In ketone body synthesis, an acetyl-CoA is split off from HMG-CoA, yielding acetoacetate, a four carbon ketone body that is somewhat unstable, chemically. It will decarboxylate spontaneously to some extent to yield acetone. Ketone bodies are made when the blood levels of glucose fall very low. Ketone bodies can be converted to acetyl-CoA, which can be used for ATP synthesis via the citric acid cycle. People who are very hypoglycemic (including some diabetics) will produce ketone bodies and these are often first detected by the smell of acetone on their breath.
Acetone is of virtually no use for energy production since it is not readily converted to acetyl-CoA. Consequently, cells convert acetoacetate into beta-hydroxybutyrate, which is more chemically stable. Though technically not a ketone, beta-hydroxybutyrate is frequently referred to as a ketone body. Upon arrival at a target cell, it can be oxidized back to acetoacetate with conversion to acetyl-CoA. Both acetoacetate and beta-hydroxybutyrate can cross the blood-brain barrier and provide important energy for the brain when glucose is limiting.