What Is Phenylketonuria (PKU)?

Phenylketonuria (PKU) is a genetic disorder, in which the body is not able to break down a type of protein called phenylalanine. Phenylalanine is one of the amino acids that help in protein formation in the body. However, in Phenylketonuria as the body is unable to process this amino acid, it begins to build up in the body and can be harmful.

Symptoms Of Phenylketonuria (PKU)

PKU is a rare genetic condition, under the umbrella of inborn errors of metabolism and its occurrence is around 1 in 10,000 cases.

The gene that is responsible for the breakdown of phenylalanine in the body gets altered in persons with PKU. As a result, phenylalanine gets excessively stored in their blood and brain. If the levels become too high, it can affect the brain and also the intellectual ability.

What Causes Phenylketonuria (PKU)?

Normally, when a person consumes proteins, the body derives the amino acid - phenylalanine from it. The body then breaks it into an enzyme called phenylalanine hydroxylase (PAH), which is responsible for further digestion of proteins. In persons with PKU, the gene that controls the function of this enzyme PAH is altered or mutated and hence the enzyme does not function properly. This leads to improper digestion of proteins and accumulation of the amino acid – phenylalanine in the body and can further affect the brain.

Symptoms Of Phenylketonuria (PKU)

Although there may be no visible symptoms in newborn babies or those who have started early treatment, if left untreated it can have serious effects on the brain. In newborns the symptoms may be visible once the baby is few months old. The severity of symptoms may range from mild to severe.

Some Of the Symptoms of Phenylketonuria (PKU) Commonly Seen Include:

  • Learning disabilities
  • Mental retardation
  • Tremors or jerky movements of limbs 
  • Skin rash or eczema
  • Epilepsy
  • Hyperactivity
  • Behavioral issues

As regards the appearance, children may have a small sized head or microcephaly. Due to the excess phenylalanine in their body, they may have a musty odor to their breath, urine or skin. They may also appear fair with lighter hair and skin.

Mild or moderate PKU are less severe forms and the children are comparatively at little risk of brain damage but special care and diet therapy is required. In severe or classic PKU the children mostly suffer from obvious brain damage and are more likely to have mental retardation.

Some adults can also have ill-effects due to high levels of phenylalanine like reduced concentration, slow reaction time; while some may not experience any difference. However, permanent brain damage due to high levels of phenylalanine is not known in adults.

Phenylketonuria or PKU in Pregnant Women

Women having PKU are at an increased risk of experiencing the ill-effects of the condition during pregnancy, which is called maternal PKU. In this, the levels of the amino acid – phenylalanine increase abnormally and can cause damage to the developing fetus in the womb.
In such cases, the babies born to mothers with PKU can have intra-uterine growth retardation (IUGR), small sized head, mental retardation, low birth weight, heart defects and behavioral issues,. These babies may not always inherit the genetic disorder but screening at birth can help in early identification.

Diagnosis of Phenylketonuria (PKU)

The best way to diagnose PKU is to determine the levels of phenylalanine in blood. In babies, the blood test is performed during early days after birth. If the test turns positive,some additional investigations may be ordered to confirm the diagnosis.

Treatment For Phenylketonuria (PKU)

PKU is generally managed in metabolic disorder clinics by experts specially trained in the field. The treatment mainly includes restricting the intake of proteins to restrict phenylalanine in the body and maintaining safe levels as recommended. This is primarily a nutritional therapy and needs regular monitoring by a nutritional expert.

Once the disorder is confirmed the protein restricted special diet is advised. Recently, some approved medical treatments are also becoming available and may be considered by the physician as applicable.

Nutritional Therapy For Phenylketonuria (PKU)

The PKU diet is mainly aimed at restricting proteins in the diet. Babies are given special formulas. The special diet for children and adults demands elimination of high-protein foods including milk, cheese, nuts, soy products, beans, legumes, egg, meat and fish. Starchy foods like bread, potatoes, corn and candies are also restricted. Aspartame, which is present in many artificial sweeteners, should also be avoided as it releases phenylalanine after being digested.

As special diets can cause nutritional deficiencies; appropriate nutritional supplementation for essential amino acids, vitamins, minerals like zinc, iron, etc and other micronutrients are considered. This diet is ideally recommended for life and termination of the diet after adolescence is generally not advisable.

Prevention Of Phenylketonuria (PKU)

This being a genetic condition is present from birth and can be identified soon after the birth by screening newborns for PKU. If the condition is identified at birth, taking appropriate measures can prevent serious health concerns.

Special attention needs to be given in case of pregnant women to maintain the phenylalanine blood levels within the recommended range to avoid harmful effects to the fetus. Women with PKU must plan their pregnancy and begin with a phenylalanine-restricted diet before conceiving under the guidance of metabolic or PKU clinic experts.

Following the phenylalanine restricted diet and other recommendations can help in preventing the harmful effects of PKU.

Written, Edited or Reviewed By:


Last Modified On: July 13, 2016

Pain Assist Inc.

Pramod Kerkar
  Note: Information provided is not a substitute for physician, hospital or any form of medical care. Examination and Investigation is necessary for correct diagnosis.

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