The mainstay of PKU treatment is a low-phenylalanine diet (i.e restriction of natural protein), and supplementation with a protein substitute (a mixture of amino acids free from phenylalanine, also containing micronutrients and vitamins) and special low-protein foods, to meet the patient's energy requirements. When diet and treatment is relaxed after childhood, adult and adolescent patients with phenylketonuria are at risk for malnutrition, depending on the compliance with treatment and the intake of amino acid supplements.