Ketone bodies are certain chemicals produced mainly in the liver; they provide much of the energy to heart tissue and during starvation also to the brain. In Type 1 diabetes, excessive ketone bodies can accumulate in the blood, causing diabetic ketoacidosis, a dangerous condition.

The chemicals acetoacetate, acetone and beta-hydroxybutyrate are collectively known as ketone bodies; note that the first two are ketones while the third is not.

Ketone bodies are formed in the mitochondria of liver cells from acetyl-coenzyme A (acetyl CoA). Acetyl CoA results from the breakdown of carbohydrates, lipids and amino acids. Normally, the acetyl group of acetyl CoA enters the citric acid cycle to generate energy in the form of ATP. Some of it can also leave the mitochondria in the form of citrate to participate in the synthesis of fatty acids. A third possible fate of acetyl CoA is the production of ketone bodies; this happens if acetyl CoA levels are high and the citric acid cycle cannot keep up. The creation of ketone bodies is also known as ketogenesis.

Ketone bodies are transported from the liver through the blood to other tissues, where acetoacetate and beta-hydroxybutyrate can be reconverted to acetyl CoA to produce energy. The heart derives much of its energy in this way although fatty acids are also important in supplying energy. Most tissues obtain the bulk of their energy from glucose. This is particularly important for the brain. If glucose levels are low, such as during starvation, the brain uses ketone bodies as an energy source instead. After a three-day fast, 30% of the energy used by the brain is from ketone bodies. And after 40 days, this goes up to 70%.

Acetone is formed from spontaneous decarboxylation of beta-hydroxybutyrate. Correspondingly, the levels of acetone are much lower than those of the other two types of ketone bodies. It cannot be converted back to acetyl CoA and is excreted in the urine and exhaled because of its high vapor pressure. The exhalation of acetone is responsible for the characteristic "fruity odor" of the breath of persons in ketotic states.

Both acetoacetate and beta-hydroxybutyrate are acidic, and if levels of ketone bodies are too high, the pH of the blood falls, resulting in a condition known as ketoacidosis. This happens in untreated Type I diabetes (see diabetic ketoacidosis) and also in alcoholics after binge drinking and subsequent starvation (see alcoholic ketoacidosis).

The ultimate reason for ketoacidosis in these cases is the same: the cell does not have enough glucose (in the case of diabetes because lack of insulin prevents the cell from taking up glucose, in the case of fasting because there is less glucose around). This means that acetyl CoA is mainly produced from the breakdown of fatty acids and fed into the citric acid cycle. The intermediates of the citric acid cycle are used for other anabolic purposes as well and have to be replenished. Normally, this is done by converting pyruvate into oxaloacetate or L-malate (the so-called anaplerotic pathways). But pyruvate is the end product of glycolysis, the breakdown of glucose, and glucose levels are lower in the cases we consider. This means that the citric acid cycle intermediates cannot be replenished, the cycle slows down, acetyl CoA accumulates and ketogenesis becomes more important.