We eat three macro nutrients – Carbohydrates, Proteins and Fat.
When we eat carbs, our primary source of fuel is glucose but things are different in keto. In keto, our goal is to keep the insulin as low as possible for as long as possible. So we reduce the amount of carbohydrates and increase the amount of fat and protein we consume.
When we consistently eat a lot of carbs, we tend to have increased levels of insulin to consume the glucose in either sending them to the cells to metabolize or convert them to fat. Medical science clearly says, refer to textbook of Physiology By Guyton, that we burn fat only in the absence of insulin. Since in keto we do not have elevated insulin at any time, our body metabolizes fat instead of carbohydrates and creates ketones. The state when body is producing ketones is knows as ketosis. In ketosis, we use ketones as the primary source of fuel.
We either supply fat from outside, to maintain healthy weight, or supply fat from body’s fat storage, to lose weight. In either case we burn fat and preserve muscles.