Conditions & syndromes · England
Understanding PKU
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PKU is a health condition you are born with.
A specialist team helps you manage it.
Protein is a nutrient found in food.
PKU affects how your body uses part of protein.
This part is called phenylalanine.
Too much phenylalanine can harm your brain.
Your body still needs a carefully planned amount.
Your specialist team makes your own food and treatment plan.
Do not copy another person’s plan.
You may need special foods and protein substitutes.
These help give you the nutrition you need.
Blood tests help your team check your treatment.
Ask for help to understand tests and results.
Aspartame is a sweetener containing phenylalanine.
It is unsuitable for people with PKU.
Ask for help checking labels and medicines.
Tell your team if treatment is difficult.
Ask for help with tastes, textures, illness or missing supplies.
People supporting you need to understand your plan.
Take the plan when you go into hospital.
Contact your specialist team before trying for a baby.
Contact them promptly if you might already be pregnant.
PKU needs care throughout your life.
Ask for help moving to an adult specialist team.
Your choices and feelings matter.
Support should help you live the life you want.
What you can do next
Ask for instructions you understand.
Do not change treatment without specialist advice.
Ask your metabolic team for support PDFThis link opens another website.
It may use harder words.
More useful links
- BIMDG: PKU information for families, 2025 PDF
Specialist metabolic dietitians explain measured intake, protein substitutes, monitoring and illness.
- NSPKU: dietary information Web page
Support with prescribed dietary products and practical resources to use with a metabolic team.
- Cambridge University Hospitals NHS: PKU and planning pregnancy Web page
Specialist advice before conception and prompt contact after an unplanned pregnancy.
These links open other websites.
They may use harder words.
PKU and a low-phenylalanine diet
Understand PKU, specialist dietary treatment, monitoring and support throughout life.
Children and adults · England · Specialist metabolic care
What PKU means
Phenylketonuria is usually shortened to PKU. It is an inherited condition affecting an enzyme, a substance that helps the body process food. The body cannot process phenylalanine normally, so it can build up and harm the brain. Phenylalanine is an amino acid, one of the building blocks of protein. PKU is not caused by parenting, and it cannot be caught from someone.
Diagnosis and learning disability
In England, newborn blood spot screening looks for PKU. A positive screening result needs further tests and prompt assessment by a specialist metabolic team. Early treatment can prevent serious effects on development. Some adults diagnosed late or treated differently in the past have learning disabilities and need accessible lifelong care. Having a learning disability does not itself mean someone has PKU or needs a low-protein diet.
Low phenylalanine does not mean none
Phenylalanine is essential: the body needs an appropriate amount for growth and repair. Too much can be harmful in PKU, but simply removing all protein is also unsafe. A metabolic dietitian balances measured natural protein with the rest of the person’s nutrition. The amount depends on the individual and can change. This guide does not give an allowance, blood target or a list to use instead of that plan.
What treatment may include
The plan may include measured portions of protein-containing foods, prescribed protein substitutes and specialist low-protein foods. A protein substitute supplies needed nutrients while limiting phenylalanine; an ordinary supplement or plant milk is not an equivalent replacement. Some people respond to medicines such as sapropterin after specialist assessment. Do not assume treatment is unavailable because of age, or change food intake because a medicine has been started.
Monitoring and appointments
The metabolic team uses blood phenylalanine results and wider checks of health, growth or weight and nutritional intake to review treatment. Ask how often tests are needed, who teaches blood spot sampling, how results reach you and who explains changes. Request support for sensory or communication needs. Do not change the plan based only on how well the person looks or on someone else’s test result.
Check for aspartame
Aspartame is a sweetener containing phenylalanine and is unsuitable for people with PKU. It may be listed as aspartame or E951, with a warning that the product contains a source of phenylalanine. Check labels, including when products change, and ask the metabolic team about anything unclear. A pharmacist should check medicines and supplements for suitability; do not stop a prescribed medicine without advice.
When eating or following the plan is difficult
Tell the metabolic team about sensory distress, swallowing problems, illness, vomiting, missed substitutes or shortages. Ask for realistic alternatives and a written plan for illness and urgent advice. Do not blame or punish someone who finds treatment difficult, force food, or compensate by making unsupervised restrictions. If swallowing is involved, the metabolic dietitian and speech and language therapist should agree one compatible plan.
Make the plan possible in everyday life
Agree who orders prescriptions, checks supplies and helps with blood tests. Plan for school, work, respite, holidays and hospital, with the person’s permission. Ask for clear pictures, accessible recipes or practical demonstrations using the person’s own prescription. Explain the plan to new staff. Low-protein foods and substitutes are medical treatment; difficulties obtaining them need prompt help from the prescriber and metabolic team.
Pregnancy needs early specialist support
Someone with PKU who is thinking about pregnancy should contact their metabolic team before trying to conceive. Raised phenylalanine can harm a developing baby even if the baby does not have PKU. If pregnancy is possible or confirmed unexpectedly, contact the team promptly for advice and support. Do not use this page to alter intake or supplements; the team must plan and monitor treatment for that pregnancy.
Care across adulthood
Agree a supported transfer from children’s to adult metabolic services, including prescriptions, tests, appointments and emergency contacts. If an adult has lost contact with care, ask the GP for referral back to a metabolic service. Returning for help is worthwhile. Review practical and emotional support as the person’s life changes, and keep their own preferences, relationships and goals central.
Words you could use
“Please help me understand my PKU treatment and make it practical. I need clear instructions, reliable supplies, accessible monitoring and a named contact when things are difficult.”
More useful links
- BIMDG: PKU information for families, 2025 PDF
Specialist metabolic dietitians explain measured intake, protein substitutes, monitoring and illness.
- NSPKU: dietary information Web page
Support with prescribed dietary products and practical resources to use with a metabolic team.
- Cambridge University Hospitals NHS: PKU and planning pregnancy Web page
Specialist advice before conception and prompt contact after an unplanned pregnancy.
These links open other websites.
They may use harder words.
Where the information comes from
Links checked 1 November 2026.
These links open other websites.
They may use harder words.
- GOV.UK: phenylketonuria, detailed information Web page
England newborn screening, diagnosis, metabolic teams and monitored dietary treatment.
- GOV.UK: newborn screening professional handbook, conditions Web page
PKU treatment controls phenylalanine while retaining the amount needed for growth and development.
- BIMDG: PKU information for families, 2025 PDF
Specialist metabolic dietitians explain measured intake, protein substitutes, monitoring and illness.
- UK metabolic dietitians: adult PKU care consensus Web page
Specialist consensus on lifelong management, nutrition and supporting adults with PKU.
- NHS: sweeteners Web page
Aspartame contains phenylalanine; product labelling and PKU.
- Cambridge University Hospitals NHS: PKU and planning pregnancy Web page
Specialist advice before conception and prompt contact after an unplanned pregnancy.
- NHS England: sapropterin access for responsive PKU Web page
Treatment depends on specialist testing and demonstrated response, not age alone.
- NSPKU: dietary information Web page
Support with prescribed dietary products and practical resources to use with a metabolic team.