Acetoacetic acid, or diacetic acid, is a naturally occurring keto acid, which is a type of molecule containing both ketone and a carboxylic acid functional groups. This compound is a beta-keto acid, so named because the ketone group is placed on the second carbon away from the acid group. Acetoacetic acid is formed by the liver and kidneys by metabolizing fatty acids. Along with other ketone bodies, this acid can be used for energy by the heart and brain, so it has its own biological importance.
Although acetoacetic acid is soluble in water, it is an unstable acid, and will break down into carbon dioxide and acetone after about 140 minutes. Under normal conditions, the brain and heart do not need acetoacetic acid for energy, and very little will be produced. Individuals suffering from conditions such as diabetes or other conditions that impair metabolism may produce an overabundance of ketone bodies. Therefore, a doctor may administer a urine test using a chemical that changes color in the presence of acetoacetic acid, and the degree of color change can be evaluated by the naked eye.
Detection of this acid, as well as other ketone bodies, is essential for diabetics. In the event that a case of diabetes is not treated, beta-keto acids may build up in the blood. Acetoacetic acid is a weak acid, but in sufficient concentrations, it may cause the blood to become acidic, itself. This condition is called ketoacidosis, and in its most extreme manifestations, it can be lethal. Smaller concentrations of ketone bodies in the blood is known as ketosis, and is not always dangerous.
While diabetes is the most well-known medical condition that can lead to the overproduction of acetoacetic acid, other medical issues may cause this process, too. Glycogen storage disease and conditions that increase metabolism, such as hyperthyroidism, may also create higher levels of beta-keto acids in the body. Perhaps the most frequent cause of this phenomenon is dietary disruptions, such as starvation, fasting, and anorexia.
When the body has lacked food for some time, tissues begin to break down their fatty acid reserves. The brain, however, does not have fatty acids to use, and instead must rely on the ketone body byproducts from other tissue. If the heart lacks fatty acids to use, it will eventually utilize ketone bodies as well. Therefore, this acid can be seen as a means of survival in extreme circumstances.