Protein metabolism in adult patients with phenylketonuria

In Vivo Metabolic Responses to Different Formulations of Amino Acid Mixtures for the Treatment of Phenylketonuria (PKU)

Phenylketonuria (PKU) is a rare autosomal recessive inborn error of metabolism where the mainstay of treatment is a Phe restricted diet consisting of a combination of limited amounts of natural protein with supplementation of Phe-free or low-Phe protein substitutes and special low protein foods. Suboptimal outcomes may be related to the different absorption kinetics of free AAs, which have lower biological efficacy than natural proteins. Physiomimic Technology^TM is a technology engineered to prolong AA (AA-PT) release allowing physiological absorption and masking the odor and taste of free AAs. The aim of these studies was to assess the impact of AA-PT formulation on selected functional and metabolic parameters both in acute and long-term experimental studies. Adult rats in fasting conditions were randomized in different groups and treated by oral gavage. Acute AA-PT administration resulted in significantly lower BUN at 90 min versus baseline. Both BUN and glycemia were modulated in the same direction as intact casein protein. Long-term treatment with AA-PT significantly reduces the protein expression of the muscle degradation marker Bnip3L (−46%) while significantly increasing the proliferation of market myostatin (+58%). Animals dosed for 15 days with AA-PT had significantly stronger grip strength (+30%) versus baseline. In conclusion, the results suggest that the AA-PT formulation may have beneficial effects on both AA oxidation and catabolism with a direct impact on muscle as well as on other metabolic pathways.

Surface Plasmon Resonance Based on Molecularly Imprinted Polymeric Film for l-Phenylalanine Detection

In this study, we designed a simple, rapid, sensitive and selective surface plasmon resonance (SPR) sensor for detection of L-phenylalaine by utilizing molecular imprinting technology. l-phenylalanine imprinted and non-imprinted poly(2-hydroxyethyl methacrylate-methacryloyl-l-phenylalanine) polymeric films were synthesized onto SPR chip surfaces using ultraviolet polymerization. l-phenyalanine imprinted and non-imprinted SPR sensors were characterized by using contact angle, atomic force microscopy and ellipsometry. After characterization studies, kinetic studies were carried out in the concentration range of 5.0-400.0 μM. The limit of detection and quantification were obtained as 0.0085 and 0.0285 μM, respectively. The response time for the test including equilibration, adsorption and desorption was approximately 9 min. The selectivity studies of the l-phenylalanine imprinted SPR sensor was performed in the presence of d-phenylalanine and l-tryptophan. Validation studies were carried out via enzyme-linked immunosorbent analysis technique in order to demonstrate the applicability and superiority of the l-phenylalanine imprinted SPR sensor.

Dietary protein and protein substitute requirements in adults with phenylketonuria: A review of the clinical guidelines

Lifelong dietary treatment is recommended in the management of phenylketonuria (PKU). Accordingly, an increasing adult population require age-specific PKU guidelines on protein requirements to support changing metabolic demands across the lifespan. Given that protein intake for dietary management of PKU is primarily (52-80%) derived from protein substitutes, the prescribing practice of protein substitutes must be underpinned by robust evidence. Whilst dietary guidelines for PKU management is evolving to incorporate adult specific protein recommendations, the scientific evidence underpinning these guidelines is currently limited. Instead, the determination of protein requirements for people with PKU have previously been extrapolated from estimates derived from the general healthy population, based on arguably outdated nitrogen balance methodology. Furthermore, a compensatory factor of 20-40% has been incorporated to account for the reduced uptake and utilisation of the elemental amino acids contained in protein substitutes. However, research informing this compensatory factor has been conducted in younger adults, with the majority of studies in non-PKU individuals. Given extensive evidence that the muscle anabolic response to ingested protein is impaired in older vs. young adults, the validity of current dietary protein recommendations for adults and older adults with PKU has been challenged. This narrative review aims to critically evaluate the existing scientific evidence underpinning current guidelines on protein requirements for adults with PKU, highlighting existing gaps in knowledge and directions for future research. We argue that current guidelines on protein requirements need updating to optimise long-term physical and functional outcomes in older adults with PKU.

