A world-record holder who runs 100-mile races says the high-fat diet Silicon Valley loves transformed his body and performance

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• Many experts think that by eating a high-fat or keto diet, it's possible to become a more efficient fat burner and get more energy from ketones.
• Intermittent fasting may have a similar effect, helping people become better at fueling themselves from fat.
• Ultramarathon world-record holder Zach Bitter recently explained how changing his diet helped improve his performance.

Our bodies might work better when we're burning fat as fuel.

From a health perspective, burning fat rather than primarily getting fuel from carbs might help stabilize blood sugar. From an athletic perspective, being a fat-burner might help some people recover more quickly and perform at a higher level.

Zach Bitter is an ultra-marathon runner who holds the world record for the longest distance run in 12 hours (101.77 miles). On a recent episode of his new podcast, Human Performance Outliers, Bitter discussed his decision to switch to a keto-like diet designed to turn him into a more efficient fat-burner.

In 2011, Bitter said, he'd been eating what most would consider a healthy, whole-food, high-carb diet that might be expected of someone running 50-mile races. But he was hurting, waking up throughout the night, seeing his energy levels fluctuate, and dealing with chronic swelling in his ankles.

Instead of cutting back on racing, he changed what he ate, embarking on a whole-food, high-fat diet. He cut out most carbohydrates, relying instead on foods like stir frys, bacon, eggs, nuts, and seeds. He often cooked with coconut oil or duck fat.

Although he still consumed some carbs while racing, Bitter said the dietary changes made him feel less need to eat while running. And overall, he felt better.

"It was pretty eye-opening to me - in the first four weeks, all of those symptoms going away, the swelling, the sleeping [problems], the energy levels throughout the course of the day," Bitter said on the podcast.

Why going high-fat or keto might help

Rob Ludacer

Becoming a more efficient fat-burner is the main idea behind the popular keto diet and behind intermittent-fasting programs as well. There are even new supplements designed to push your body to become a supercharged, fat-burning - or "fat-adapted" - machine.

Most of us burn sugar for fuel first. Our bodies burn through easily accessible glycogen energy stores, which we get from breaking down carbohydrates. After we burn through our supply of those, our bodies can eventually start getting energy via ketones, which are produced from fatty acids (basically, energy from fat).

People whose bodies are more used to burning fat tend to get more energy from ketones on a regular basis, not just after they run out of sugar fuel.

Fasting may be the most efficient way to get your body more accustomed to fueling itself via ketones. But a low-carb, high-fat diet can do the same thing over time.

Courtesy Andrew Neugebauer Xendurance

Becoming fat-adapted for performance's sake

The effect that fat adaptation has on performance has been debated over time.

In 2015, sports nutrition researcher Louise Burke wrote in the journal Sports Medicine that although she had thought researchers put the "nail in the coffin" on the idea that fat adaptation was beneficial, there did seem to be evidence that low-carb, high-fat diets may help in certain cases.

Bitter credits his dietary switch with helping him build the strength to become a world-record holding racer.

Within a few weeks of the change, he said he noticed improvements. But it took about two years before things really "clicked" with regard to his performance, Bitter said on the podcast.

"In the fall and winter of 2013, I was able to race and recover and race again and hit some training blocks in between at a frequency that I never would have thought possible earlier," he said.

That December, he set his world record.

When it comes to nutrition, however, approaches that work for some people don't work for everyone. Bitter understands that.

"Everyone is different - it was developing what worked for me, for my lifestyle," he previously told Business Insider.

Still, he said most of the runners he's coached have gotten at least some boost from trying a high-fat approach during their training.

"From middle- to back-of-the-packers to people who are looking to podium ... if they follow the program right, I have not seen any athlete that has come to me not have a successful outcome from it," he said.