Introduction

Definition of phenylketonuria (PKU)

Phenylketonuria (PKU) is a rare genetic disorder that disrupts the body's ability to metabolise phenylalanine, an amino acid found in many protein-containing foods.² About 1 in 10,000 babies in the UK are born with this condition.³ PKU is inherited in a manner where both parents must pass down a faulty gene for a child to develop the disorder. This deficiency of the enzyme phenylalanine hydroxylase, which converts phenylalanine to tyrosine, leads to the accumulation of phenylalanine in the blood and brain, potentially causing severe intellectual disabilities and other neurological issues if not managed properly.² 

The primary management strategy for PKU is a strict, lifelong diet low in phenylalanine, supplemented by specialised medical foods and dietary supplements.¹ Horst Bickel’s group working at the Birmingham Children’s Hospital in the UK in 1953 first reported the effectiveness of a low-phe diet in reducing phe concentrations and improving behavior in a child with PKU.¹

Overview of PKU

Symptoms and complications of untreated PKU 

PKU can cause severe symptoms if untreated, such as developmental delays, intellectual disabilities, and seizures. Newborns with PKU typically appear normal at birth, but symptoms begin to emerge within a few months as phenylalanine levels rise.² Other potential complications include eczema, a musty odor in the breath, skin, or urine, and lighter skin and hair due to impaired melanin synthesis. Prompt treatment can prevent these symptoms, while untreated adults may experience difficulty concentrating and slow reaction times.³

Diagnosis 

In the UK, newborns are offered blood spot screening for PKU at 5 days old.³ This screening program began in the 1960s.¹ If elevated levels of phenylalanine are detected, further confirmatory tests, including genetic testing, are performed to identify mutations in the PAH gene.²

Dietary management of PKU

Low-phenylalanine diet 

The cornerstone of PKU management is a low-phenylalanine diet, typically limited to around 500 mg of phenylalanine per day.¹

Food restrictions

Natural foods are severely restricted to limit protein intake while providing adequate phenylalanine. These include animal proteins, legumes, nuts, and certain grains. Patients must also limit starchy vegetables, certain fruits, and foods with moderate phenylalanine levels.¹ Aspartame, an artificial sweetener, should also be restricted.³

Acceptable foods 

Low-protein bread and pasta products made from starch provide needed energy and variety. An amino acid-based, phenylalanine-free formula (600–900 ml per day) or medical food is essential to provide adequate protein, vitamins, minerals, and energy due to the restriction in natural foods. Intake of vegetable oils rich in linolenic acid, such as canola or soybean oil, is encouraged ¹.

Importance of early and lifelong dietary adherence

Lifelong adherence to the highly restrictive low-phenylalanine diet, supplemented with an amino acid formula, is crucial. Studies show significant differences in cognitive performance in children and neurological function in adults who discontinue the low-phenylalanine diet compared with those who remain on it.¹

Role and types of dietary supplements in PKU

Purpose and necessity of supplements

Dietary supplements are important, as a restrictive diet can lead to deficiencies in essential nutrients. Supplements can provide the amino acids, yet other vitamins and minerals that are typically obtained from a more varied diet.

Types of supplements used 

Amino acid supplements 

These are phenylalanine-free formulations that provide essential amino acids needed for protein synthesis. Tailored to the specific needs of individuals with PKU, they prevent deficiencies and support normal metabolic functions. The current amino acid-based low-phenylalanine diet provides approximately 80% of protein needs from synthetic amino acids and 20% from the intact protein found in fruits and vegetables.¹

Medical foods

These specialized foods are designed to be low in phenylalanine and nutritionally balanced. In the UK, individuals with PKU are prescribed low-protein versions of common foods such as pasta and rice ³. They include metabolic formulas and low-protein products that help patients maintain a balanced diet without exceeding their phenylalanine tolerance. 

Metabolic formulas

These are complete nutritional supplements that provide essential nutrients without phenylalanine. They are often used as a primary source of nutrition, especially in infants and young children.

Phenylalanine-free protein substitutes 

Phenylalanine-free protein substitutes are crucial for individuals with PKU as they provide the necessary protein without the risk of phenylalanine accumulation. It is available in various forms, including powders, liquids, and bars. They are formulated to be as palatable as possible to encourage adherence and help meet daily protein requirements while keeping phenylalanine intake within safe limits.

