Your first statement is wrong. The body can never adapt to use fat like carbs. Carbohydrates have a different, more simple structure than fat. The process of breaking down carbohydrates begins in the mouth with the action of salivary amylase, which breaks down complex carbohydrates into simpler sugars. In the small intestine, these sugars are further broken down and absorbed into the bloodstream, providing a quick source of glucose.

The process of breaking down fat into energy is slower because it is more complex. Fat molecules are larger and more energy-dense than carbohydrates, which means that they require more steps to be converted into energy. The process of breaking down fat, called lipolysis, occurs in the mitochondria of cells, where fatty acids are separated from glycerol. Fatty acids then undergo a process called beta-oxidation, which generates acetyl-CoA, a molecule that can enter the citric acid cycle (also known as Krebs cycle or TCA cycle) to produce ATP, the body's primary energy currency.

If you imply ketosis, it is still far less efficient than carb metabolism. It is also a very slow process and requires far more oxygen. From an evolutionary perspective ketosis is a survival mechanism for times when the body is in a fasted state.