The notion that your body can literally run on fat, rather than carbohydrates, is an attractive one. The ketogenic diet has gained momentum in recent years, with ultra athletes like Western States 100 winner Timothy Olson lauding the benefits of the high-fat regime. Just because you can go keto, though, doesn’t mean you should. We examined the latest research and chatted with registered dietitian Cara Harbstreet to give you the cost-benefit analysis of running on the ketogenic diet.
What is ketosis?
In the average diet, carbohydrates supply glucose that the body uses for energy. Ketosis is a metabolic state in which the body doesn’t have carbs to burn, so it turns to ketone bodies—fatty acids made in the liver—for energy instead.
“The metabolic shift occurs as the body becomes more efficient at burning fat,” Harbstreet says. She notes that as the body becomes adapted to burning fat, it becomes less adapted to burning carbs. “You have to stay committed to the diet long enough to achieve stable ketosis,” she says. “If a person is bouncing in and out of ketosis, you don’t quite achieve that metabolic efficiency, and performance can suffer.”
The advantage to ketosis, though, is simple. The body can only store 2,000 to 2,500 calories worth of carbohydrates, but each pound of fat we carry represents about 3,500 calories of energy. In other words, you carry more energy in a pound of fat than in an entire fuel belt’s worth of gels and goos.
How does your body burn carbs versus fat?
The problem with the ketogenic diet for runners is that the body burns carbohydrates and fats quite differently. As leading fat researcher Louise Burke writes in Sports Nutrition: More Than Just Calories, “Fat oxidation (burning) is inhibited at exercise intensities above [approximately] 75 percent of maximum aerobic capacity.” Anything over 75 percent of max effort and the body switches to burning carbs because the fat oxidation process—fat mobilization, glucose breakdown, and glycolysis—takes more time than the body can sustain an intense effort.
For example, if you’re running a 5K at 90 percent of aerobic capacity, you’ll finish before your body has a chance to use onboard fat stores as energy. This is true no matter how fast you run. Even elite marathoners can’t really use fat as fuel. Running sub-three hours puts most humans above 75 percent of VO2 max.
However, ultramarathoners and marathoners whose pace doesn’t exceed 75 percent of VO2 max can burn fat as a primary source of fuel rather efficiently while in ketosis. In a 2015 study by Ohio State University fat researcher Jeff Volek, elite endurance athletes on diets of 10 percent carbohydrates, 19 percent protein, and 70 percent fat burned fat more than twice as efficiently than elite high-carb runners.
That said, Volek’s study and similar experiments have yet to find a performance benefit from ketogenic diets. Ultra athletes like Olson prefer them because fattier fuel can make you feel fuller and prevent sugar highs and crashes, but as far as we know, they don’t make you any faster.
Which runners can benefit?
First off, you need to run really far in order to avoid a negative outcome. You’re essentially starving your running body of its primary energy source after all. “Carbohydrates are just the most efficient source of fuel, especially for endurance athletes like runners,” Harbstreet says. “You’re essentially using your glycogen, stored as glucose until those stores are depleted over one or two hours of exercise.”
So, in order to kick your fat burners into gear, you need to run for a while—and not exceed 75 percent of aerobic capacity. But to get that far, you’ll need to put your body into a ketosis, and that doesn’t happen overnight. In Volek’s study, the elite endurance athletes on ketogenic diets restricted carbs for at least six months before the test. Although you can reach ketosis in less time, ketones expert Ken Ford told Outside “nothing good happens in five days.”
What’s more, you’ll need to be strict to maintain a low carb and moderate protein consumption. “For the average person, you would be drastically changing your day-to-day routine,” Harbstreet says. “Many people mistakenly believe they’re following a ketogenic diet; a true ketogenic diet is 5 percent total calories from carbohydrates.”
So should you try it? Well, be honest about your commitment to a low-carb diet and the time and duration of your runs. There’s no shame in not being an elite athlete.