Amino Acid Plasma Profiles from a Prolonged-Release Protein Substitute for Phenylketonuria: A Randomized, Single-Dose, Four-Way Crossover Trial in Healthy Volunteers

Several disorders of amino acid (AA) metabolism are treated with a protein-restricted diet supplemented with specific AA mixtures. Delivery kinetics impacts AA absorption and plasma concentration profiles. We assessed plasma profiles after ingestion of an AA mixture engineered to prolong AA absorption with Physiomimic Technology^TM (Test) in a randomized, single-dose, four-way crossover trial in healthy volunteers (Trial Registration: ISRCTN11016729). In a two-step hypothesis, the primary endpoints were (i) significant reduction in peak plasma concentrations (C_max) of essential amino acids (EAAs) while (ii) maintaining EAA bioavailability (AUC_{0-300 min}) compared to a free AA mixture (Reference). Secondary endpoints included effects on plasma profiles of other AA groups and effects on several metabolic markers. Thirty subjects completed the study. Both co-primary endpoints were met: C_max for EAAs was 27% lower with the Test product compared to the Reference product (ratio, 0.726, p < 0.0001); overall plasma EAA levels from the two AA mixtures was within the pre-specified bioequivalence range (AUC_0-300min ratio, 0.890 (95% CI: 0.865, 0.915)). These findings were supported by the results of secondary endpoints. Prolongation of AA absorption was associated with modulation of several metabolic markers. It will be important to understand whether this can improve the long-term management of disorders of AA metabolism.

Casein-derived peptides as an alternative ingredient for low-phenylalanine diets

Introduction: casein-derived peptides can be liberated both in vivo via normal digestion of casein, as well as in vitro via enzymatic hydrolysis. These peptides were suggested to have biological activity. Objectives: the aim of this study was to describe the production and characterization of casein peptides and to explore the potential of these peptides as an option for low-phenylalanine diets. Methods: peptides were produced by tryptic hydrolysis of sodium caseinate and acid precipitation with HCl, followed by precipitation with ethanol or aggregation of CaCl2 or ZnSO4. Results: the amino acid analysis revealed a significant reduction in the amount of phenylalanine from the original protein. Conclusion: casein-derived peptides could be a future alternative of short chain peptides to low-phenylalanine formulations.

Optimising amino acid absorption: essential to improve nitrogen balance and metabolic control in phenylketonuria

It has been nearly 70 years since the discovery that strict adherence to a diet low in phenylalanine prevents severe neurological sequelae in patients with phenylalanine hydroxylase deficiency (phenylketonuria; PKU). Today, dietary treatment with restricted phenylalanine intake supplemented with non-phenylalanine amino acids to support growth and maintain a healthy body composition remains the mainstay of therapy. However, a better understanding is needed of the factors that influence N balance in the context of amino acid supplementation. The aim of the present paper is to summarise considerations for improving N balance in patients with PKU, with a focus on gaining greater understanding of amino acid absorption, disposition and utilisation. In addition, the impact of phenylalanine-free amino acids on 24 h blood phenylalanine/tyrosine circadian rhythm is evaluated. We compare the effects of administering intact protein v. free amino acid on protein metabolism and discuss the possibility of improving outcomes by administering amino acid mixtures so that their absorption profile mimics that of intact protein. Protein substitutes with the ability to delay absorption of phenylalanine and tyrosine, mimicking physiological absorption kinetics, are expected to improve the rate of assimilation into protein and minimise fluctuations in quantitative plasma amino acid levels. They may also help maintain normal glycaemia and satiety sensation. This is likely to play an important role in improving the management of patients with PKU.

The Indicator Amino Acid Oxidation Method with the Use of l-[1-13C]Leucine Suggests a Higher than Currently Recommended Protein Requirement in Children with Phenylketonuria

BACKGROUND

Phenylketonuria is characterized by mutations in the Phe hydroxylase gene that leads to the accumulation of Phe in plasma and the brain. The standard of care for phenylketonuria is nutritional management with dietary restriction of Phe and the provision of sufficient protein and energy for growth and health maintenance. The protein requirement in children with phenylketonuria is empirically determined based upon phenylketonuria nutritional guidelines that are adjusted individually in response to biochemical markers and growth.

OBJECTIVE

We determined dietary protein requirements in children with phenylketonuria with the use of the indicator amino acid oxidation (IAAO) technique, with l-[1-¹³C]Leu as the indicator amino acid.

METHODS

Four children (2 males; 2 females) aged 9-18 y with phenylketonuria [mild hyperphenylalanemia (mHPA); 6-10 mg/dL (360-600 μmol/L)] were recruited to participate in ≥7 separate test protein intakes (range: 0.2-3.2 g ⋅ kg^-1 ⋅ d^-1) with the IAAO protocol with the use of l-[1-¹³C]Leu followed by the collection of breath and urine samples over 8 h. The diets were isocaloric and provided energy at 1.7 times the resting energy expenditure. Protein was provided as a crystalline amino acid mixture based on an egg protein pattern, except Phe and Leu, which were maintained at a constant across intakes. Protein requirement was determined with the use of a 2-phase linear-regression crossover analysis of the rate of l-[1-¹³C]Leu tracer oxidation.

