Phenylketonuria, or PKU, is a rare, inherited condition. Babies with PKU are born missing an enzyme called phenylalanine hydroxylase, which the body needs to break down an amino acid called phenylalanine. If left undiagnosed, the condition will cause a buildup of phenylalanine in the body that is harmful to the central nervous system. Many states require PKU testing for all newborns. Because the body takes phenylalanine in from the proteins consumed, infants with PKU must be fed a strict low-protein diet.
Significance
Without phenylalanine hydroxylase, an infant's body is incapable of converting phenylalanine from an essential amino acid into a nonessential amino acid called tyrosine. The body's inability to break down phenylalanine results in a high level of phenylalanine in the blood and phenylketones in the urine. Without treatment, PKU can be devastating. PKU symptoms include delayed mental and social skills, hyperactivity, seizures and mental retardation. PKU is treatable by following a low-phenylalanine diet based on an infant's individual needs and monitored closely by a doctor or dietitian.
PKU Infant Formula
While infants with PKU must have their phenylalanine intake restricted, their bodies still need some amount of the amino acid for normal growth and development. Prior to the addition of solid foods, infants with PKU are prescribed a diet of breast milk or regular infant formula combined with a special phenylalanine-restricted commercial formula. Regular blood tests monitor the infant's phenylalanine level, and the amount of phenylalanine given to the child is adjusted as necessary.
Solid Foods
Once an infant is ready to start eating solids, low-phenylalanine foods can be introduced following the same schedule recommended for regular infants. The phenylalanine in these foods will be calculated by the doctor or dietitian to replace what the infant was receiving while on breast milk or regular formula. Infants with PKU should continue to drink phenylalanine-restricted commercial formula in addition to solid foods, as these formulas contain the vitamins, minerals and essential amino acids needed for normal growth and development. Low-protein foods allowed on the PKU diet include fruits, vegetables and some grain products. A dietitian or doctor will teach parents which specific fruits, vegetables and grains are right for their child.
Foods to Avoid
As MayoClinic.com notes, infants with PKU must avoid all high-protein foods, such as milk, cheese, eggs, soybeans, nuts, beans, chicken, steak, fish, chocolate and peas. In addition, certain foods will need to be limited, such as pasta, rice, bread, cookies and some fruits and vegetables. Foods and medications containing aspartame should also be avoided. Aspartame, found in many sugar-free products and sodas, releases phenylalanine when digested.
Considerations
Infants diagnosed and started on a PKU diet shortly after birth typically have no symptoms and experience normal brain development. Children with PKU will need to continue a low-phenylalanine diet throughout their lives. According to About Kids Health, discontinuing the diet can lead to serious problems, such as deterioration of IQ, learning disabilities, hyperactivity, irritability, tremors, eczema and personality disorders. In addition, a buildup of phenylalanine in pregnant women with PKU who do not follow their diets can damage their unborn children.
