Phenylketonuria (PKU) is a rare genetic disorder that results in the amino acids phenylalanine getting deposited in the system. Amino acids are the building blocks of protein that are essential for growth and development. Phenylalanine is present in all protein foods and some artificial sweeteners.

Phenylalanine hydroxylase is an enzyme the body uses to convert phenylalanine into tyrosine which the body requires to make neurotransmitters like epinephrine, norepinephrine, and dopamine. PKU is caused by a defect in the genes that aid the production of phenylalanine hydroxylase, when this enzyme is inadequate the body cannot break down phenylalanine and this results in the accumulation of phenylalanine in the system. When an individual with this disorder consumes foods rich in protein or aspartame, an artificial sweetener that contains phenylalanine it can eventually lead to severe health issues.

People with PKU need to follow a strict diet that eliminates phenylalanine, which is present in most protein-rich foods. Prompt diagnosis and treatment can help alleviate symptoms of PKU and avert brain damage.

Also Read: Protein Building Blocks: Here’s Why You Need Essential Amino Acids In Ample Amounts

Symptoms Of Phenylketonuria

Generally, PKU symptoms range from mild to severe. The most severe of this condition is termed classic PKU. An infant born with PKU may appear normal for the first few months of life but if left untreated they will start to develop the following symptoms:

  • Seizures
  • Tremors
  • Stunted growth
  • Hyperactivity
  • Eczema
  • Bad breath
  • Foul odour from skin or urine

If PKU isn’t diagnosed at birth and treatment is delayed, then the condition can cause:

  • Irreversible brain damage and intellectual disability
  • Behavioural issues
  • Seizures

A less severe form of PKU is termed as variant PKU. This develops when the baby has excess phenylalanine in the body and infants with this form exhibit mild symptoms, however, they need to be on a special diet regimen to avert any intellectual disabilities.

Once the child is on a specific diet plan combined with other treatments, symptoms start to settle. Also, individuals with PKU who manage their diet effectively don’t exhibit any symptoms.

Causes Of Phenylketonuria

PKU is an inherited disorder caused by a defect in the PAH gene and this gene aids production of the enzyme phenylalanine hydroxylase responsible for breaking down phenylalanine. A very high accumulation of phenylalanine can happen when a person eats protein-rich foods like eggs and meat.

However, only when both parents pass on a defective version of the PHA gene to their offspring, then the child inherits this disorder. If one parent passes on an altered gene, the child wouldn’t show any symptoms but will be a carrier of the gene.


Newborn screening helps to identify all cases of phenylketonuria. Most countries routinely screen babies for PKU.

If you have a family history of it or health care provider may suggest screening tests before pregnancy or birth. Also, it’s possible to determine PKU carriers through blood work.

Testing Baby After Birth

PKU screening is done a day or two after the baby’s birth. For precise results, the test is performed after 24 hours of the baby's birth and after the baby has been breastfed.

A lab technician collects a few drops of blood from the baby’s heel.

The sample taken is run for certain other metabolic disorders, including PKU.

If the test indicates that the baby may have PKU, then further blood work is done to confirm the diagnosis, including more blood and urine tests.

Genetic testing is done to identify the gene alteration for PKU.

Also Read: Genetic Testing: What Is It, How It Is Done And What To Expect?


It is very essential to start treatment early and continuing treatment throughout life can help avert intellectual disability and other severe health concerns.

The chief goal of treatments for PKU include:

A customized diet regimen to be followed for a lifetime that limits the intake of phenylamine-rich foods.

Nutritional supplement is provided to ensure that the person gets an adequate amount of essential protein and other nutrients required for normal growth and health.

Doctors also prescribe medications for a few people with PKU and control symptoms.

A safe amount of phenylalanine differs from person to person with the condition and can vary over a period. In general, the main aim is to have only the right amount of this amino acid required for normal growth and development, but no more. The healthcare provider can assess a safe limit of phenylalanine through:

Constant review of food records and growth charts.

Blood works to monitor blood phenylalanine levels, mainly during childhood growth spurts and pregnancy.

A certified dietitian may help a patient learn more about the PKU diet and make alterations to the diet as and when needed and provide suggestions on ways to cope with PKU diet challenges.