Phenylketonuria (also known as Phenylalanine hydroxylase deficiency, and PKU) is an inborn error of metabolism involving impaired metabolism of the amino acid phenylalanine. Phenylketonuria is caused by absent or virtually absent phenylalanine hydroxylase (PAH) enzyme activity. The condition is also known as phenylalanine hydroxylase deficiency.

Protein-rich foods or the sweetener aspartame can act as poisons for people with phenylketonuria. The role of PAH is to break down excess phenylalanine from food. Phenylalanine is a necessary part of the human diet and is naturally present in all kinds of dietary protein. It is also used to make aspartame, known by the trade name Nutrasweet, which is used to sweeten low-calorie and sugar free soft drinks, yogurts, and desserts. In people without PKU, the PAH enzyme breaks down any excess phenylalanine from these sources beyond what is needed by the body. However, if there is not enough of the PAH enzyme or its cofactor, then phenylalanine can build up in the blood and brain to toxic levels, affecting brain development and function. PKU is rare, but important to identify, because if caught early it is very treatable. It is not contagious, and it is lifelong, but with early diagnosis and consistent treatment, the damaging effects can be minimal or non-existent.

Untreated PKU can lead to intellectual disability, seizures, and other serious medical problems. The best proven treatment for classical PKU patients is a strict phenylalanine-restricted diet supplemented by a medical formula containing amino acids and other nutrients. In the United States, the current recommendation is that the PKU diet should be maintained for life. Patients who are diagnosed early and maintain a strict diet can have a normal life span with normal mental development.

PKU is an inherited disease. When an infant is diagnosed with PKU, it is never the result of any action of the parents or any environmental factor. Rather, for a child to inherit PKU, both of his or her parents must have at least one mutated allele of the PAH gene. Most parents who are carriers of PKU genes are not aware that they have this mutation because being a carrier causes no medical problems. To be affected by PKU, a child must inherit two mutated alleles, one from each parent.


PKU is commonly included in the newborn screening panel of most countries, with varied detection techniques. Most babies in developed countries are screened for PKU soon after birth. Screening for PKU is done with bacterial inhibition assay (Guthrie test), immunoassays using fluorometric or photometric detection, or amino acid measurement using tandem mass spectrometry (MS/MS). Measurements done using MS/MS determine the concentration of Phe and the ratio of Phe to tyrosine, the ratio will be elevated in PKU.

Because the mother's body is able to break down phenylalanine during pregnancy, infants with PKU are normal at birth. The disease is not detectable by physical examination at that time, because no damage has yet been done. However, a blood test can reveal elevated phenylalanine levels after one or two days of normal infant feeding. This is the purpose of newborn screening, to detect the disease with a blood test before any damage is done, so that treatment can prevent the damage from happening.

If a child is not diagnosed during the routine newborn screening test (typically performed 2–7 days after birth, using samples drawn by neonatal heel prick), and a phenylalanine restricted diet is not introduced, then phenylalanine levels in the blood will increase over time. Toxic levels of phenylalanine (and insufficient levels of tyrosine) can interfere with infant development in ways which have permanent effects. The disease may present clinically with seizures, hypopigmentation (excessively fair hair and skin), and a "musty odor" to the baby's sweat and urine (due to phenylacetate, a carboxylic acid produced by the oxidation of phenylketone). In most cases, a repeat test should be done at approximately two weeks of age to verify the initial test and uncover any phenylketonuria that was initially missed.

Untreated children often fail to attain early developmental milestones, develop microcephaly, and demonstrate progressive impairment of cerebral function. Hyperactivity, EEG abnormalities, and seizures, and severe learning disabilities are major clinical problems later in life. A characteristic "musty or mousy" odor on the skin, as well as a predisposition for eczema, persist throughout life in the absence of treatment.

The damage done to the brain if PKU is untreated during the first months of life is not reversible. It is critical to control the diet of infants with PKU very carefully so that the brain has an opportunity to develop normally. Affected children who are detected at birth and treated are much less likely to develop neurological problems or have seizures and intellectual disability (though such clinical disorders are still possible.)

In general, however, outcomes for people treated for PKU are good. Treated people may have no detectable physical, neurological, or developmental problems at all. Many adults with PKU who were diagnosed through newborn screening and have been treated since birth have high educational achievement, successful careers, and fulfilling family lives


PKU is an autosomal recessive metabolic genetic disorder. As an autosomal recessive disorder, two PKU alleles are required for an individual to exhibit symptoms of the disease. If both parents are carriers for PKU, there is a 25% chance any child they have will be born with the disorder, a 50% chance the child will be a carrier, and a 25% chance the child will neither develop nor be a carrier for the disease.

