Ketones are organic compounds produced when the body breaks down fats for energy, especially during periods of low carbohydrate intake, such as in a ketogenic diet or fasting. They serve as an alternative energy source for cells, particularly in the brain, when glucose levels are low. The three main types of ketones are acetoacetate, beta-hydroxybutyrate, and acetone. Ketones can be measured in the blood, urine, or breath, and their presence indicates a state of ketosis, which can have various effects on metabolism and energy levels.