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PKU is purely genetic, and both parents must have the defective gene for it to be passed on. People can be carriers of the gene without developing the disorder, so it is difficult to try and predict whether or not a baby will be born with PKU.

Also known as PKU, phenylketonuria is an inherited disorder that is rare and often picked up by doctors at birth. This disorder causes a buildup of an amino acid, phenylalanine, in the body, as the defect occurs in the gene that helps to produce the enzyme that would normally break down this amino acid. Phenylalanine is commonly found in foods containing protein, and consumption of these foods by someone with PKU can lead to serious medical complications.

Signs and Symptoms

When a baby is first born, there are usually no symptoms or signs present. Many countries now test babies immediately after birth to identify the presence of PKU. When symptoms do occur, they can range from mild to the severe. These can include:

  • Delayed development
  • Intellectual disability
  • Psychiatric disorders
  • Behavioral problems
  • Social and emotional problems
  • Seizures and other neurological problems
  • Weak bones
  • Hyperactivity
  • Eczema and rashes
  • Unusual breath odor
  • Musty smell on the skin and in the urine
  • Blue eyes and fair skin
  • Microcephaly
Classic PKU is the name given to the severest form of this disorder, where the more serious complications are likely to occur. For those with milder forms, the risk of brain damage is still present if a strict no-protein diet isn’t followed, even though their damaged enzyme may still function to some degree.

Genetic Risk

PKU is purely genetic, and both parents must have the defective gene for it to be passed on. People can be carriers of the gene without developing the disorder, so it is difficult to try and predict whether or not a baby will be born with PKU. If just one parent has the gene, the disorder will not develop in the child, but the child could become a carrier.

PKU and Pregnancy

The health risks are greater in pregnant women with PKU, particularly if they don’t follow their special diet. If the phenylalanine levels increase during pregnancy, there is a chance that the woman will miscarry, or cause serious harm to the unborn child. This is also true for women who only have mild PKU.

Although there is a low risk of the baby of a mother with PKU to inherit the disorder, there can still be serious complications and consequences following the birth, which are almost the same as having PKU. These include:

  • Delayed development
  • Intellectual disability
  • Microcephaly
  • Heart defects
  • Low birth weight
  • Behavioral problems

Many of these babies will often need to spend time in intensive care units following their birth. It is therefore vitally important for pregnant women with PKU to strictly follow the special diet during their pregnancy to protect themselves and their baby. Regular monitoring and checkups by a health professional throughout pregnancy are also essential. The sicker the mother gets, the greater the risk to the child.

Treatment And Diet

Baby Formula

Breast milk and regular baby formula contain phenylalanine, so they are not suitable for a baby with PKU. A special formula is available that is free of phenylalanine, and with the aid of a specialist dietitian, in some cases this can be mixed with a small amount of breast milk or regular formula.

Child and Adult Formula

Because the PKU diet is void of protein, children and adults alike will need a special protein substitute formula which is consumed every day. This formula is different from the baby formula and the amount and regularity of consumption must be calculated by a doctor or dietitian to ensure you are getting the right amounts each day. People with PKU will need to take this formula for the rest of their lives to replace the protein which is essential for body health.


To date there is only one medication that has been approved by the FDA for use as a treatment for PKU. This drug is called sapropterin, and in increases the body’s tolerance to phenylalanine. However, it is not always successful, and the FDA has directed for more studies to be completed.

Protein Foods to Avoid

Some foods will need to be avoided completely because of their phenylalanine levels. These high-protein foods include:

  • Eggs
  • Milk
  • Cheese
  • Beans
  • Nuts
  • Soybeans
  • Beef
  • Fish
  • Chicken
  • Pork
  • Diet soda
  • Artificial sweeteners

Other foods that should be taken in limited quantities include:

  • Potatoes
  • Grains
  • Vegetables that contain protein

Low-Protein Foods

Many of the daily food basics are now being produced in low-protein varieties. These include rice, bread, flour and pasta, and many health food stores will have these available. They can be a little more expensive than the regular varieties, but it allows the person with PKU to have regular looking lunches, especially for kids that are in school and want to be like everyone else.

Special Diets and Children

It’s hard enough to try and follow a special diet when you’re an adult, but for a child, it’s even worse. They see what everyone else is eating, and many just want to fit in and be like everyone else, so it can be difficult to ensure the child doesn’t stray. Children also don’t want to eat the same thing over and over again, and it’s hard to explain to a child that they have to follow the diet rules to stay healthy. At a young age, they are incapable of looking ahead, they live only in the moment, and if they see another kid eating something they are not allowed, the temptation will be very strong.

It’s a good idea to talk to your dietitian about ways to make meals exciting for children. Low-protein foods can be a little bland, so try experimenting with spices and herbs to add more flavor. Using these can also make a meal taste different depending on what you add, so you can basically cook the same meal but change how it looks and tastes. It’s a bit of trickery, but it’s better than having a child that is frustrated and despondent about their diet.


PKU is a serious condition that affects the person for their entire lifetime. It needs to be monitored regularly, and the person must stick to their specialized diet without straying. Not following the diet could be devastating.

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