The Surprising Diet Trick of the Fastest Man to Run 100 Miles

Cristina Goyanes
by Cristina Goyanes
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The Surprising Diet Trick of the Fastest Man to Run 100 Miles

It turns out that carb-loading with a big bowl of pasta the night before an endurance event may not be the best fueling strategy. Zach Bitter prefers bacon pre- and post-race, and there’s good reason to follow his culinary cue. The 31-year-old, pro ultra-runner, who lives in Sacramento, California, holds the American record for the fastest 100 miles — which he first set on a track in 2013 and then re-broke in 2015, running 7 minutes faster, clocking an impressive time of 11 hours, 40 minutes and 55 seconds. That’s a 7:01-minute-mile for 100 miles! His secret to success — besides training 20+ hours a week during peak season — is a low-carb, high-fat diet.


Five years ago, Bitter began cutting carbs after meeting with Jeff Volek, PhD, RD, a professor in the Department of Human Sciences at Ohio State University who famously studies ketogenic diets (high-fat and protein, low-carb). At the time, Bitter was looking for an alternative energy source when he realized his food choices — seemingly healthy, unprocessed stuff — were slowing him down and affecting his moods.

“I was training hard and racing a lot, and I was realizing that I wasn’t sleeping well at night. I’d have lingering inflammation and huge energy swings throughout the day. I wasn’t breaking down altogether, but it was something that I could see in my lifestyle that wasn’t sustainable,” explains Bitter, who followed a more traditional high-carbohydrate diet back then. “So I started to investigate my nutritional options, and began implementing a modified Atkins diet into my training. Right away, I noticed improvements, like sleeping deeper throughout the night and having less inflammation and swelling.”


It’s easy to assume Bitter just had a gluten-sensitivity, but the truth is, a high-carb diet may be affecting more people than we think, including you. “When you’re training hard, and you’re fueling yourself primarily with carbohydrates, which I would consider a high-octane fuel source, you’re also causing your body to produce a ton of oxidative stress. It’s a lot more stressful on the body to metabolize a carbohydrate than fat,” says Bitter, who averages 100 miles a week throughout the year.

In other words, carbs aren’t solely hard on people with food sensitives, but everyone, really. Bitter discovered this while working with Volek, who has extensively studied the effects of a low-carb, high-fat diet on all walks of life.


“The most important effect a low-carb diet has on the body is that it increases the body’s ability to burn fat — and that’s true whether you’re Zach Bitter, a world-class ultra-endurance athlete, or a Type 2 diabetic, or someone who’s obese. When you restrict carbohydrates to a certain level, your body increases its ability to burn fat by two-fold at rest and during exercise,” Volek says. “This is why the diet has health benefits, but also why it helps athletes perform better and recover faster. The adaptations are quite extensive in terms of how the body does this, but in the end, it manifests in this better ability to access your body fat and oxidize it for fuel.

“Increasingly, we’re seeing that this type of dietary lifestyle is associated with less inflammation and oxidative stress, it certainly helps people lose weight without having willpower because there’s much greater satiety levels tied to it,” Volek adds. “Also, when you get carbs low enough, you increase ketone levels in the body, and there’s an overwhelming amount of evidence coming out that ketones have a lot of positive effects on cellular processes.” Volek recommends working with a nutritionist who has experience in low-carb dieting to help you fine-tune a plan that’s right for your needs and goals.


Another cool advantage to switching from high-carb to high-fat, it’s near-impossible to bonk during a race, Bitter claims. “Bonking is a scenario that occurs when your glycogen stores are really low, and you’re so dependent on them that you can’t think straight. So one thing that I’ve noticed from eating high-fat is that I don’t bonk from a nutritional side of things. You really can’t almost. Your body knows how to utilize fat. Even if your glycogen stores get depleted, and you’re running a little slower, your body recognizes that you have enough fuel on board that it won’t put you in these altered states,” he says.

This “altered state” is what Volek confirms is an “energy crisis in the brain.” “When you’re hitting the wall about two and a half hours into continuous exercise, which corresponds to when you’d run out of carbohydrates stores, what’s happening is really dramatic fatigue. That means the brain — an energetically expensive organ — doesn’t have this source of glucose, and it shuts down. The brain is literally starving and that manifests in fatigue in athletes, which is when brain fog and disorientation sets in during a race,” Volek says. “When you’re fat-adapted, the brain can use ketones, so you have a near-infinite supply of fuel that can protect you from bonking.”

If you’re curious to give this eating style a try to see how it helps you perform, sleep and feel better, plus become bonk-proof, consider these six tips from Bitter, who is on track to set a new PR at the Western States 100-Mile Endurance Run, the oldest trail race on the planet, on June 24.


“If you’re planning to make the switch and you’re an athlete, pick the right time of year. If you have a big race in four weeks, now’s not the time to overhaul anything. The offseason is the ideal time to try this new approach because you’re lowering your stress down and recovering. That gives your body a chance to adapt,” Bitter suggests.


“I tell people to first focus on viewing fat as your primary fuel source, whether that means adopting the Atkins diet or trying to eat more fat than carbohydrates,” Bitter says. If you’re not so worried about performance, and you just go to the gym a couple of days a week, then you can make these changes a lot quicker without worrying about putting too much stress on your body, he adds.


Getting used to the new diet could take up to four weeks, and during that time, you might not feel great, Bitter warns. “From a performance side of things, you’re asking your body to completely re-hardwire its view on fuel sources. So I spent 2–4 weeks trying not to do anything too crazy with my training, keeping the intensity really low to let my body get used to this new diet. When the body re-programs, you end up feeling much better and with more sustained energy in the long-run,” he says.


We’re not just talking about the demands of your workout. If you’re constantly feeling stressed about work or relationships or other issues, it can play a big role in how your body reacts to this new diet. “Our body’s fight-or-flight response is heavily rooted in activating glucose for energy. So if you’re in that state constantly, you’re going to struggle to get your body to accept that fat is a fuel source, especially when you’ve relied mostly on carbs in the past,” Bitter says.


“If you’re not ready to go super low-carb, then don’t put something that big on your plate,” Bitter warns. “Take it one step at a time. A lot of times, people will do a hard-reset and go super low-carb for the first 4–6 weeks, and then start to look at how many carbs are good for your lifestyle,” says Bitter, who’s carb intake fluctuates between 10–30% depending on where he is on his training plan (i.e., he allows more carbs on days when volume or intensity are high). Still, the largest macro group in his food log will always be fat, he says. “Once I got going on the high-fat diet, my body stopped craving sugary, high-carb options,” he adds.


Bitter’s go-to sources of pure fat include coconut oil, coconut milk, clarified butter, olive oil and avocado oil. For fat and protein combos, he prefers grass-fed beef, chicken, liver, any kind of organ meat, salmon, rib-eye and eggs. For vegetables, he sticks to the non-starchy kind — including broccoli, spinach, arugula, beet leaves, cabbage, cauliflower, cucumbers and homemade fermented foods like beets, carrots and kimchi — to keep his carbs extra low.

About the Author

Cristina Goyanes
Cristina Goyanes
Cristina Goyanes is a NYC-based freelance editor and writer who covers topics including sports and fitness, health and lifestyle, and adventure travel for various national men’s and women’s magazines and websites. When she’s not feverishly typing stories at her desk, she’s exploring the world, from the Arctic to Antarctica and plenty of countries in between. Follow her adventures and more at


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