Ketone body: A chemicals that the body makes when there is not enough insulin in the blood and it must break down fat instead of the sugar glucose for energy. The ketone bodies -- acetone, acetoacetate, and beta-hydroxybutyrate -- are toxic acidic chemicals. They build up in the blood and then spill over into the urine. The body can also rid itself of acetone through the lungs. This gives the breath a fruity odor.
The presence of ketone bodies in the blood is termed ketosis and the presence of ketone bodies in the urine is called ketonuria.