What is PKU?

Pauline -

I have been asked to explain PKU many times; by friends, colleagues, teachers, hospitality workers… I’ve even briefed medical staff on the ins & outs of PKU. A world in which everyone has their speciality can reduce awareness of other conditions.

This means that I often consent to medical students joining my clinic appointments. Patient to clinician time is key to understanding rare diseases. During one of these sessions, my doctor asked: ‘Pauline, how would you describe your PKU to my colleague?’

Describing PKU

Deep breath in: “I have inherited a mutated gene meaning my body cannot produce a particular enzyme which breaks down an essential amino acid. Amino acids are the building blocks of protein, so I have to dramatically restrict the amount of protein I eat every day.

If I eat too much protein, the amino acid builds up in my blood like a toxin affecting my neurological and central nervous systems. Ultimately, too much protein in my food can lead to brain damage.”

I have to dramatically restrict the amount of protein I eat every day.

At this point most people say “Woah, what did you call it again!” Or I might say: “You know that warning on the back of soft drink cans ‘May contain Phenylalanine’? Well, it’s me they are warning.”

Foods not allowed on a low-protein diet

  • Meat
  • Fish, including Shellfish
  • Eggs
  • Cheese (some new vegan cheese is ok, but not all vegan cheese)
  • Nuts and Seeds
  • Flour-based foods (bread, cakes, biscuits etc.)
  • Soya & Tofu
  • Quorn
  • Any medicine or food containing Aspartame (an artificial sweetener)

It is complicated, and the list goes on. Usually, someone interrupts to exclaim, “What do you actually eat?”

What do you actually eat with PKU?


“Well, I’m allowed to eat most, but not all, fruit and vegetables. There are medically produced pasta and rice substitutes which I get on prescription. There is also a medical flour allowing me to bake my own bread. To supplement this diet, I need to take prescribed supplements every day which make sure I get things like iron and calcium as I can’t have meat & dairy.”

This isn't a choice, PKU is not a lifestyle diet. It is a critical medical condition.

At some point during this explanation, someone will ask: "Why you can’t eat all that?"
"Because I’ll get brain damage."
"Yeah, but, like, why can’t you save up and have a steak one night?"
"Nope, it doesn’t work that way."
"What if it is a special occasion? Like, just take one night off!"
"This isn't a choice, it doesn't work that way."
"Yeah, but…"
"I can’t have a day off. It doesn’t matter whether it is your birthday or Christmas. If I eat too much protein, then it will build up in my blood to levels which harm my brain."
For some reason, even though this isn’t my fault, I will then say "sorry".

PKU is nobody's fault


It is nobody’s fault that we were dealt this hand. There was nothing our parents or grandparents did, or didn’t do. It just is what it is. And it sucks. All we can do is make the best of it, and press for better treatments.

Support for Living with PKU

There are patient organisations which help to support people with PKU and actively promote new research and treatments.

UK: NSPKU - The National Society for Phenylketonuria

Ireland: PKU Association of Ireland

Europe: ESPKU - The European Society for PKU and allied disorders treated as PKU. A grouping of national and regional associations which were created by parents across 31 countries. Find your national or regional organisation here: https://www.espku.org/who-we-are/interesting-stuff/

USA: NPKUA - National PKU Alliance

Canada: CanPKU

Australia & Aotearoa/NZ: ASIEM - Australasian Society for Inborn Errors of Metabolism

My posts about PKU are all collected here.

*First published 1 May 2019, updated 28 February 2025