Olympic Athletes Brag About High-Fat Diets. Take It With a Grain of Salt.

On Monday night, 17-year-old American phenom Chloe Kim dazzled audiences with thrilling flips and turns that earned her a gold in women’s halfpipe. She also charmed Twitter following as she chronicled her food intake.

Kim isn’t the only athlete who has showcased her love of high fat food while performing amazing Olympic feats: Sometimes, it can seem as if the old adage “you are what you eat” is turned on its head when it comes to Olympians. While we’re dieting and trying to keep up with our New Year’s resolutions, the elite athletes in Pyeongchang, South Korea might consume what nutritionists call “fat bombs,” yet not resemble balls of lard. These competitors, on average, have much lower body-fat percentages than average, even though they eat plenty of the finger-licking stuff.

That’s because, of course, they’re using that fat—along with other nutrients like proteins and carbs—to ski, skate, and luge their way to gold. But some think fat is more than just a part of a balanced diet—it might be key to winning.

Far from being a settled question, exactly how much fat, carbs, and other foods athletes should eat is hotly debated, and for good reason: When you’re a high-level athlete bound for the Olympics, you have a different relationship to your body and nutrition than the rest of us. What you eat has long- and short-term consequences. The wrong snack can cause you to cramp up, tire out in the last mile, or run to the bathroom when you haven’t a moment to spare. The wrong foods or the wrong combinations of foods can mean the difference between taking home a gold medal or going home empty-handed.

So while plenty of the general public has dallied with low-carb or low-fat diets for weight loss, athletes—and the people who work closely with them—have been experimenting with inputs and outputs with much-higher stakes in mind.

One of the biggest challenges for athletes of any kind is running out of energy to perform at peak levels. So keeping energy up is job one when it comes to sports nutrition.

And traditionally, that’s been the carb’s job. “For a long time, carb-loading, especially for endurance sports, was the accepted practice. However, in recent years, scientists and athletes have studied (and practiced) training athletes to use fat as a primary source of fuel,” Sylvia Tara, a biochemist and author of The Secret Life of Fat: The Science Behind the Body’s Least-Understood Organ and What It Means for You, told The Daily Beast. The theory is that fats are a longer-lasting source of energy, so they can more effectively and consistently power longer-duration athletic events like cross-country skiing or hockey.

While it’s well known that carbs are great for high-intensity, short-duration exercise, there are limits—the body can only store a limited amount off of them. “Depletion of carbs can be a major cause of fatigue,” Tara pointed out. So longer-term, consistent energy is key, but how do athletes cultivate it?

The answer? Fat instead of carbs.

Why Fat Isn’t Necessarily an Ideal Energy Source

Well, it’s more complicated than that, actually. “While carbohydrates account for the majority of energy during short-duration exercise, fats make up the majority of energy during longer or more intense workouts,” says Tara. But simply eating more fat isn’t the easy answer to performing at a higher level—an athlete has to retrain the body to “use fat more effectively” as a major source of energy, according to Tara: Key to this fat-centric training plan is restricting carbs. The combination of few carbohydrates and a lot of fats teaches an athlete’s body to utilize fat, which, in theory should provide longer-lasting, more consistent energy. But that shift takes time.

“If you take carbs away from the diet, the whole regulated pathway that’s involved in fat burning has to adapt,” Louise Burke, head of sports nutrition for the Australian Institute of Sport, said. She and her team have run several tests of low-carb, high fat (LCHF) on athletes.

If the LCHF diet sounds suspiciously like the Atkin’s Diet or keto, that’s because it’s been popular for the last few years among non-athletes looking to slim down. Carbs are severely curtailed (no grains, no fruit, no sugar) and certain fats are prioritized: “There’s a preference for mono unsaturated and saturated fats like cream, butter, and lard, and omega-3 polyunsaturated fats are important,” Burke said. Those omega-3s are especially relevant for athletes since they “help control inflammation—a byproduct of training that can induce muscle damage,” Tara said.

But some nutrition experts have doubts about whether these diets really improve athletic performance. Some athletes sing the benefits of LCHF, winning Olympic medals to boot. When Burke looked for the hard science on how true these claims were, she came up mostly empty-handed; the connection between fat consumption and performance was lacking.

After hearing arguments from anti-carb advocates for years, including testimonials from both camps insisting their method improved athletic performance—especially vehement and popular discussions on social media—Burke and her team put LCHF diets to the test.

Burke found some answers in a 2017 study, where 21 male race-walkers were tracked for 3 weeks. During the research, all meals and trainings were done under supervision. “The study involved a pragmatic blend of rigorous scientific control and research methodology with real-world allowances needed to accommodate elite athlete populations,” Burke wrote.

The results were clear: Despite improvements in peak aerobic capacity, “…adaptation to an LCHF diet impairs performance in elite endurance athletes.” Not only was there no improvement in performance with the low-carb, high fat diet, it actually led to poorer performances in a closely tested group of endurance athletes.

Burke says her team is now “periodizing” carb restriction, where athletes only consume carbs during a “high-intensity” part of their day but have “normal” diets otherwise. “How much [the athletes] consume depends on the day and the type of training they do,” she said.

When Chris Froome, the three-time Tour de France winner posted a pic of his no-carb breakfast, followers went wild pointing out the connection between his success and LCHF. But that was just what he’d eaten on one rest day, and was misleading, Burke pointed out. A varied approach is what modern sports nutrition is all about,” Burke said. They key might lie not in either/or when it comes to these diets, but a both/and plan, at least for athletes.

For the rest of us, Burke thinks the high-protein diets are going to soon be experiencing a backlash. “In any of the new diets, from high-carb to high fat, people tend to lose weight right away because they are changing their diets and cutting out food.” But she says, most restrictive diets are hard to maintain, and weight is regained.

None of us can eat just anything, fats or otherwise. But unlike even the weekend warriors among us regular folk, what athletes eat has much higher stakes than just fitting into their old jeans. With variability between human bodies, that means there’s never a one-size-fits-all approach. Every athlete is, in some ways, and experiment of one when it comes to nutrition guidelines for optimal performance.

Despite all the research, there is still at least some part of being a successful high-level athlete that’s ineffable, maybe even a little magical: Not only are bodies unique, but there’s plenty of variation in what’s eaten among both winning and losing athletes in the Olympics: “Not everyone who wins a gold are doing what they should do to optimize,” Burke said. Even she admits that sometimes willpower—or something else entirely—the Olympic spirit, perhaps, pushes athletes to win.