Editor's Note: This story was updated on Friday, Aug. 12 at 4:45 p.m. E.T.
The sports world has been abuzz in recent years with the idea that athletes could improve their performance by following an ultra high-fat, low-carbohydrate diet.
Fans of this diet plan said it allows them to run, swim or bike endless miles without needing to refuel with sugary foods. Others, like mixed martial artist George St. Pierre, said they use a low-carb diet to drop 20 to 30 lbs. (9 to 14 kilograms) in a five-day span before weigh-ins, qualifying for lower weight classes in their bouts. (Doing this can lead to deadly dehydration and should never be attempted without medical supervision.)
But is a low-carb diet safe, and does it actually improve athletic performance?
Though safe, restricting carbohydrates won't help athletes hit their peak, especially in high-intensity activities like a marathon, said Asker Jeukendrup, the director of mysportscience. [Dieters, Beware: 9 Myths That Can Make You Fat]
"If you're trying to run your best possible time, carbohydrate is going to be the main fuel and not fat," said Jeukendrup, who has done some of the pivotal studies on how macronutrients affect exercise performance. "That is a well-established fact."
However, a low-carb diet could work for those exercising at lower intensities, such as ultramarathoners who slowly jog hundreds of miles, Jeukendrup said. However, whether people prefer to gnaw on beef jerky or granola bars during an ultramarathon would likely be a matter of personal preference, he said. That preference likely doesn't point to any concrete performance advantage for those who rely on a fat-adapted diet, he said.
Proposed benefits of fat-adapted diets
The idea behind the low-carb or "fat-adapted" diet is simple: Get the body to burn fat, not carbs, to fuel a workout.
Typically, a fat-burning diet includes 65 percent of its calories from fat, less than 25 percent from carbs and the remainder from protein, according to a 2015 study in the journal Sports Medicine. Some of these diets restrict carbohydrate intake even further, to just 20 grams (or about 80 calories) a day (for a 2,000 calorie diet, this would be just 4 percent of calories from carbohydrates). At this level, the body switches to a metabolic state called ketosis, in which it burns fat as fuel. [Infographic: How the Body Uses Energy]
Because the body has a nearly unlimited supply of fat stores, some people have argued that carb-limited diets can help runners avoid a phenomenon known as "hitting the wall," in which they can experience a sudden, intense feeling of fatigue during their races, Jeukendrup said.
Hitting the wall occurs because although the carbohydrates (called glycogen) stashed in liver and muscle can fuel high-intensity exertion, "your carbohydrate sources are very limited," Jeukendrup told Live Science. "And that amount is not quite enough to run a marathon."
Because of this, marathon runners need to refill their glycogen stores, often midrace.
They'll chug sports drinks, swallow candy bars, or eat special goos and gels while they're racing to keep their glycogen stores up, said Dr. David Ludwig, a professor of nutrition at Harvard University in Massachusetts and the author of "Always Hungry?: Conquer Cravings, Retrain Your Fat Cells and Lose Weight Permanently" (Grand Central Life & Style, 2016).
"Unfortunately, the fuel that many athletes rely on is processed, fast-acting carbohydrate: sugar," Ludwig said.
For elite athletes, those quick, sugary calories get burned off quickly, but people doing more average levels of exercise may "become dependent upon constant, frequent infusions of carbohydrate" to fuel their performance, Ludwig said.
As a result, "a garden-variety athlete could easily take in more calories than they burn off, just to keep themselves feeling adequately fueled, and that defeats a basic purpose of physical activity," Ludwig told Live Science. Others complain of gastrointestinal distress when eating all those carbs during a marathon, Ludwig said.
Processing carbs also requires insulin, the hormone that allows sugar in the bloodstream to pass into the cells as fuel. But insulin also promotes fat storage, he said.
Another reason athletes switch to low-carb diets is to lower their water weight, or the amount of water their body is holding on to.
Each molecule of stored glycogen binds to three water molecules, said Jordan Moon, the program director of sports and health sciences and sports management at American Public and American Military University, and chief science officer at the fitness-tracking website FitTrace.com.
Some mixed martial artists and wrestlers, for example, will adopt a low-carb diet before weigh-ins to deplete their glycogen stores so they can drop water weight and qualify for a lower weight class, Moon said. Then, after the weigh-in but before the match, they'll switch to a high-carb diet, he said.
Do fat-adapted diets work?
But despite the hype, little evidence suggests that fat-adapted diets really improve athletic performance.
When the body doesn't have enough carbohydrates, it does increase its breakdown of fat, according to a 2015 study in the journal Metabolism Clinical and Experimental. In that study, ultra-endurance marathoners who were on an extremely low-carb diet could burn fat at twice the rate of those who were on a high-carb diet.
Still, low-carb, high-fat diets almost always lead to lower performance, according to more than a dozen studies conducted from 1960s to the 2000s. Because fat metabolism requires more chemical reactions in cells than metabolizing carbs does, it takes longer to produce the same amount of energy, meaning people who switch to burning fat can only exercise at a lower intensity compared to those who are burning carbs. [Infographic: What Is VO2 Max?]
In some of the most definitive work on this subject, Louise Burke, an exercise physiologist at the Australian Institute of Sport, and her colleagues conducted a study of low-carbohydrate and high-carbohydrate diets for elite race walkers. Her work has found that low-carb diets reduced performance.
And cycling between a low- and high-carb diet, as many mixed martial artists may do, probably doesn't help performance either, said Melinda Manore, a biologist and nutrition scientist at Oregon State University who studies how exercise affects nutritional needs.
That's because going on a low-carb diet changes how many enzymes the body makes to burn carbohydrates, and it can take several days to reverse this. On the day of a fight, for instance, people on a low-carb diet won't be able to utilize their glycogen stores as well to perform at high intensity, she said.
Fat-adapted diets may work OK for some ultramarathoners or extreme endurance athletes who need to work out at low intensity for long periods of time. However, studies also suggest that people on high-carb diets can get excellent results in similar endurance challenges, Jeukendrup said.
Beyond that, low-carb diets are often difficult to follow. Consuming no carbs means no fruits, veggies or whole grains, Manore said. One of the competitive race walkers in Burke's study took to eating sticks of butter, according to a recent interview.
"Most athletes hate it. They can't stay on it. They don't feel good," Manore said. "It's just not practical."
Editor's Note: This story was edited to update Asker Jeukendrup's affiliation. He is no longer with the Gatorade Sports Science Institute.
Original article on Live Science.
Live Science newsletter
Stay up to date on the latest science news by signing up for our Essentials newsletter.
Tia is the managing editor and was previously a senior writer for Live Science. Her work has appeared in Scientific American, Wired.com and other outlets. She holds a master's degree in bioengineering from the University of Washington, a graduate certificate in science writing from UC Santa Cruz and a bachelor's degree in mechanical engineering from the University of Texas at Austin. Tia was part of a team at the Milwaukee Journal Sentinel that published the Empty Cradles series on preterm births, which won multiple awards, including the 2012 Casey Medal for Meritorious Journalism.