What you should know:
Ketosis is a condition in which your body produces and uses a greater amount of a type of chemical called ketones as fuel. Unlike fat, ketones are a kind of fuel that your brain can use. But in reading descriptions of ketosis one can get the impression that, as brain fuel, glucose is entirely replaced by ketones, and that’s definitely not true. The body’s operation is much more nuanced than that.
The truth is that maintenance of blood glucose within a certain range is absolutely necessary for survival, and the body will go to great lengths to achieve it. That starts with storing the extra glucose that generally comes from the carbs in a meal in the form of glycogen in your muscles and liver, then using the glycogen in your liver as a source of glucose to replenish blood sugar.
If the glycogen runs out before next mealtime, glucose can be generated from certain components of fat and from proteins, and the body also burns more and more fat to conserve glucose. But since that fat cannot be used as fuel in the brain, your body (specifically, your liver) will also produce ketones so that the brain doesn’t require as much glucose.
The bottom line is that at any given time your body is using all three fuels—glucose, fat, and ketones—for energy, adjusting the relative amounts of each so as to obey the imperative of maintaining a healthy glucose level. Once you understand that imperative, much of what you hear or read about energy management in your body will make more sense.