Medium-chain acyl-coenzyme A dehydrogenase (MCAD) deficiency is a rare condition that prevents the body from converting certain fats to energy, particularly during periods without food (fasting). People with this disorder have inadequate levels of an enzyme that breaks down (degrades) a certain group of fats called medium-chain fatty acids. Typically, initial signs and symptoms of this disorder occur during infancy or early childhood and may include vomiting, lack of energy (lethargy), and low blood sugar (hypoglycemia). Serious complications such as seizures, breathing difficulties, liver problems, cardiac arrest, brain damage, coma, and sudden unexpected death are also a risk. Episodes of medium chain acyl-coenzyme A dehydrogenase deficiency can be triggered by periods of fasting or illnesses such as viral infections, particularly when eating is reduced.