PKU is characterized by homozygous or compound heterozygous mutations in the gene for the hepatic enzyme phenylalanine hydroxylase (PAH), rendering it nonfunctional. This enzyme is necessary to metabolize the amino acid phenylalanine (Phe) to the amino acid tyrosine (Tyr). When PAH activity is reduced, phenylalanine accumulates and is converted into phenylpyruvate (also known as phenylketone), which can be detected in the urine.

Carriers of a single PKU allele do not exhibit symptoms of the disease but appear to be protected to some extent against the fungal toxin ochratoxin A. This accounts for the persistence of the allele in certain populations in that it confers a selective advantage—in other words, being a heterozygote is advantageous.

The PAH gene is located on chromosome 12 in the bands 12q22-q24.1. More than 400 disease-causing mutations have been found in the PAH gene. This is an example of allelic genetic heterogeneity.

Phenylketonuria can exist in mice, which have been extensively used in experiments into finding an effective treatment for it. The macaque monkey's genome was recently sequenced, and the gene encoding phenylalanine hydroxylase was found to have a sequence that, in humans, would be considered a PKU mutation.


An enzyme assay can determine if parents carry the gene for PKU. Chorionic villus sampling can be done during pregnancy to screen the unborn baby for PKU.

It is very important that women with PKU closely follow a strict low-phenylalanine diet both before becoming pregnant and throughout the pregnancy, since build-up of this substance will damage the developing baby even if the child has not inherited the defective gene.


PKU can be easily detected with a simple blood test. All states in the US require a PKU screening test for all newborns as part of the newborn screening panel. The test is generally done by taking a few drops of blood from the baby before the baby leaves the hospital.

If the initial screening test is positive, further blood and urine tests are required to confirm the diagnosis.


The outcome is expected to be very good if the diet is closely followed, starting shortly after the child's birth. If treatment is delayed or the condition remains untreated, brain damage will occur. School functioning may be mildly impaired.

If proteins containing phenylalanine are not avoided, PKU can lead to mental retardation by the end of the first year of life.


Severe mental retardation occurs if the disorder is untreated. ADHD (attention-deficit hyperactivity disorder) appears to be the most common problem seen in those who do not stick to a very low-phenylalanine diet.


PKU is not curable. However, if PKU is diagnosed early enough, an affected newborn can grow up with normal brain development by managing and controlling phenylalanine ("Phe") levels through diet, or a combination of diet and medication.

When Phe cannot be metabolized by the body, a typical diet that would be healthy for people without PKU causes abnormally high levels of Phe to accumulate in the blood, which is toxic to the brain. If left untreated, complications of PKU include severe intellectual disability, brain function abnormalities, microcephaly, mood disorders, irregular motor functioning, and behavioral problems such as attention deficit hyperactivity disorder, as well as physical symptoms such as a "musty" odor, eczema, and unusually light skin and hair coloration. In contrast, PKU patients who follow the prescribed dietary treatment from birth, may have no symptoms at all. Their PKU would be detectable only by a blood test.

To achieve these good outcomes, all PKU patients must adhere to a special diet low in Phe for optimal brain development. Since Phe is necessary for the synthesis of many proteins, it is required for appropriate growth, but levels must be strictly controlled in PKU patients.

PKU is not a food allergy or a digestive problem. Eating "forbidden" foods does not cause an immediate reaction. However, some people with PKU can be very sensitive to quick changes in Phe levels causing a "Protein High". Quick changes can be caused by absorption of PKU formula causing the level of phenylalanine to drop suddenly or a quick rise in Phe level after a meal high in protein. The phenylalanine from that food remains in the person's system, however, and as Phe accumulates over time they may experience concentration, confusion and mood problems, as well as eczema and other symptoms. For children, developmental problems may occur if levels are elevated frequently or remain elevated for a significant amount of time. Changes in Phe levels, while sleeping at night, can prevent some people with PKU from getting enough rest, making it more difficult to concentrate during the day.

Optimal health ranges (or "target ranges") are between 120 and 360 µmol/L or equivalently 2 to 6 mg/dL, and aimed to be achieved during at least the first 10 years, to allow the brain to develop normally.

In the past, PKU-affected people were allowed to go off diet after approximately eight, then 18 years of age. Today, most physicians recommend PKU patients must manage their Phe levels throughout life. For teens and adults, somewhat higher levels of Phe may be tolerable, but restriction is still advised to prevent mood disorders and difficulty concentrating, among other neurological problems.

