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Before Dr. Robert Atkins launched his low-carb diet in 1972, there was Banting, the fat British undertaker who designed coffins for England’s elite in the 1800s. According to , the guy needed to drop a few kilos, so his doc put him on a high-fat-low-carb (HFLC) diet and, presto change-o, he lived to 82 and was buried in a skinny man’s coffin.

It may seem silly to talk about Banting now, but he’s back from the dead, courtesy of a controversial sports scientist who has been vehemently championing the Brit’s diet for the past few years. , from the Sports Science Institute of South Africa at the University of Cape Town, even published that sold out not long after hitting shelves earlier this year. There’s just one problem with the diet: It’ll make you slower.

Matt Fitzgerald, author of the new book , has some sobering words for athletes who try to train on fat. “Decades of research indicate that high-carb diets are optimal for endurance and that ingesting carbohydrates during endurance exercise enhances endurance,” Fitzgerald says.

Sports nutrition scientist and European Journal of Sport Science editor in chief , for example, recently outlining carb needs during exercise to enhance endurance, suggesting athletes take in small amounts of carbs during training sessions lasting an hour, 60 grams of carbs per hour for exercise lasting two to three hours, and 90 grams per hour for exercise lasting longer than that—regardless of body weight or training status. , on the other hand, advocates eating as little as 25 to 50 grams of carbs per day. While upping your healthy fat intake to around 40 percent of your total daily calories is fine, Noakes promotes a diet that’s 80 percent fat and only 10 percent carbs. Most endurance athletes, Fitzgerald says, should not be cutting carbs.

It’s not just science that shows carbs make athletes better. There’s a lot of anecdotal evidence as well. Eating carbs “is almost a universal practice among the world’s best endurance athletes,” Fitzgerald says. “The typical Kenyan diet is 78 percent carbs, and they destroy the rest of the world in distance running.”

Clearly, there’s a lot going on behind Kenyan running prowess, but the carbs can’t be discounted. “If you care about your performance as an endurance athlete, the safe way to go is a high-carb diet,” Fitzgerald says. “If you go on the high-fat bandwagon, it’s a crapshoot.”

Proponents of HFLC claim the Banting diet is the key to weight loss and improved health and encourages the body to burn fat for fuel. But studies comparing HFLC to nonrestrictive diets found that, over time, people lost no more weight “banting” than they did otherwise. Celeste Naude, a researcher from the Center for Evidence-Based Health Care at Stellenbosch University, told the that “the dietary pattern and food choices promoted with a low-carbohydrate/high-fat diet are not well aligned with healthy dietary patterns and food choices known to, along with a healthy lifestyle, reduce the risk of diseases like heart disease, stroke, diabetes, and cancers.”

Why the fervor? Fitzgerald says the desire to follow a restrictive diet is fulfilling mental, rather than physical, needs. He calls the phenomenon “sour grapes syndrome,” after Aesop’s fable about the fox who couldn’t reach some hanging grapes. Rather than admit he couldn’t jump high enough to get them, the fox decided they must be sour anyway. As Fitzgerald writes on , the victims of sour grapes syndrome:

…are endurance athletes who cannot cope psychologically with being slower than they would like to be and who resolve this cognitive dissonance by replacing the goal of doing their sport well with that of doing it “right.” The syndrome is being spread by various movements that promote alternative methods that are contrary to those practiced by the most successful athlete … By latching onto [HFLC], athletes can claim a kind of victory over superior competitors.

At some point, Fitzgerald writes, when a movement grows large enough, “it begins to win converts among athletes who are not, in fact, wracked with jealousy of faster athletes but who simply don’t know any better.” To enlighten those athletes, he cites a study recently published in the journal Nutrients in which researchers compared HFLC athletes with those eating a balanced diet. While HFLC made athletes leaner, it also caused them to lose power due to “impairment of the muscles’ ability to burn carbs.”

The athletes had, essentially, trained their bodies to use fat for fuel and decrease reliance on carbs. The result, Fitzgerald writes, is a reduced tolerance for high-intensity training and impaired “performance in all races except perhaps ultra-endurance events such as 100 km trail runs.” If you want to go fast, your body needs carbs for fuel. Training on a Banting diet makes your body less efficient at doing so, ultimately hobbling most athletes on race day.

The next time you’re thinking about cutting carbs, check your motives. If you’re doing it to look lean or run 62 miles or more at once, HFLC may work for you. But if you want to perform optimally at nearly any other endeavor, don’t ditch your bagels.