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As you transition from burning sugar to burning fat, you may notice a temporary decline in your athletic performance. This phase typically lasts about two weeks. When glycogen stores are depleted, your muscles need time to adapt to using fat as a fuel source. During this period, you may experience reduced vitality, muscle strength, and overall exercise endurance, but your performance will eventually recover. This process is often referred to as keto-adaptation. The advantage of following a very low-carbohydrate or ketogenic diet and exercising in a fasted state is that it trains your muscles to burn fat, although this adaptation requires time. The calories stored as fat are significantly more than those stored as glycogen. For endurance athletes, switching to burning fat means having access to a much larger pool of available calories, which offers a considerable advantage.