Keto-adaptation (also sometimes called “fat-adaptation”) is the process the body goes through on a ketogenic diet as it changes from using primarily glucose for energy to using primarily fat for energy. The “keto” part refers to ketones, which are water-soluble molecules that the liver makes when metabolizing fats, particularly when carbohydrate intake is low. Ketones can be used for energy by most tissues in our bodies, including the brain (which cannot use fatty acids directly).
Our bodies are always using a mix of fat and glucose for energy, but in a non-keto-adapted state the body reaches for glucose first, since only low amounts of ketones are normally generated during fat metabolism and these are preferred by other tissues such as the heart
Since the brain cannot use fat, it is dependent on glucose when we are in a non-keto-adapted state. Because of this, when we go on a low-carb diet we can sometimes experience what I call “carb crash” or is sometimes referred to as “the Atkins flu” when our bodies run out of glycogen stores (this is the main way our bodies store glucose). It is when the glycogen stores get low that the body begins the process of keto-adaptation.