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While it is believed that carbohydrate intake after exercise is the most effective way of replacing depleted glycogen stores, studies have shown that, after a period of 2–4 weeks of adaptation, physical endurance (as opposed to physical intensity) is unaffected by ketosis, as long as the diet contains high amounts of fat, relative to carbohydrates. Some clinicians refer to this period of keto-adaptation as the "Schwatka imperative" after Frederick Schwatka, the explorer who first identified the transition period from glucose-adaptation to keto-adaptation.
Western diets are high in carbohydrates, leading to high blood sugar levels, whereas the ketogenic diet enforces a small amount of carbs to be consumed. It can take some weeks of keto dieting to reach increased levels of blood ketones. For others, after 2 - 4 days of low-carbohydrate, high-fat ketogenic dieting, ketone levels in the blood can increase to ~1 - 2 mM, achieving ketosis.5
An interview with University of Florida researcher and scientist Dominic D’ Agostino. In that episode, “A Deep Dive Into Ketosis: How Navy Seals, Extreme Athletes & Busy Executives Can Enhance Physical and Mental Performance With The Secret Weapon of Ketone Fuel“, Dominic highlights his research into the use of ketones to enhance breathhold time and reduce the brain’s requirements for oxygen.
“Muscle loss on the ketogenic diet is an ongoing area of research,” says Edwina Clark, RD, a dietitian in private practice in San Francisco. “Small studies suggest that people on the ketogenic diet lose muscle even when they continue resistance training. This may be related to the fact that protein alone is less effective for muscle building than protein and carbohydrates together after exercise.” Meanwhile, according to a small study published in March 2018 in the journal Sports, people following the keto diet for three months lost about the same amount of body fat and had about the same muscle mass changes as people following normal diets. Yet the folks on keto did lose more leg muscle.