For years, we were told to fear fat. Filling your plate with the F word was seen as an express ticket to heart disease. Low-carb, high-fat diets like Atkins were ridiculed for causing high cholesterol and giving followers license to gorge on damaging red meats and full-fat cheeses. Meanwhile, carb-loading became a religion to endurance athletes hoping to avoid the feared hitting-of-the-wall.
Then, trends began to change. Common criticism of the Atkins diet was debunked: Popular science suggested that a low-carb, high-fat diet actually improved HDL, or “good” cholesterol, and didn’t worsen LDL, or “bad” cholesterol. And in the '80s, Stephen Phinney—a MIT medical researcher—noticed that the carb-loading math just didn’t add up. Our bodies only have a limited store of glycogen, or the fuel in your muscles, about 2, 500 calories of carbs in reserve at all times—and this can be quickly depleted on long runs. But our bodies have about 50, 000 calories of fat stored—a much deeper pool to pull from. Phinney wondered if athletes could train their bodies to burn fat instead of carbs. Your body naturally burns carbs to keep your muscles moving—and carbs are the quickest form of fuel to convert into energy. But “think of glycogen as the gas in the tank of the car, ” says Pam Bede, R.D., sports dietitian for Abbott’s EAS Sports Nutrition. When that gas is low, you need to refuel, which is where gels and GUs come in. If your body could burn fat, Phinney thought, you could go a heck of a lot longer before refueling. (Try these 6 All-Natural, Energizing Foods for Endurance Training.)
So Phinney put a small group of elite male cyclists on a low-carb diet to test it out—forcing their bodies to tap into the fat stores. While plenty of studies show that a low-carb, high-fat diet results in lower peak power and VO2 max—meaning it more or less makes you slower—he found that cyclists indeed performed just as well on a two and a half hour ride when they ate a diet low in carbs and high in fat as when they ate their traditional training diet. (Check out these 31 Biking Tips from Elite Female Cyclists.)
Out of this, the low-carb, high-fat (LCHF) diet was born. What is it? With an ideal meal plan, you’re taking in roughly 50 percent of your calories from healthy fats, 25 from carbs, and 25 from protein, explains Bede. (The current government recommendation, for comparison, is 30 percent of calories from fat, 50 to 60 percent from carbs, and 10 to 20 from protein.)
The problem? Phinney’s model was imperfect: When he tested cyclist’s sprinting capabilities on the LCHF diet, he noticed fat-fueled athletes clocked in at a slower time than normal. Fast forward some 40 years, though, and medal-winning-triathletes like Simon Whitfield and Ben Greenfield have renounced the church of carbs in favor of a low-carb, high-fat diet. Kim Kardashian famously went low-carb to shed her baby weight. Melissa McCarthy attributed her impressive 45-pound weight loss to a similar eating plan. (Check out 10 Unforgettable Celeb Diets Through the Years.)
But with mixed research and confusing star-studded testimonials—does the diet work? And, furthermore, is it healthy?
Can It Improve Your Fitness?
The effect of a low-carb, high-fat diet on athletic performance has only been looked at in a handful of studies since Phinney’s original experiment. And when it comes to high speeds, Bede says it makes sense why LCHF would slow you down: “Carbs are a fairly efficient way to burn fuel, so if you’re running at high speeds and need that energy immediately, carbohydrates are going to be a better source of fuel, ” Bede explains. Because it takes longer for your body to access the energy in fat, you won't be able to perform as quickly.
If you’re focused on distance and not speed, though, don’t write off LCHF so soon. It actually helps with that moment every runner dreads: hitting the wall. “In endurance athletes, adapting as much as possible to use fat can help those who struggle with bonking. It can help delay that significant onset of fatigue, which is favorable because it enables an athlete to rely less on carbohydrate gels or fluid carbohydrates—and to go faster for longer, ” says Georgie Fear, R.D., author of Lean Habits For Lifelong Weight Loss. Another added bonus: You’ll avoid the all-too-common side effect of gastric distress from race gels and GUs. (Gross! Avoid these 20 Foods that Can Ruin Your Workout too.)
But like much of the LCHF research, the scientific evidence is mixed—it’s still a vastly under-researched area. The most promising study to date is expected to come out later this year from Jeff Volek, Ph.D., R.D., at the Ohio State University, the second most prolific researcher on the topic next to Phinney.
Beyond the research, there’s also a growing wave of triathletes and ultra-runners who attribute their success to jumping on the fat-fueling bandwagon. Fitness coach Ben Greenfield finished the 2013 Ironman Canada in under 10 hours while consuming almost no carbs, while ultra-runner Timothy Olson set a record for fastest completion of the Western States 100-mile course on a LCHF diet. “Athletes I work with say that once they got used to the diet, they feel better than they ever have before, their performance is potentially better—but certainly no worse—and they don’t have sugar cravings or mood swings like when they were trying to fuel with carbs, ” Bede says. (Sound familiar? Until you go LCHF, try these 6 Foods to Fix Your Mood.)
Whether or not it improves performance, teaching your body to pull from your fat reserves—which you can do by simply switching to the diet—does offer better blood sugar stability, Fear adds. This helps prevent hypoglycemia, or low blood sugar (which is the reason Hyvon Ngetich collapsed and had to—now famously—crawl across the finish at this year’s Austin Marathon).