RESULTS

The mean protein requirement was determined to be 1.85 g ⋅ kg^-1 ⋅ d^-1 (R² = 0.66; 95% CI: 1.37, 2.33). This result is substantially higher than the 2014 phenylketonuria recommendations (1.14-1.33 g ⋅ kg^-1 ⋅ d^-1; based on 120-140% above the current RDA for age).

CONCLUSIONS

To our knowledge, this is the first study to directly define a quantitative requirement for protein intake in children with mHPA and indicates that current protein recommendations in children with phenylketonuria may be insufficient. This trial was registered at clinicaltrials.gov as NCT01965691.

Minimally invasive (13)C-breath test to examine phenylalanine metabolism in children with phenylketonuria

BACKGROUND

Phenylketonuria (PKU) is an autosomal recessive disorder caused by deficiency of hepatic phenylalanine hydroxylase (PAH) leading to increased levels of phenylalanine in the plasma. Phenylalanine levels and phenylalanine hydroxylase (PAH) activity monitoring are currently limited to conventional blood dot testing. 1-(13)C-phenylalanine, a stable isotope can be used to examine phenylalanine metabolism, as the conversion of phenylalanine to tyrosine occurs in vivo via PAH and subsequently releases the carboxyl labeled (13)C as (13)CO2 in breath.

OBJECTIVE

Our objective was to examine phenylalanine metabolism in children with PKU using a minimally-invasive 1-(13)C-phenylalanine breath test ((13)C-PBT).

DESIGN

Nine children (7 M: 2 F, mean age 12.5 ± 2.87 y) with PKU participated in the study twice: once before and once after sapropterin supplementation. Children were provided 6 mg/kg oral dose of 1-(13)C-phenylalanine and breath samples were collected at 20 min intervals for a period of 2h. Rate of CO2 production was measured at 60 min post-oral dose using indirect calorimetry. The percentage of 1-(13)C-phenylalanine exhaled as (13)CO2 was measured over a 2h period. Prior to studying children with PKU, we tested the study protocol in healthy children (n = 6; 4M: 2F, mean age 10.2 ± 2.48 y) as proof of principle.

RESULTS

Production of a peak enrichment (Cmax) of (13)CO2 (% of dose) in all healthy children occurred at 20 min ranging from 17-29% of dose, with a subsequent return to ~5% by the end of 2h. Production of (13)CO2 from 1-(13)C-phenylalanine in all children with PKU prior to sapropterin treatment remained low. Following sapropterin supplementation for a week, production of (13)CO2 significantly increased in five children with a subsequent decline in blood phenylalanine levels, suggesting improved PAH activity. Sapropterin treatment was not effective in three children whose (13)CO2 production remained unchanged, and did not show a reduction in blood phenylalanine levels and improvement in dietary phenylalanine tolerance.

CONCLUSIONS

Our study shows that the (13)C-PBT can be a minimally invasive, safe and reliable measure to examine phenylalanine metabolism in children with phenylketonuria. The breath data are corroborated by blood phenylalanine levels in children who had increased responses in (13)CO2 production, as reviewed post-hoc from clinical charts.

Practices in prescribing protein substitutes for PKU in Europe: No uniformity of approach

BACKGROUND

There appears little consensus concerning protein requirements in phenylketonuria (PKU).

METHODS

A questionnaire completed by 63 European and Turkish IMD centres from 18 countries collected data on prescribed total protein intake (natural/intact protein and phenylalanine-free protein substitute [PS]) by age, administration frequency and method, monitoring, and type of protein substitute. Data were analysed by European region using descriptive statistics.

RESULTS

The amount of total protein (from PS and natural/intact protein) varied according to the European region. Higher median amounts of total protein were prescribed in infants and children in Northern Europe (n=24 centres) (infants <1 year, >2-3g/kg/day; 1-3 years of age, >2-3 g/kg/day; 4-10 years of age, >1.5-2.5 g/kg/day) and Southern Europe (n=10 centres) (infants <1 year, 2.5 g/kg/day, 1-3 years of age, 2 g/kg/day; 4-10 years of age, 1.5-2 g/kg/day), than by Eastern Europe (n=4 centres) (infants <1 year, 2.5 g/kg/day, 1-3 years of age, >2-2.5 g/kg/day; 4-10 years of age, >1.5-2 g/kg/day) and with Western Europe (n=25 centres) giving the least (infants <1 year, >2-2.5 g/kg/day, 1-3 years of age, 1.5-2 g/kg/day; 4-10 years of age, 1-1.5 g/kg/day). Total protein prescription was similar in patients aged >10 years (1-1.5 g/kg/day) and maternal patients (1-1.5 g/kg/day).

CONCLUSIONS

The amounts of total protein prescribed varied between European countries and appeared to be influenced by geographical region. In PKU, all gave higher than the recommended 2007 WHO/FAO/UNU safe levels of protein intake for the general population.

