Ketones are produced when the body doesn't have enough glucose to meet its fuel needs. The liver breaks down fat to produce ketones, releases them into your bloodstream and your muscles and other tissues use those ketones for fuel!
In human biology, there are three molecules referred to as ketones: beta-hydroxybutyrate (BHB), acetoacetate (AcAc) and acetone. These biological ketones are a fuel source that our bodies can naturally produce from breaking down fat.
BHB and AcAc are the most important ketones for providing fuel to the muscles and brain. The evolutionary function of ketones is fuel the brain when blood glucose falls, because fats in their unprocessed form can’t fuel the brain. However, ketones can be used as a fuel by virtually all the other tissues in the body. Acetone is a by-product of AcAc breakdown and is mainly released in the breath, and not used for fuel.
Our bodies produce ketones when we fast or follow a strict low-carbohydrate diet. That said, the time it takes for your ketone levels to rise depends on your individual metabolism and the methods you're using to achieve ketosis.