Vitamins and minerals 

Due to diet restriction, nutrient deficiencies are common, especially vitamin B, D, calcium, iron and long chain fatty acid.⁵ Therefore, the AA formula and/or vitamin and mineral supplements needed to be consumed daily. Research shows that while supplements of long-chain polyunsaturated fatty acids can benefit visual function, the impact on neurocognition remains unclear.⁴ Interestingly, even with adequate iron intake, studies have shown iron deficiency in those with PKU.¹

Nutritional considerations

Monitoring phenylalanine levels

Regular monitoring of blood phenylalanine levels is essential to ensure dietary intake remains within safe limits. This involves periodic blood tests and adjustments to the diet and supplements as needed. Frequent monitoring of plasma concentrations of phenylalanine and tyrosine in relation to phenylalanine intake, often based on a 3-day record of food intake before blood sampling.¹

Adjusting supplements based on age, growth, and metabolic needs

As individuals with PKU grow and their metabolic needs change, their dietary requirements and supplement regimens must be adjusted accordingly. The UK Medical Research Council Working Party on PKU recommends a total protein intake of at least 3 g/kg body weight per day for children under 2 years of age and 2 g/kg body weight per day for children over 2 years of age.¹

Advances in PKU dietary supplements

Development of more palatable and varied supplements 

Given the poor dietary compliance with the low-phenylalanine diet after early childhood, new approaches are needed. Administration of BH4 and large neutral amino acids (LNAAs) are two new approaches to therapy that have recently been shown to be effective. Glycomacropeptide (GMP), a natural protein found in sweet cheese whey, is uniquely suited to the PKU diet because it is the only known dietary protein that contains no phenylalanine in its pure form.¹

Innovations in formulation 

Improved taste and texture

Innovations in the formulation of PKU supplements have led to products that are more palatable and easier to incorporate into the diet. This includes better-tasting amino acid supplements and low-protein foods that more closely resemble their regular counterparts.

Convenience 

New product formats, such as ready-to-drink shakes and portable snacks, offer greater convenience for individuals with PKU, making it easier to adhere to dietary restrictions while on the go. Distribution of amino acid formula throughout the day improves protein retention and reduces blood phe levels in those with PKU ¹

Personalized nutrition approaches

Personalized nutrition approaches are becoming more common in the management of PKU. This can involve genetic testing to determine specific metabolic profiles and the use of advanced technologies to track and manage dietary intake and blood phenylalanine levels more accurately.

Challenges and considerations

Compliance and adherence issues

One of the major challenges in managing PKU is ensuring compliance with the strict dietary regimen. Despite the success of dietary therapy for the treatment and management of PKU patients, there is mounting evidence regarding issues with dietary compliance and nutritional deficiencies.⁵ The restrictive nature of the diet and the need for regular supplement intake can be difficult for patients, particularly as they transition from childhood to adolescence and adulthood. Strategies to improve adherence include providing education and support, developing more palatable supplements, and addressing psychological and social factors.¹

Cost and accessibility of supplements

The high cost of special medical foods and formula, as well as frequent visits to health professionals, pose a substantial financial burden on PKU patients and their carers. The main components of PKU treatment cost are the special formula foods and amino acid supplements, and depending on the age of the patient, it has been estimated to cost £4,000 per year in the UK.⁵ Furthermore, effective dietary compliance may be hindered by a lack of health insurance or non-coverage of certain expenses by insurance companies in countries without a nationalized health system.¹

Support and resources

Role of healthcare providers 

Dietitians offer education, support, and regular adjustments to help individuals with PKU meet their nutritional needs and control phenylalanine levels. Metabolic specialists, who focus on managing metabolic disorders such as PKU, also play a crucial role. Support groups and communities provide invaluable resources and emotional support for those with PKU and their families, offering a platform for sharing experiences, strategies, and encouragement.

Conclusion

Effective management of PKU through a low-phenylalanine diet and dietary supplements is crucial for preventing severe neurological complications and ensuring the best possible quality of life for individuals with the disorder. Despite the challenges, including dietary adherence and financial burdens, ongoing advances in dietary supplements and personalized nutrition approaches offer hope for improved management and outcomes. Continuous support from healthcare providers, access to educational resources, and community support play vital roles in helping individuals with PKU and their families navigate the complexities of this lifelong condition. Ongoing research and innovation are essential to further enhance dietary management strategies and improve the lives of those affected by PKU.

FAQs

Is there a cure for PKU?

There is no cure for PKU, but it can be effectively managed with a strict low-phenylalanine diet and dietary supplements.

What is the role of tyrosine in PKU management?

Tyrosine becomes an essential amino acid in PKU patients because phenylalanine, which normally converts to tyrosine, must be restricted.

Can PKU patients follow a vegan or vegetarian diet?

Yes, but it requires careful planning to ensure adequate nutrition while adhering to low-phenylalanine requirements.