Medically Reviewed by Minesh Khatri, MD on January 27, 2022
6 min read

Phenylketonuria (PKU) is a rare disorder you inherit from your parents. It affects the way your body handles an amino acid called phenylalanine (Phe for short).

Phe is one of many amino acids that join together to form proteins. It’s in lots of common foods, but it’s highest in protein-packed foods like meat, eggs, fish, and milk. You can also find it in the artificial sweetener aspartame.

Babies born with PKU don't have the enzyme needed to break Phe down. This means it builds up in the blood and starts to harm a child’s growing brain. There’s no cure, but a low-protein diet can help you stay healthy.

PKU happens when there's a problem with a gene that's involved in the breakdown of Phe. To get PKU, you need two copies of the gene -- one from each parent. People who have only one faulty gene are called carriers. They don’t have symptoms but can pass the gene on to their children. If a carrier has a child with another carrier, the chance that child will have PKU goes up.

PKU can come with mild symptoms or more-serious ones.

The most severe form of the disorder is called classic PKU. It happens when you have little or none of the enzyme that helps process Phe and there is too much Phe in your blood. Symptoms of untreated classic PKU include:

  • IQ loss
  • Delayed mental and physical development
  • Seizures
  • Emotional problems
  • Memory loss
  • Heart defects
  • A skin rash called eczema
  • A musty odor in your breath, skin, or urine

In milder forms of PKU, your Phe levels are lower and there’s less chance of serious harm to the brain. Some people have a very slight case of PKU that might not cause problems or need treatment.

If you’re a mom-to-be with the disorder, it’s important to keep your Phe levels in a safe range (2% to 6%). High levels of it in the womb can cause your child to have:

  • Low birth weight
  • Slowed growth
  • Behavior problems
  • Heart trouble


Many countries, including the U.S., test newborns for PKU. It's done through a simple blood test that happens a day or two after they're born. A health care worker at the hospital usually takes the blood through a prick in your baby’s heel. It’s put on a special paper and sent to a lab. Your baby’s doctor will share the results with you.

PKU is very serious, so it’s important to learn everything you can. You can start the conversation with questions like:

  1. What causes PKU?
  2. What should my diet look like?
  3. Do I have to follow this diet for life?
  4. What happens if I go off the diet or skip a couple of days?
  5. How will I get all the protein and nutrients I need?
  6. Are there treatments besides the PKU meal plan?
  7. How often should I have my Phe levels checked? Can I check them at home?
  8. Does PKU make it more likely I’ll feel anxious or depressed?
  9. Does PKU cause other health problems?
  10. Can I have children if I have PKU? Can I breastfeed?


The main way to treat PKU is with a low-protein diet.

Babies with PKU start on a Phe-free infant formula right away. They may be able to have some breast milk, too. When your baby’s old enough, they may follow the same meal plan as older kids and adults. But diets for some kids are stricter than for others. A dietitian who specializes in PKU will choose the right meal plan for your child.

In general, the PKU diet:

  • Avoids all high-protein foods, including meat, eggs, cheese, milk, nuts, beans, and soy
  • Doesn't allow anything sweetened with aspartame
  • Limits some veggies like artichokes, asparagus, avocado, broccoli, corn, kale, and potatoes
  • Carefully measure portions of grains, cereals, and pasta.
  • Includes a special formula (also called a medical food) that you have every day. It provides the Phe-free protein, vitamins, and minerals that you need for good health. This formula is a key part of treating PKU.

You should stay on treatment for life. If you go off the diet, you may have some memory loss, trouble concentrating, or you might feel anxious or depressed. If you stick with or go back on the diet, your mind will be sharper and you’ll feel better.

Two medicines are approved to treat PKU. One is a pill called sapropterin (Kuvan) that helps process Phe. It’s more likely to work if you have a mild form of PKU. You have to stay on your meal plan, too. But you might be able to eat more types of food.

Pegvaliase (Palynziq) is given as a shot. It may help lower your Phe levels. It’s expensive, and it can have serious side effects. It’s usually tried only if you can’t control your levels with your diet.

The PKU diet cuts out many of the foods most people eat. This can make life tough for both kids and adults. If your child has the disorder, you may worry they’ll miss out on birthday parties and trips, or that they'll fall off the diet because of pressure from their friends. If you have PKU, you might be concerned that it could affect your job or social life.

But don’t worry. There are lots of things you can do to ease the way for you and your child.

For parents: Talk to your child’s teachers, principal, and school nurse as soon as possible. Explain what PKU is. Let them know that your child has special food needs and that they must drink a low-protein formula during the day. Give them a list of foods your child can eat. This can vary a lot from child to child. Offer to bring treats your child can eat for birthdays and other school functions. Also tell them that PKU might affect how well your child does in school. Bring up Section 504 of the Rehabilitation Act of 1973 and individualized education programs (IEPs). These ensure that kids with special needs get the help they need.

For other grownups: These days, there’s nothing odd about being on a special diet. People follow plans that are vegan, gluten-free, keto, paleo, and other things. It’s not likely your PKU plan will stand out or attract much notice. Many restaurants now cater to people on special diets, too. There also are lots of products that you can swap out for high-Phe ones. This includes dairy foods made from rice, coconut, and oat milks. There are low-protein pastas, breads, and baking mixes, too.

The medical food you need can cost $10,000 or more a year. Some states give food and formula free of charge to people with PKU. These programs may have age or cost limits, though. Learn the laws in your state. Also, find out if your insurance covers PKU food and supplies.

You need to check the Phe levels in your blood often. This shows if your treatment is working. It also lowers the chance of brain, mood, and social problems. You usually check an infant’s levels once or twice a week by taking the blood sample from a tiny prick in the heel. In older kids and adults, you take the blood from a fingertip. You put the drop of blood on a card and mail it to a lab. If your level is off, your treatment team will work with you to bring it into a safe range.

From time to time, you or your child might get bored with their meal plan and want to eat what everyone else does. This is normal, and it shouldn’t make you feel anxious or guilty.

Many people with PKU turn to close friends and family for support. But it can also help to talk to other people with the disease because they know what you’re going through. Some studies have shown that people in support groups know more about their disease and are more likely to stick with treatment.

Your doctor can refer you to a face-to-face or telemedicine support group. There are also lots of online communities to choose from.