About halfway through the training cycle for one of my clients’ first marathon attempts, I got an email with one simple question: “Is it helpful to eat fewer carbs and more fats for energy on my long runs?”

This was a few years ago, and I had not heard much about distance runners following a low-carb, high-fat (LCHF) diet. This is sometimes referred to as the ketogenic diet (although the two may be different). Given this runner’s training progress at that point, I didn’t want to switch it up. Given the limited data on the benefits of a LCHF or ketogenic diet for runners, I didn’t see any reason to recommend either one.

But we can burn fat for fuel, right?

Yes. Long-distance runners—training for half, full or ultra-marathons—are much more likely than their sprinting counterparts to use fat for fuel while running. Training for endurance often means that many training runs are “easy,” or “long and slow,” with relatively low intensity. Some refer to this as the “fat-burning zone” for cardio exercise, or an aerobic workout—low-intensity exercise, during which the body is able to take the time to burn fat for fuel. Conversely, in an anaerobic state–i.e. high-intensity exercise, like sprinting—the body naturally turns to carbohydrates for fuel, because it can quickly and efficiently do so to provide bursts of energy that both the brain and muscles need to continue pushing hard.

Does science support a LCHF or ketogenic diet for runners?

Research has yet to show that a LCHF or ketogenic diet is beneficial to sport performance or health (for the average adult) in the long term. That said, a recent study through Ohio State University did demonstrate that endurance athletes who followed a “low-carb” diet (for an average time of 20 months) “burned more than twice as much fat as high-carb athletes during maximum exertion and prolonged exercise.” This was in comparison to endurance athletes of similar skill and fitness levels that followed a diet where at least half of their energy came from carbohydrates. But it’s worth noting a few things: this was a small sample size (20 athletes total, 10 in each group), focused on ultra-marathon and Ironman-level athletes, that only tested each athlete in two athletic scenarios (one maximal graded exercise test, one long treadmill run).

What’s missing? Sample diversity, longevity and a variety of sport performance tests.

What’s relevant from their conclusion? The suggestion that our human bodies may have a higher fat-burning capacity than has been seen, or recognized, in research before.

Their conclusion doesn’t indicate that this approach should be the go-to for any distance runner, though. And it doesn’t conclude that the fat-burning capacity enhanced performance.

Another recent study followed “healthy adults” on a ketogenic diet for six weeks, and not only found their oxygen uptake had decreased but also that LDL-cholesterol levels (aka the “bad cholesterol”) had increased. In their conclusion, study authors suggested a ketogenic diet “may be a matter of concern in competitive athletes.”

In short, Washington D.C.-based dietitian and certified specialist in sports dietetics (CSSD), Stephanie Mull, says, “I don’t advise the ketogenic diet for any athlete because carbohydrate is an essential nutrient for energy production especially during high-intensity exercise and powerful movements.”

Anecdotally, a handful of professional endurance runners have become known for their fat-adapted approach to distance running. Most of these examples are found with ultramarathon runners and Ironman triathletes–sports in which athletes may have to train for 10 or more hours of activity on race day. In this case, their goal to burn fat more efficiently to provide fuel to their body allows them to eat less both before and during these long workouts (or races). Instead, the body is relying on its own energy stores. Some have seen breakthroughs in their performances—such as Timothy Olson, a two-time Western States 100-Mile winner and course record-holder. But the long-term effects and efficacy of the ketogenic diet for runners remains unknown.

What’s the harm in trying a LCHF or ketogenic diet for runners?

Jonathan Levitt, an avid runner and curious experimenter, recounted his experience in trying a LCHF diet for about six weeks while training for a 5K PR in 2014. “I did PR in my 5K, but I felt like garbage while following the diet,” he recalls.

He aimed for about 70 percent of calories from fat, using a food tracking app to stay within range. After a breakfast coffee with butter, and fatty mid-morning snack (which sometimes included more butter “straight from the spoon”), his energy levels would plummet by early afternoon. He attributes the PR to more diligent training and months of focusing on the 5K distance, not the diet. (A LCHF approach isn’t ideal for a short-and-fast race.)

“Now, I’m eating more carbs than ever and my running is better than it ever has been,” he says. He mostly focuses on longer distances, and has little to no interest in diet restrictions.

Levitt notes that through his job at InsideTracker, they’ve seen some runners on the LCHF diet experience lower testosterone levels (compared to their normal), along with increased cortisol (aka stress hormone). He also adds that he has seen it “work for some people,” but “what works for some doesn’t work for all.”

I personally caution runners against experimenting with a fat-adapted approach without the guidance of a sports nutritionist or CSSD. Without a standard definition of “low carb,” or individualized recommendations for achieving ketosis, personal experiments with a LCHF or ketogenic diet may not yield desired or expected results.

Additionally, if an athlete does test out the ketogenic diet for running, they’re in for a lot of calculating. Keeping carbohydrate intake under 50 grams per day, or under 10 percent of total daily calories, requires diligent food tracking on a daily basis. While this may eventually become habitual, it allows very little (if any) flexibility. A rigid dietary regimen is unsustainable for many people, including runners. “I’ve never had an athlete be able to adopt a ketogenic diet, improve their body composition, and stick to it,” says Mull.

Word to the wise: If you’re a curious long-distance runner and want to give the LCHF or ketogenic diet a try, consult a sports dietitian before you start eating butter from a spoon every morning.