No, you cannot develop phenylketonuria (PKU) later in life. PKU is a genetic condition present from birth, caused by inherited mutations in a specific gene. Every person with PKU was born with it. However, some people are not diagnosed until childhood or even adulthood, which can create the impression that the condition appeared later. Understanding why this happens, and what it looks like when PKU goes undetected, matters for anyone asking this question.
Why PKU Is Always Present From Birth
PKU is caused by mutations in the PAH gene, which provides instructions for making an enzyme that breaks down an amino acid called phenylalanine. Hundreds of different mutations in this gene have been identified, and all of them reduce or eliminate the enzyme’s ability to do its job. Without enough working enzyme, phenylalanine builds up in the blood and brain to toxic levels.
Because PKU is encoded in your DNA, it cannot be “triggered” by diet, illness, medication, or aging. You either have two copies of the faulty gene (one from each parent) or you don’t. There is no acquired form of PKU. The severity depends on how much enzyme activity your particular mutations leave intact. People with classic PKU have little to no enzyme activity, while those with milder forms retain some function.
How PKU Gets Missed at Birth
Newborn screening for PKU became widespread in the United States during the 1960s. By 1965, 32 states had enacted screening laws. By the mid-1970s, screening was routine in nearly every industrialized nation. But the system isn’t perfect, and some cases slip through.
The most common reason for a missed diagnosis is timing. If a blood sample is collected too early, phenylalanine levels may not have risen enough to flag the test. Data from the CDC shows that roughly 16% of PKU cases could be missed when screening happens in the first 24 hours of life. That rate drops to about 2% on the second day and 0.3% by the third day. Samples taken before 72 hours carry the highest risk of a false negative. Administrative failures, like a sample never being collected or processed, also account for missed cases.
Anyone born before universal screening was implemented, born in a country without a screening program, or born during a gap in the system could reach adulthood without ever being tested. These individuals have had PKU their entire lives but may only learn about it when symptoms become severe enough to prompt investigation.
What Undiagnosed PKU Looks Like in Adults
Babies with PKU appear healthy at first. Without treatment, symptoms typically emerge within the first few months of life as phenylalanine accumulates. But the damage is cumulative and ongoing, so adults who were never diagnosed or treated often have significant neurological and psychiatric problems that may have been attributed to other conditions.
A large meta-analysis of adults with late-treated or untreated PKU found striking rates of neuropsychiatric symptoms. About 68% experienced problems with inattention, 49% had significant anxiety, 35% had depression, and 34% showed hyperactivity. Neurological symptoms were also common: 40% had tremors and 21% had epilepsy or seizures. Other features include intellectual disability, a musty body odor caused by excess phenylalanine, lighter skin and hair than family members (because phenylalanine interferes with pigment production), and skin conditions like eczema.
For someone who has lived decades without a diagnosis, these symptoms may have been managed individually with psychiatric medications or attributed to other causes entirely. A blood test measuring phenylalanine levels can confirm the diagnosis at any age. Classic PKU is defined by blood phenylalanine levels above 1,200 micromoles per liter, mild PKU falls between 600 and 1,200, and milder elevations below 600 are classified as non-PKU hyperphenylalaninemia.
Starting Treatment as an Adult
The cornerstone of PKU management is a strict, lifelong low-phenylalanine diet. This means severely limiting high-protein foods like meat, dairy, eggs, nuts, and beans, and relying on specially formulated medical foods to get adequate nutrition. Current guidelines recommend maintaining this diet for life, not just through childhood as was once believed.
For adults diagnosed late, starting a low-phenylalanine diet can still produce meaningful improvements. Multiple studies of untreated adults placed on a low-phenylalanine diet found that caregivers reported reductions in aggression, irritability, hyperactivity, and self-injurious behavior. One study found that within three to eight weeks of starting the diet, 46% of participants showed improved behavior, including reductions in irritability (67%), attention problems (59%), and hyperactivity (51%). Some participants were able to reduce their use of psychiatric medications. The improvements were not universal, and people with more severe, longstanding brain damage saw smaller gains, but the diet consistently helped with behavioral symptoms even when started late.
Beyond diet, a medication approved in 2018 offers another option for adults whose phenylalanine levels remain too high. It works by providing a substitute enzyme that breaks down phenylalanine through a different chemical pathway than the one affected by PKU. It’s given as an injection and is specifically indicated for adults with blood phenylalanine above 600 micromoles per liter who aren’t adequately controlled on existing management. An older medication works by boosting the activity of whatever residual enzyme a person still produces, but it only helps the subset of patients with milder mutations.
Why This Question Comes Up for Women
One situation that brings PKU to attention in adulthood is pregnancy. Women with PKU who relaxed or abandoned their diet years ago may not be thinking about phenylalanine levels until they become pregnant or plan to. But high phenylalanine during pregnancy poses serious risks to the developing baby, even though the baby itself may not have PKU.
Elevated maternal phenylalanine increases the chance of the baby being born with heart defects, an unusually small head, low birth weight, intellectual disability, and seizures. These risks apply to the fetus because phenylalanine crosses the placenta freely. Women who bring their phenylalanine levels into the recommended range before conception and maintain control throughout pregnancy are not expected to have a higher chance of birth defects. This makes preconception planning critical for any woman with PKU, including those who haven’t thought about their diagnosis in years.
Adults Who Stopped the Diet
A related group sometimes confused with “developing PKU later” are adults who were diagnosed and treated as children but stopped following the diet in their teens or twenties. For decades, medical advice suggested the diet could be safely discontinued after childhood brain development was complete. That guidance has since been reversed. Current recommendations call for lifelong dietary treatment.
Adults who go off the diet often experience a gradual return of symptoms: difficulty concentrating, increased anxiety and depression, tremors, and mood instability. These problems can emerge slowly enough that the person doesn’t connect them to PKU. The neuropsychiatric data bears this out, with even early-treated adults showing higher rates of inattention, anxiety, and depression than the general population when diet control lapses. Returning to dietary treatment typically improves these symptoms, though the timeline varies from weeks to months depending on the individual.