In December 2015, Zach Bitter ran exactly 402.5 laps around the track at the Desert Solstice Invitational. At the end of the 100 miles, he had not only bested the previous American record by six-and-a-half minutes, but also maintained an astonishing seven-minute mile pace for almost the entirety of the 11 hours, 40 minutes, and 55 seconds.

What's even more surprising, though, is that Bitter trains and competes on almost no carbs. At times, carbs account for as little as 5% of his diet—and Bitter insists that even we non-endurance record-holders can do the same.

Ahead of his October run in the Javelina Jundred, we asked him how he does it.

Men’s Fitness: You’ve said that you originally switched your diet because of health problems. What were they?

Zach Bitter: In high school and college, I had a whole-food type of approach to nutrition, like a lot of endurance athletes do. At that time, most of the research you'd find was based in a high-carbohydrate diet, so I skewed my nutrition that way. My diet was clean, but probably 60% carbs. Then, in 2010, I started participating in ultra-endurance events and noticed that things weren't ideal—not being able to sleep consistently through the night, having big energy swings during the day, chronic inflammation in my ankles, things like that.

And that's when you decided to change your diet?

Well, I was at a crossroads. One option was to scale back on training, and that wasn't necessarily something I wanted to do. The other option was to look at what I could do nutritionally, and that's when I met Jeff Volek, M.D., Ph.D., who's been very influential in the high-fat approach, and I made the switch.

How did it feel at first?

It wasn't like I had this crazy energy drop on the first day. For the first two to four weeks, there were a couple of days when I felt lethargic when I exercised, but I didn't really notice a huge energy deficit when I was just doing routine activities, like walking around or writing on the computer or something like that.

Zach Bitter

Did you start to feel better?

Absolutely. The inflammation went away really quickly, and within a month I was sleeping way better, like getting through the night. And I noticed that my energy levels were consistent throughout the day.

What about your training?

I actually dropped the intensity in my training for about a month. I look at it like this: Stress is something you can get from all aspects of life, whether it's from training or family or work. Drastically switching your diet is also a stressor—at least as your body first tries to learn—so I didn't want to add another stress.

Especially for an endurance athlete, though, four weeks is a long time to take off.

I planned it for during my off-season, which is a bit of time in the middle of summer and a bit of time in those deep winter months, like January and February. Most athletes then are hitting the reset button in general, so I decided to go for it then.

How did your diet change when you started to compete again?

When I'm racing, I'm able to cut my calorie intake in half. Because my body is so much more metabolically efficient at burning fat, I can rely on that a lot more readily than I would've been able to in the past.

But you're working out for 20 hours a week. You aren't nervous about losing too much weight or “bonking”?

That's another misconception with the high-fat diet. Some people say, "Well, I'm skinny. I can't afford to do that.” That's misleading, really. If you do the math, even the leanest athletes you see out there—who are, like, 4–5% body fat—have enough body fat to last them for a long endurance event. If you teach your body to metabolize fat when energy levels start to dip, then your body can turn to that fat as a fuel source.

Maybe for elite athletes—but what about the rest of us?

I actually think it's easier for those folks because they're not asking their bodies to maximize their top performance. If I weren’t running as much, I would eat way fewer carbohydrates, and I would consume more fat because I'm not asking myself to do anything in that high-performance space. You can easily do daily tasks by metabolizing fat.

Zach Bitter

And what was the most difficult food for you to give up?

Sweet potatoes. After a month, though, I stopped even wanting them. Actually, when I'm in my higher training points, the hard part is bringing back a carbohydrate because I've gotten so used to enjoying some of the more savory foods.

What does a typical day of eating look like for you?

For breakfast, instead of having cereal or oatmeal, I'll have eggs and bacon with maybe some spinach. Lunch is usually a Cobb salad, and my favorite dinner is a tri-tip steak or something like that.