A low-carb diet forces the body to use fat as its primary fuel; some endurance athletes are hoping that will help them stay energized for longer.
A low-carb diet forces the body to use fat as its primary fuel; some endurance athletes are hoping that will help them stay energized for longer. (EpicStockMedia/Thinkstock)
Sweat Science

The High-Fat Diet for Runners

Long shunned by endurance athletes looking to stay lean, the enemy is having a moment

Conventional wisdom for endurance athletes has been to avoid fat and load up on carbohydrates. Research has shown things aren't quite so simple. Read more.

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In 1879, more than a century before ultra-endurance races became a thing, Arctic explorer Frederick Schwatka and 12 Inuit companions embarked on a record-setting 3,200-mile sled journey across the frozen tundra. To fuel themselves during the next 11 months, the men killed and ate reindeer, musk oxen, polar bears, and seals. In short: tons of fat and practically zero carbs.

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Not surprisingly, Schwatka initially noted in his journal “an apparent weakness and inability to perform severe exertive, fatiguing journeys.” But after two or three weeks the feeling passed. The secret? The low-carb diet had forced his body to adapt and use fat as its primary fuel.

These days the Schwatka diet—or modern variations on it—is making an unlikely comeback among ultra-endurance athletes, led by record-breaking runners, alpinists, and Ironman triathletes. The data bolstering the trend doesn’t exactly date from Schwatka’s time, but neither is it new.

Back in 1983, an MIT medical researcher named Stephen Phinney decided to test the hunch that fat adaptation might be useful during prolonged exercise. Phinney’s hypothesis was based on simple math: a typical person can store about 2,500 calories of carbohydrates, enough to last a couple of hours at most, but carries a reservoir of perhaps 50,000 from fat. If athletes could train their bodies to tap this secondary source, he thought, they’d never bonk.

“Nobody in the maritime industry will touch him,” says his lawyer. “Who could afford to take the PR risk? I mean, bang a dock and it's on the front page: Hazelwood Does It Again!”

The hypothesis is based on simple math: A typical person can store about 2,500 calories of carbohydrates but carries a reservoire of perhaps 50,000 from fat.

Inspired by Schwatka’s journals, Phinney put five elite cyclists through a four-week diet that provided more than 80 percent of their calories from fats. Contrary to the era’s prevailing carbo-loading wisdom, the average performance of Phinney’s cyclists in a 2.5-hour time trial remained unchanged. The study could have altered endurance sports forever had it not been for one small thing: their sprinting capabilities were compromised. That’s because fat-adaptation diets don’t just ramp up fat burning, they actually throttle carb burning by decreasing the activity of an enzyme called pyruvate dehydrogenase. For many athletes, that’s a deal breaker.

“It doesn’t make sense to nobble a fuel-producing pathway,” says Louise Burke, head of sports nutrition at the Australian Institute of Sport, who conducted some of the early studies on fat adaptation. “I think any endurance sport that requires high-intensity performance will suffer.”

So for years, athletes have continued to load up on carbs. But more recently, prominent scientists like Tim Noakes, the author of the 1986 pro-carb sports-science bible Lore of Running, have given Phinney’s findings a second look. One reason is that ultra-endurance events are booming, and athletes who participate in them tend to care more about making it to the finish of a grueling course than outsprinting a competitor. “The benefits of low carbs really start to distinguish themselves when you get beyond the marathon, because you’re definitely running out of carbs then,” says Jeff Volek, a researcher at Ohio State, who with Noakes and Phinney authored an editorial in the British Journal of Sports Medicine earlier this year questioning the claim that carbohydrates are essential for athletes.

Of course, the danger of running out of carbs is lessened by the fact that virtually all long competitions have numerous aid stations where racers can refuel. But there are no aid stations in the mountains, which is why alpinist Steve House recently increased his fat intake sixfold. And if you’re running a 100-mile race that lasts 15 to 20 hours, your stomach may rebel at the thought of yet another grape-flavored gel. By reducing your dependence on carbs, Volek says, you decrease your chances of bonking. This proved true for fitness coach Ben Greenfield. In 2013, he finished Ironman Canada in under ten hours while consuming almost no carbs.

The challenge for many athletes is harnessing the sustained energy of fat adaptation without sacrificing the ability to push hard—and that takes a more sophisticated training plan than just loading up on burgers. While Volek and Phinney suggest eating 50 grams of carbohydrates per day, a few ultrarunners have adopted a more moderate approach. Both Zach Bitter, who set a U.S. track 100-mile record last year, and Timothy Olson, who captured the course record at the Western States 100 in 2012, keep overall carbohydrate intake low but ramp it up before and during long training runs and races. Olson eats a relatively high-fat diet but dines on sweet potatoes the night before long runs and takes in one or two gels per hour during races. The diet has helped him reduce inflammation and recover faster, he says. And it certainly hasn’t put any flab on his wiry frame.

Many sports nutritionists have turned to an alternate approach dubbed “train low, race high.” They suggest triggering fat adaptation by eating a conventional high-carb diet but performing certain training sessions in a low-carb state: a hard run first thing in the morning before breakfast, or two workouts in one day without replenishing carb stores in between. These targeted sessions, Burke says, are enough to boost the body’s fat-burning ability without compromising carb burning, giving the athlete “metabolic flexibility” to tap into whatever fuel they need in a given situation.

Research is ongoing in the fat-as-fuel debate, but one thing is certain: for many, the days of skim milk are over.