The substances beta-hydroxybutyric acid, acetoacetic acid, and acetone, which are produced by fatty acid and carbohydrate metabolism in the liver in approximately a 78:20:2 ratio. Acetoacetate is produced from acetyl-CoA. Most is enzymatically converted to beta-ketobutyrate, but a small amount is spontaneously decarboxylated to acetone. The ketone bodies can be used as fuels by muscle and brain tissue. In starvation and uncontrolled diabetes mellitus, large quantities are produced, causing metabolic acidosis and elevated blood and urine levels of all three ketone bodies.