The diet has quietly become the rage among ultra-endurance athletes and elite soldiers, and it's a surprisingly yummy way to fuel up.
In the past several years, as measured by Google Trends, interest in an unusual style of eating called the ketogenic diet has tripled, and chances are you have a friend or coworker who’s tried it. Early adopters are typically people who run or ride a lot and want a food plan that doesn’t just fill their tanks but also boosts performance. Followers scarf eggs, cheese, and olive oil in hunger-killing quantities, turning their backs on just about every carb other than vegetables. They don’t use half-and-half in their coffee—they use heavy cream. Still, they’re likely to look a little lean, since the ketogenic diet turns them into 24/7 fat burners. (Even while surfing the couch.) And don’t be surprised if they report feeling better and stronger than ever.
Outside Fitness Special
127 radical tips for total health.
Ketones are a type of organic substance that includes ketone bodies, a collective name for the three molecules that are produced naturally by the liver when it breaks down fat for energy, a process that the ketogenic diet jump-starts. Under normal circumstances—that is, if you’re eating a standard, balanced diet—your body gets most of its energy by turning carbohydrates into glucose, which cells then convert to energy. If you significantly reduce carb intake (typically to less than 50 grams per day), your body undergoes a fundamental change: it starts relying on fat-generated ketone bodies as its primary energy source. The brain, heart, and muscles can all burn ketone bodies efficiently if you’ve been eating this way for a month or so. This metabolic state is called ketosis.
A Day in Food on the Ketone Diet
How endurance athlete Patrick Sweeney puts away nearly 3,000 calories a day on the ketone diet.
Historically used as a driver of weight loss, carb restriction has recently gained favor in ultra-endurance circles and the military’s Special Forces. The idea is to radically crank up fat burning so that athletes and soldiers are in ketosis during grueling, survival-like situations. The biochemistry of how ketone bodies aid performance is complicated, but the processes and benefits are summed up well for laypeople by Dr. Ken Ford, a ketones expert who runs the Florida-based Institute for Human and Machine Cognition (IHMC), a nonprofit research outfit that’s funded by organizations like DARPA, the National Science Foundation, and the Army, Navy, and Air Force.
“During ketosis, the liver produces ketone bodies that are converted into substances that feed cellular energy production,” Ford says. “So basically, an athlete in ketosis can access additional fuel. Though there’s no scientific reason to believe that a ketogenic diet would increase anaerobic power or muscular strength, there is reason to believe that aerobic capacity and muscular endurance could be improved when sufficient ketone bodies are present to complement glucose.” The upshot is that for lower-intensity, longer-range exertion, ketone bodies offer the physiological equivalent of solar power.
There’s more. Ketone bodies apparently switch on specific genes responsible for a flurry of molecular upgrades, enhancing health and lengthening lifespan. Scientists are now investigating their use for treating everything from traumatic brain injury to cancer.
This broadband interest is new. The diet itself isn’t. Ketosis got a foothold in medicine in the 1920s, when it was used successfully to treat children with epilepsy who didn’t respond to drugs. Labeled the hyperketogenic diet, the regimen gave patients 90 percent of their daily calories from fat to help prevent seizures. “No one knew how it worked,” Nobel Prize–winning biophysicist Rod MacKinnon says. “They just knew it worked.”
More recently, there’s been a keto buzz among endurance athletes. It started in 2012, when Timothy Olson, a runner who follows a ketosis-friendly diet, broke the record at the Western States 100, the rugged, revered annual trail race in the Sierra Nevada. Last year, Zach Bitter, another ketones-adapted runner, set the American record for 100 miles on a track—11 hours 40 minutes 55 seconds. Data from a study conducted by Ohio State human-sciences professor Jeff Volek showed that during Bitter’s runs, as much as 98 percent of his energy can come from fat and only 2 percent from carbs. Your body can store a maximum of around 2,500 carbohydrate calories. But if you’re carrying around, say, 25 pounds of stored fat, that’s the equivalent of roughly 100,000 potential calories. So a fat-adapted runner can, in theory, chug along indefinitely.
In May, I visited Ford at the IHMC campus in Pensacola. The lab’s big thing, he said, is “the extension and leveraging” of human capacities and resilience. Recently featured in Scientific American for programming a semiautonomous robot that can traverse the rubble of a simulated nuclear disaster, IHMC is also studying the ketogenic diet. Leaders in the field of ketosis—like Volek and University of South Florida associate professor Dom D’Agostino—are linked in with IHMC’s inner circle.
Ford is the hub of the current ketones conversation. A baritone-voiced polymath with expertise in computer science, artificial intelligence, and biochemistry, he has held leadership posts at NASA and the National Science Board, which advises the president and Congress on science and engineering issues. When Ford isn’t traveling, a typical day involves gulping shots of espresso (“the elixir of the mind,” as he calls it) while whizzing around the IHMC campus, checking in with his scientists as they tinker in their labs.