The diet requires severely restricting or eliminating foods high in Phe, such as soybeans, seal meat, eggwhites, shrimps, chicken breast, spirulina, watercress, fish, whale, nuts, crayfish, lobster, tuna, turkey,legumes, elk meat and lowfat cottage cheese. Starchy foods, such as potatoes and corn are generally acceptable in controlled amounts, but the quantity of Phe consumed from these foods must be monitored. A food diary is usually kept to record the amount of Phe consumed with each meal, snack, or drink. An "exchange" system can be used to calculate the amount of Phe in a food from the protein content identified on a nutritional information label. Lower-protein "medical food" substitutes are often used in place of normal bread, pasta, and other grain-based foods, which contain a significant amount of Phe. Many fruits and vegetables are lower in Phe and can be eaten in larger quantities. Infants may still be breastfed to provide all of the benefits of breastmilk, but the quantity must also be monitored and supplementation for missing nutrients will be required. The sweetener aspartame, present in many diet foods and soft drinks, must also be avoided, as aspartame contains phenylalanine.

Different patients can tolerate different amounts of Phe in their diet. Regular blood tests are required to determine the effects of dietary Phe intake on blood Phe level. Patients typically work with a professional dietitian to find a diet that meets their nutritional needs without causing their blood Phe level to exceed the target range.

Supplementary "protein substitute" formulas are typically prescribed for Classical PKU patients (starting in infancy) to provide the amino acids and other necessary nutrients that would otherwise be lacking in a low-phenylalanine diet. Tyrosine, which is normally derived from phenylalanine and which is necessary for normal brain function, is usually supplemented. Consumption of the protein substitute formulas can actually reduce phenylalanine levels, probably because it stops the process of protein catabolism from releasing Phe stored in the muscles and other tissues into the blood. Many PKU patients have their highest Phe levels after a period of fasting (such as overnight), because fasting triggers catabolism. A diet that is low in phenylalanine but does not include protein substitutes may also fail to lower blood Phe levels, since a nutritionally insufficient diet may also trigger catabolism. For all these reasons, the prescription formula is an important part of the treatment for patients with classic PKU.

The oral administration of tetrahydrobiopterin (or BH4) (a cofactor for the oxidation of phenylalanine) can reduce blood levels of this amino acid in certain patients. The company BioMarin Pharmaceutical has produced a tablet preparation of the compound sapropterin dihydrochloride (Kuvan), which is a form of tetrahydrobiopterin. Kuvan is the first drug that can help BH4-responsive PKU patients (defined among clinicians as about 1/2 of the PKU population) lower Phe levels to recommended ranges. Working closely with a dietitian, some PKU patients who respond to Kuvan may also be able to increase the amount of natural protein they can eat. After extensive clinical trials, Kuvan has been approved by the FDA for use in PKU therapy. Some researchers and clinicians working with PKU are finding Kuvan a safe and effective addition to dietary treatment and beneficial to patients with PKU.

Biomarin is currently conducting clinical trials to investigate another type of treatment for PKU. PEG-PAL (PEGylated recombinant phenylalanine ammonia lyase or ‘PAL’) is an enzyme substitution therapy in which the missing PAH enzyme is replaced with an analogous enzyme that also breaks down Phe. PEG-PAL is now in Phase 2 clinical development to treat patients who do not adequately respond to KUVAN.

Dietary supplementation with large neutral amino acids(LNAAs), with or without the traditional PKU diet is another treatment strategy. The LNAAs (e.g. leu, tyr, trp, met, his, ile, val, thr) compete with phe for specific carrier proteins that transport LNAAs across the intestinal mucosa into the blood and across the blood brain barrier into the brain .

Studies have demonstrated that PKU patients given daily supplements of LNAAs have decreased plasma phe levels and reduced brain phe concentrations measured by magnetic resonance spectroscopy.

Another interesting treatment strategy for PKU patients is casein glycomacropeptide (CGMP), which is a milk peptide naturally free of Phe in its pure form CGMP can substitute the main part of the free amino acids in the PKU diet and provides several beneficial nutritional effects compared to free amino acids. The fact that CGMP is a peptide ensures that that the absorption rate of its amino acids is prolonged compared to free amino acids and thereby results in improved protein retention and increased satiety compared to free amino acids. Another important benefit of CGMP is that the taste is significantly improved when CGMP substitutes part of the free amino acids and this may help ensure improved compliance to the PKU diet.

Furthermore, CGMP contains a high amount of the phe lowering LNAAs, which constitutes about 41 g per 100 g protein and will therefore help maintain plasma phe levels in the target range.

Other therapies are currently under investigation, including gene therapy.