fat-burning mode isn’t like flipping a switch. When you go Keto, your body needs time to fat-adapt.
Without Keto, your body relies more on carbs (glucose) for energy. But a low-carb diet takes away your carbs, so you switch to burning fat for energy. This fat-adaptation, also called Keto-adaptation, can take anywhere from 2 to 8 weeks, depending on the state of your metabolism.[*]
So if you’re on the Keto diet to lose weight, you need to be patient. That’s the first principle. Being patient gives your body time to fat-adapt, so you start burning through your body fat for energy.
During fasting, or when very little carbohydrate is eaten, the body first pulls stored glucose from the liver and temporarily breaks down muscle to release glucose. If this continues for 3-4 days and stored glucose is fully depleted, blood levels of a hormone called insulin decrease, and the body begins to use fat as its primary fuel. The liver produces ketone bodies from fat, which can be used in the absence of glucose.