“We wouldn’t be here as a species without ketones,” Ford told me. He said we’ve known since 1965 that the brain, the body’s most energy-hungry organ, is ravenous for them. A study done that year by Harvard Medical School researcher George Cahill scuttled what was then the conventional belief that the brain can burn only glucose. “In fact the brain will metabolize ketones preferentially,” Ford said.
Cahill conducted a series of starvation studies with divinity students, monitoring fasts lasting up to 40 days. The protocols wouldn’t pass today’s ethical standards. The students didn’t eat: their intake was reduced to water augmented with electrolytes and vitamins. “If Cahill tried to do those studies now, he’d be thrown in jail,” Ford said, only half joking.
Cahill measured the students’ blood glucose as well as the presence of two ketone bodies, betahydroxybuterate and acetoacetate. “Betahydroxybuterate,” he wrote, “is not just a fuel, but a superfuel.” His studies helped illuminate the mechanism that allows humans to survive long periods without food. As he showed, when your glucose supply runs low, insulin drops, which switches on ketone production.
But ketosis is more than a backup generator, Ford said. Harking back to our caveman days, ketosis signals to the body and brain—as in, “Hey, there’s no food!”—that they need to improve resilience and efficiency in cells and neurons.
Ford, who is now 61, has practiced a ketogenic diet himself since 2006 and says he has consistently experienced improved cognition. His body fat is under 10 percent. He prefers not to drop out of ketosis, but sometimes he does. “Once when I was in Italy, I ate a half-bowl of pasta after a workout,” he said. “I felt like a zombie.”
These days, D’Agostino and others are researching new medical applications for ketone bodies. With epilepsy as a starting point, their neuroprotective functions have inspired a whole new field.
For six years, D’Agostino has studied why ketone bodies are anticonvulsive, flowing the data into metabolic-therapy models that he hopes will prove useful in the management of neurodegenerative diseases like ALS and Parkinson’s. With money from the Office of Naval Research, D’Agostino’s lab is also closing in on a solution for Navy SEALs who use diving rebreathers to eliminate telltale bubbles during missions. The devices can lead to oxygen toxicity that can cause seizures. D’Agostino’s team has produced encouraging results in testing ketone esters, an exogenous form that you can drink or eat to boost ketone bodies with or without a ketogenic diet.
Another research area is the treatment of traumatic brain injury. According to the Department of Defense, TBI has been a serious problem for some 340,000 American soldiers. In May, I sat down with a former Special Forces medic (who asked not to be named) who served multiple combat tours. He said that at the most elite levels of the U.S. military, people aren’t waiting for research to confirm the benefits of a keto diet. “I’d say more than a third of the guys are doing it, for the endurance and also for the cognition,” he said. As research by D’Agostino and others indicates, the anti-inflammatory benefits of ketone bodies on the brain may add a measure of injury protection. In fact, the medic told me that he used the keto diet to supplement his own TBI treatment at Walter Reed hospital.
“Ketosis appears to be beneficial in a surprisingly broad range of seemingly unrelated diseases,” Ford said. “At first blush, nutritional ketosis can sound like snake oil—that it’s good for whatever ails you.” The thing is, there’s some truth to that generalization. “Ketosis represents a profound and fundamental shift in metabolism,” he said, “which has broad epigenetic effects as well as energetic effects.”
These effects are having an impact in the ultra-endurance world. At the 2012 Western States 100, Volek brought a team of graduate students to study runners. He picked the right year: Timothy Olson, making only his second start at the race, became the first person to cover the epic course—which involves 18,090 feet of ascents and 22,970 feet of descents—in under 15 hours. Volek’s subsequent studies, conducted on runners and triathletes who had been fat-adapting for six months or longer, recorded fat-burning rates close to Zach Bitter’s 1.7 grams per minute.
Not everybody buys in, of course. One detractor of lockstep low-carb, high-fat protocols for athletes is Louise Burke, head of sports nutrition at the Australian Institute of Sport. Burke published a 2006 review in the Journal of Applied Physiology called “‘Fat Adaptation’ for Athletic Performance: The Nail in the Coffin?” (She softened her stance in 2015 with “Re-Examining High-Fat Diets for Sports Performance: Did We Call the ‘Nail in the Coffin’ Too Soon?”) Burke maintains that a low-carb, high-fat diet drains power by interfering with production of an enzyme called pyruvate dehydrogenase, or PDH. The PDH gene gets disrupted, according to Burke, and study subjects rapidly run out of gas. But peers like Volek say these studies are flawed in part because the subjects went through fat-adaptation phases of as little as five days. “Nothing good happens in five days,” Ford told me. In endurance athletes who have spent months in ketosis, skeletal-muscle samples don’t show any decrease in PDH.
