A phenylketonuric is a person with the inherited metabolic disorder phenylketonuria (PKU). This condition affects how the body processes protein from food, centering on an amino acid called phenylalanine, which is a building block of proteins. For a person with PKU, the body is unable to properly break down phenylalanine, causing it to accumulate in the bloodstream and other tissues. This buildup can interfere with normal body functions if not managed. The disorder is present from birth and requires lifelong attention to prevent health complications.
The Genetic Basis of Phenylketonuria
Phenylketonuria arises from mutations within the PAH gene. This gene contains the instructions for creating an enzyme called phenylalanine hydroxylase, which is produced in the liver. The primary function of this enzyme is to initiate the breakdown of phenylalanine by converting it into a different amino acid named tyrosine.
In an individual with PKU, mutations in the PAH gene mean the phenylalanine hydroxylase enzyme is either deficient or nonfunctional. Without a working enzyme, the metabolic pathway is blocked. As a result, phenylalanine from dietary protein cannot be processed effectively, leading to its accumulation in the body.
The condition is inherited in an autosomal recessive pattern, meaning a child must inherit one non-working copy of the PAH gene from each parent. The parents are carriers who have one functional and one non-functional copy of the gene and do not show signs of the condition. When both parents are carriers, there is a one-in-four chance with each pregnancy that the child will have PKU.
Identifying Phenylketonuria
The identification of phenylketonuria occurs within the first few days of a newborn’s life due to routine newborn screening programs. The goal of this screening is to find babies with PKU before they show any outward signs of the disorder, allowing for immediate management.
The screening process involves a “heel prick test.” Performed between 24 and 72 hours after birth, a healthcare provider collects a few drops of blood from the baby’s heel onto a special filter card. This blood sample is then sent to a laboratory to be analyzed for phenylalanine levels.
A result showing an abnormally high level of phenylalanine indicates the baby may have PKU and requires follow-up. Further diagnostic blood tests are then performed to confirm the diagnosis and determine the severity of the enzyme deficiency. This swift process ensures a management plan can be put into place right away.
Health Implications of Untreated Phenylketonuria
If phenylketonuria is not treated shortly after birth, the consequences can be severe. The persistent, high concentration of phenylalanine in the bloodstream becomes toxic, particularly to the developing brain. Nerve cells are sensitive to these toxic levels, and prolonged exposure leads to irreversible brain damage.
The most profound effect of untreated PKU is permanent intellectual disability. Infants may appear normal for the first few months, but as phenylalanine accumulates, developmental milestones are delayed. Without intervention, this progresses to significant cognitive impairment and other neurological problems like seizures and hyperactivity.
Untreated individuals also develop a distinct musty or “mousy” odor in their breath, skin, and urine. This smell is caused by the buildup of phenylacetic acid, a byproduct from an alternative metabolic pathway. Skin conditions like eczema and lighter skin and hair pigmentation compared to other family members are also frequently observed.
Management and Dietary Control
The management of phenylketonuria is centered on lifelong dietary control to keep blood phenylalanine levels within a safe range. The basis of this treatment is a strict diet that is very low in phenylalanine, which requires the severe restriction of high-protein foods. Foods that must be avoided include:
- Meat
- Poultry
- Fish
- Milk
- Cheese
- Eggs
- Nuts
- Legumes
To ensure adequate nutrition, individuals with PKU rely on a special medical formula. This formula provides all the other amino acids the body needs for growth and repair, along with vitamins and minerals, but without the phenylalanine. This formula is a main source of protein from infancy throughout adulthood.
The diet also includes carefully measured amounts of certain fruits, vegetables, and special low-protein breads and pastas. The exact amount of phenylalanine a person can tolerate varies, so the diet must be individualized. Regular blood tests are necessary to monitor phenylalanine levels, allowing a dietitian to make adjustments to the diet as needed.
It is also necessary for individuals with PKU to avoid the artificial sweetener aspartame. This sweetener, found in many diet sodas and low-calorie products, is a source of phenylalanine. Adherence to this dietary plan allows a person with PKU to prevent health complications and lead a healthy life.