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Endurance Athletes Burn Twice as Much Fat on Low-Carb Diets

Low-carb diets with an emphasis on calories from fat help runners torch fat without hurting exercise performance or recovery.

You may bear witness to a tectonic shift in sports nutrition very soon, according to researchers at Ohio State University. At least if your primary goal in exercise is to burn fat...

Carbs, typically an endurance runner’s fuel of choice may be slogging 'em down, they say. Turns out, the endurance athletes who “go against the grain” and opt for little to no carbs burn twice as much fat as high-carb athletes, according to new a new study

Researchers enlisted 20 ultra-endurance runners between the ages 21 and 45 who were top competitors in running events of 31 miles or more. 

Half of these runners were placed on a ketogenic diet—diets that reduce carbs enough in the body to allow excess fat stores to serve as the primary source of fuel—and consistently ate meals comprising 10 percent carbs, 19 percent protein, and 70 percent fat for about 20 months (it takes weeks or longer for the body to fully adjust to a low-carb diet, so athletes were only eligible if they had been carb-restricting for at least six months, according to a press release). The other half consumed more than half their calories from carbs, with a diet ratio of 59 percent carbs, 14 percent protein, and 25 percent fat.

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Then came the tests. On day one of two, the athletes did a brief high-intensity workout on a treadmill so the researchers could determine their maximum oxygen consumption and peak fat-burning rates. On the second day, the athletes ran on a treadmill for three hours at an intensity equal to 64 percent of their maximum oxygen capacity. Before the run, athletes consumed either low- or high-carb nutrition shakes consisting of about 340 calories, but during this test, they consumed nothing but water.

By measuring gas exchange repeatedly during the treadmill test, the researchers determined the athletes’ maximum oxygen intake to, in turn, measure their carb- and fat-burning rates. On average, the two groups didn’t really differ in how much oxygen they consumed or how much energy they exerted, but fat-burning rates during prolonged exercise were about twice as high in low-carb athletes than high-carb athletes (1.5 versus .67 grams per minute, respectively). And the average contribution of fat as fuel during the long-distance run was 88 percent and 56 percent, respectively.

"The low-carb guys go beyond what you can achieve with good genetics and extensive training," said lead researcher Jeff Volek in the press release. "The high-carb runners were very healthy, and were awesome fat burners by conventional standards—yet their peak fat burning is less than half that of endurance athletes eating low-carb diets. This shows that we have far underestimated how much fat humans can burn. There is a large reserve capacity that can only be tapped if carbs are restricted."

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Carb-restricting isn’t limited to runners, either; the researchers say many different athletes from sports teams are experimenting. And maybe more should since it doesn’t seem to hinder recovery, either.  

“Despite their low intake of carbs, these fat-burning athletes had normal muscle glycogen levels—the storage form of carbohydrates—at rest," Volek said. "They also broke down roughly the same level of glycogen as the high-carb runners during the long run, and synthesized the same amount of glycogen in their muscles during recovery as the high-carb athletes.” 

Burn twice as much fat, and dodge any negative effects on performance and recovery by a simple diet tweak? Could be worth a shot. 

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