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After adaptation phase the low-carb athlete changes into a fat incinerator

Endurance athletes who have been on a low-carbohydrate diet for quite some time will burn considerably more fat than endurance athletes with a traditional carbohydrate-rich diet. Even at an intensity where their muscles can not derive their energy from fat according to the textbooks, these athletes mainly burn fat. American sports scientist Jeff Volek reported this in Metabolism in 2016.

Volek studied 20 well-trained endurance athletes. Some of them were ultrarunners, some Ironman triathletes. Ten athletes followed a traditional diet: 60 percent of their energy came from carbohydrates, 15 percent from protein and 25 percent from fat. Ten other athletes had switched autonomously to a low-carbohydrate diet at least 9 months earlier: 70 percent of their energy came from fat, 20 percent from protein and the remaining 10 percent from carbohydrates.

Volek had his test subjects run on a treadmill with increasing intensity. For the athletes who followed a traditional diet, fat burning was maximal at an intensity of 55 percent of the VO2max. With more intensive exercise, the muscles had to burn more carbohydrates. That is more or less in accordance with the textbooks.

The athletes who followed a low-carbohydrate diet, however, burned most fats per minute at an intensity of 70 percent of the VO2max. And that is definitely not in accordance with the textbooks.

After adaptation phase the low-carb athlete changes into a fat incinerator

On another occasion, Volek let his subjects run for 3 hours with an intensity of 64 percent of their VO2max. Just before that, the subjects were given a shake with a nutritional profile that was consistent with their diet.

The low-carbohydrate athletes mainly burned fat, the carbohydrate athletes mainly burned carbohydrates.

After adaptation phase the low-carb athlete changes into a fat incinerator

After adaptation phase the low-carb athlete changes into a fat incinerator

Before and after this test, Volek determined the glycogen concentration in the muscle cells of the athletes. This concentration was - contrary to what you might expect - almost identical in both groups of athletes. After endurance efforts, muscle cells recover optimally if they contain sufficient glycogen.

After adaptation phase the low-carb athlete changes into a fat incinerator

"These results provide the first documentation of the metabolic adaptations associated with long-term consumption of a very low-carbohydrate/high-fat diet in highly trained keto-adapted ultra-endurance athletes", Volek wrote. "The enhanced ability to oxidize fat during exercise across a range of intensities is striking, as is the ability to maintain 'normal' glycogen concentrations in the context of limited carbohydrate intake."

"Keto-adaptation provides an alternative to the supremacy of the high-carbohydrate paradigm for endurance athletes."

Metabolism. 2016 Mar;65(3):100-10.

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