, some of whom have used the diet to help stabilize their conditions.

But Weiss said there hasn't yet been enough scientific research to know whether the public should try the high-fat plan. It's possible, he said, that the athletic drawbacks recorded in his study are just the tip of the keto iceberg.

"We might find out down the road that this causes huge problems that are life and death matters," he said. "We just don't know the answers. Is it safe or not?"

Read more of Business Insider's ongoing coverage of the keto diet:

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