VLCAD deficiency: a condition in which the body is not able to break down very long chain fatty acids (VLCADs), a type of fat. The condition is caused by mutations in a gene known as ACADVL. It is inherited in an autosomal recessive manner, meaning an affected person must receive a defective copy of the gene from each parent. There are different forms of the condition, and symptoms can begin in infancy, childhood or adulthood. Symptoms can include low blood sugar (hypoglycemia, muscle weakness, and decreased energy. Liver and heart problems can develop in people with the most severe form of the condition.