On Phenylketonuria (PKU)

I’ve been interested in restricted diets and food hypersensitivities for around two decades, and to my regret — with the exception of the odd passing mention — I’ve yet to say a word about phenylketonuria (PKU).

This, today, International PKU Day, is a modest, humble attempt to put that right.

I’m writing this partly for myself, then, to ease my guilt, and partly to give a deserved shout-out to the PKU community and its advocates, but perhaps mostly for people with coeliac disease and food allergies and their carers — two groups who have become very aware of one another’s needs and realities over the last decade, but are far less likely to know about some of the less common restricted-diet medical conditions out there, nor the people who live with them, with whom they doubtless have some shared experiences.


What is PKU?

PKU is very rare — 100 times rarer than coeliac disease — effecting very roughly one person in 10,000 — but it is arguably far tougher to deal with and more serious. Typically diagnosed early in life (babies are generally tested for it soon after birth), PKU is an inherited metabolic disorder where insufficient enzyme is produced to break down a particular essential amino acid, called phenylalanine, found in protein-based foods.

In that sense, it could be considered a food intolerance, although that expression considerably understates it: because if you consume too much protein, the resulting phenylalanine build-up in the blood can lead to toxicity, potentially harming the nervous system and brain. 

As far as diet is concerned, this essentially rules out all meat, fish, eggs, dairy and high-protein plant-based staples, such as legumes and nuts. 

Most fruit and vegetables are, on the whole, fine, but some have to be moderated. 

The sweetener aspartame breaks down into phenylalanine in the body and is poison to someone with PKU. “Contains a source of phenylalanine” appearing on your diet soft drink bottle? That’s there to warn folk with PKU. A legal requirement, I know, but I’ve always found it heartening that even such a small minority of people have been protected in law in this way.

Severity varies among people with PKU, and because phenylalanine is an essential amino acid, very small quantities must be consumed, so it’s not strictly a ‘free from’ diet, but one of finding an ideal balance. Each PKU patient is permitted a restricted number of daily ‘exchanges’ (portions of foods with 1g of protein in them), and must make up other daily protein requirements with special phenylalanine-free supplements. 

Furthermore, food, protein and supplement intake must be spread fairly evenly across the day, meaning meticulous planning is required, and any spontaneous deviation from the plan can require considerable revision of the rest of the day’s food and drink intake.

It is fiendishly complicated and stressful. Eating out? Salad and chips, or rice pasta and tomato, are some of the few options, potentially, but most set dishes will be off the menu for someone with PKU.


Find out more about PKU …

There is a national charity, The National Society for Phenylketonuria (NSPKU), who will be celebrating their 50th anniversary next year, and whose website has lots of information for parents and patients dealing with the day-to-day challenges of a diagnosis. 

But roughly half of what I’ve personally learned about PKU has been in the last week, and that has largely been thanks to Pauline O’Connor’s new book, Living with PKU, a self-published title which is rare in that it is well-written, as well as wise, and never once does it run out of steam or become dull to a non-knowledgeable reader. 

It’s been pleasing to learn that both the ‘free from’ and vegan booms have had knock-on benefits for people with PKU, with gluten-free flours and baking cropping up in PKU cuisine, and the vegan trend, with its coconut yogurt, its jackfruit and its banana blossom, offering up new welcome additions to the sometimes same-y and unvaried PKU diet. I’ll write more about this another time.

Like many authors, I’ve a potential list of ‘to do’ books, with draft outlines in grubby notepads or forgotten flash drives, but I’m such a slow worker that I only get to cross one off every several years. Still on it are very niche proposals on fragrance sensitivities, PPD allergy, and others.

Also somewhere on that list is ‘Something on PKU’. It’ll be scrubbed out now, and with some pleasure and relief, because it has been done, and been done better than I could possibly have managed, and I would urge anyone involved in food hypersensitivity or ‘free from’ — be it professionally or personally — to go buy Pauline’s book in order to understand a condition which you may not have realised even existed. 

As she puts it: People with PKU have to “Avoid 85% of foodstuffs or risk brain damage”.

It is that stark, and I feel strongly that more of us ought to know about it.


  1. C. Hawthorne

    For one who has seen the development of Pauline’s book and the wprk that went in to it & not having previously known of PKU I am delighted to see it promoted here. the book is so readable and well presented.

    1. Alex G (Post author)

      I agree and thanks for your comment. The post has been well received, about which I’m very happy!


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