The HADH gene provides instructions for making an enzyme called 3-hydroxyacyl-CoA dehydrogenase that is important for converting certain fats to energy. This enzyme is involved in a process called fatty acid oxidation, in which several enzymes work in a step-wise fashion to break down (metabolize) fats and convert them to energy. The role of 3-hydroxyacyl-CoA dehydrogenase is to metabolize groups of fats called medium-chain fatty acids and short-chain fatty acids. These fatty acids are found in foods such as milk and certain oils and are produced when larger fatty acids are metabolized.~3-hydroxyacyl-CoA dehydrogenase functions in mitochondria, the energy-producing centers within cells. This enzyme is especially important for the normal functioning of the heart, liver, kidneys, muscles, and pancreas. The pancreas makes enzymes that help digest food, and it also produces insulin, which controls how much sugar (glucose) is passed from the blood into cells for conversion to energy.~3-hydroxyacyl-CoA dehydrogenase is essential in the process that converts medium-chain and short-chain fatty acids to ketones, the major source of energy used by the heart and muscles. During prolonged periods without food (fasting) or when energy demands are increased, ketones are also important for the liver and other tissues.