When the body breaks down fat for energy, three intermediate products are formed. These products, collectively referred to as ketones, are acetone, acetoacetic acid, and beta-hydroxybutyric acid.

Normally, the body gets the energy it needs from carbohydrates in the diet. However, stored fat is broken down and ketones are produced and appear in the urine if the diet does not contain enough carbohydrate to supply the body with glucose for energy or if the body cannot use glucose properly.