Burke’s theory, that high power output is impaired by a ketogenic diet, is not uncommon. Biochemist Robb Wolf, author of the bestselling book The Paleo Solution, told me that even though he loves how he thinks and feels when he’s in ketosis, he struggles with power outages in his sport of choice, jujitsu. He fares better on the mat when he works sweet potatoes and cashews into his diet.
Ford’s contention is that while there’s no reason to believe ketosis will increase anaerobic power or muscular strength, a well-formulated diet—given time to take hold—shouldn’t decrease power or strength and will improve aerobic capacity and muscular endurance. Ford, an aficionado of high-intensity resistance training, thinks that generating more ketones through diet may be a partial answer to the power-loss problem. “I like to go into my hard interval workouts with higher ketone levels, at least two millimoles per liter of blood. Otherwise I get smoked,” Ford said.
The takeaway from conversations with Ford, Volek, and D’Agostino is that the ketogenic diet isn’t about making the Olympics. As Volek told me, “My real interest is in how the diet can help solve obesity and other health problems. But bias against anything low-carb makes it tough to get funding.”
Volek’s comment reminded me of my early introduction to the topic. My wife’s aunt Martha, her husband, Ray, and their adult sons struggled for years with severe obesity. In less than three months on a keto diet, they lost an average of 35 pounds each. I was floored when I saw them at a family gathering, not recognizing them at first. They told me about their weight-loss plan, which involved no exercise, no calorie counting, and a lot of bacon.
Sounds great. But is the payoff worth the lifestyle price?
Even if it’s the right path to follow, low-carb eating in a high-carb world is tricky. Outside Online’s editor, Scott Rosenfield—a long-distance mountain biker—tried the diet earlier this year, leaning heavily on canned sardines and staying under 50 grams of carbs per day. He liked the results. “One day I did a 100-mile solo ride on my fat-tire bike,” he said. “I felt like Superman.”
The sustained power was one thing; sustaining the diet was another. “It got monotonous,” Rosenfield said. Another problem was ordering “weird” food at restaurants and having to explain the diet to bewildered friends and service staff. Predictably, eating sardines became a chore. He fell off.
I told Rosenfield about Ford’s advice: if you stick with the ketogenic diet for six months or so, you can stay in ketosis at 100 grams per day. He brightened. “That seems more doable,” he said. “I could have a tortilla with my eggs.” The basic parameters of the diet are simple. Restrict your daily carb intake to 50 grams or less. (A Starbucks blueberry muffin contains 53 grams.) Don’t overdo it with meat, either—too much will drive up insulin levels and boot you out of ketosis. Low-carb diets increase dehydration and provoke electrolyte loss; Volek says to drink a lot of water and increase salt intake. When it comes to fat, feel no fear—olive oil, butter, and chicken skin are all just fine. Make fatty fish a staple. Eat some vegetables, but take it easy on the fruit. Get a keto cookbook to avoid monotony. Buy a blood-ketone tester from a drugstore or Amazon, and check your levels periodically. A measurement of over 0.5 millimole of ketones means you’ve crossed into a state of ketosis.
When you’re in ketosis, Ford explained, you see real benefits. Drop out of it and you don’t. Bottom line: stay in it as best you can. Consistency, Ford and other advocates insist, earns you a new metabolism.
I asked Ford about the potential hazards of eating a diet that mandates a lot of fat. He responded by sending me a massive independent review of the subject, put out by Credit Suisse Research, which analyzed more than 400 studies on fat intake in the human diet. “This comes to the inescapable conclusion that fat is not the enemy,” Ford said.
Talking with D’Agostino led me to Patrick Sweeney, a tech millionaire who uses the ketogenic diet for health and performance. A former Olympic rower now in his forties, Sweeney gravitated to ketosis after being diagnosed with a rare form of leukemia in 2004.
“I’m sure it was because I’d been working 75 hours a week, drinking at night, and getting up early because I felt guilty for the drinking,” he told me. When informed by a doctor that eating a single grapefruit could reduce the effects of chemotherapy, he began reading, which led him to information about low-carb eating. For Sweeney, the main draw of this route was that most cancer cells metabolize only glucose. Not ketones.
If there’s a dietary middle ground, Sweeney may be plotting it. To prep for long mountain-bike rides, he goes strict with keto. “I’ll hammer the three months before a race or an adventure like Kilimanjaro to make sure my ketone levels are high, above one millimole per liter,” he said. “In between adventures, I’ll be less strict.”
He’s also not afraid to let down his guard from time to time, like during the holidays. On such occasions, he said, “I fall in love again with pale ales, French wine, and Irish whiskey.”