Ketosis is a metabolic state in which blood ketone levels reach around 0.5 mmol/L. People will typically enter ketosis when they restrict carbohydrates (i.e., by following the ketogenic diet) or eat little to no calories (i.e., fasting for multiple days).
In response to sugar restriction and glycogen depletion (when you have almost no stored sugar left), the liver will begin turning fatty acids from your fat cells and your diet (and ketogenic amino acids from protein) into ketones. These ketones will then enter the circulation and provide cells with a much needed alternative fuel source so that the body doesn’t need to rely so much on burning sugar for fuel.
The primary evolutionary purpose of ketosis is to provide the brain with a reliable and efficient source of energy when sugar is no longer in abundance. As a result of burning this new fuel, many changes occur throughout the body and brain that improve our health in ways that no other way of eating can mimic.
For a more in-depth look at ketosis and its benefits, click here.