Protein substitute for children and adults with phenylketonuria

BACKGROUND

Phenylketonuria is an inherited metabolic disorder characterised by an absence or deficiency of the enzyme phenylalanine hydroxylase. The aim of treatment is to lower blood phenylalanine concentrations to the recommended therapeutic range to prevent developmental delay and support normal growth. Current treatment consists of a low-phenylalanine diet in combination with a protein substitute which is free from or low in phenylalanine. Guidance regarding the use, dosage, and distribution of dosage of the protein substitute over a 24-hour period is unclear, and there is variation in recommendations among treatment centres. This is an update of a Cochrane review first published in 2005, and previously updated in 2008.

OBJECTIVES

To assess the benefits and adverse effects of protein substitute, its dosage, and distribution of dose in children and adults with phenylketonuria who are adhering to a low-phenylalanine diet.

SEARCH METHODS

We searched the Cochrane Cystic Fibrosis and Genetic Disorders Group Trials Register which consists of references identified from comprehensive electronic database searches and hand searches of relevant journals and abstract books of conference proceedings. We also contacted manufacturers of the phenylalanine-free and low-phenylalanine protein substitutes for any data from published and unpublished randomised controlled trials.Date of the most recent search of the Group's Inborn Errors of Metabolism Trials Register: 03 April 2014.

SELECTION CRITERIA

All randomised or quasi-randomised controlled trials comparing: any dose of protein substitute with no protein substitute; an alternative dosage; or the same dose, but given as frequent small doses throughout the day compared with the same total daily dose given as larger boluses less frequently.

DATA COLLECTION AND ANALYSIS

Both authors independently extracted data and assessed trial quality.

MAIN RESULTS

Three trials (69 participants) are included in this review. One trial investigated the use of protein substitute in 16 participants, while a further two trials investigated the dosage of protein substitute in a total of 53 participants. Due to issues with data presentation in each trial, described in full in the review, formal statistical analyses of the data were impossible. Investigators will be contacted for further information.

AUTHORS' CONCLUSIONS

No conclusions could be drawn about the short- or long-term use of protein substitute in phenylketonuria due to the lack of adequate or analysable trial data. Additional data and randomised controlled trials are needed to investigate the use of protein substitute in phenylketonuria. Until further evidence is available, current practice in the use of protein substitute should continue to be monitored with care.

Spectrofluorimetric determination of phenylalanine based on fluorescence enhancement of europium ion immobilized with sol-gel method

The analysis of phenylalanine (Phe) in serum is widely performed for the screening of newborn phenylketonuria (PKU). In this work, a novel spectrofluorimetric method for the determination of Phe was developed based on the fluorescence enhancement of Ruhemann's purple, the reaction product between Phe and ninhydrin, upon coordination with Eu(3+). A filter paper disc containing immobilized reactants (ninhydrin and Eu(3+)) was fabricated by sol-gel method. The experimental parameters affecting the determination of Phe, such as the concentrations of immobilized reagents, the pH value, the reaction time and temperature were optimized. Under optimum conditions, the fluorescence intensity of Phe-ninhydrin-Eu(3+) system was linearly proportional to the concentration of Phe in the range from 5×10(-5) to 2×10(-3) mol L(-1), and the limit of detection was found to be 5.2×10(-6) mol L(-1). The relative standard deviation was 2.6% for ten replicate measurements of 1.5×10(-4) mol L(-1) of Phe. The method has merits of sensitivity, simplicity and low cost, and has been applied to the determination of Phe in artificial serum.

Suboptimal outcomes in patients with PKU treated early with diet alone: revisiting the evidence

BACKGROUND

The National Institute of Health (NIH) published a Consensus Statement on the screening and management of Phenylketonuria (PKU) in 2000. The panel involved in the development of this consensus statement acknowledged the lack of data regarding the potential for more subtle suboptimal outcomes and the need for further research into treatment options. In subsequent years, the approval of new treatment options for PKU and outcome data for patients treated from the newborn period by dietary therapy alone have become available. We hypothesized that a review of the PKU literature since 2000 would provide further evidence related to neurocognitive, psychosocial, and physical outcomes that could serve as a basis for reassessment of the 2000 NIH Consensus Statement.

METHODS

A systematic review of literature residing in PubMed, Scopus and PsychInfo was performed in order to assess the outcome data over the last decade in diet-alone early-treated PKU patients to assess the need for new recommendations and validity of older recommendations in light of new evidence.

RESULTS

The majority of publications (140/150) that contained primary outcome data presented at least one suboptimal outcome compared to control groups or standardized norms/reference values in at least one of the following areas: neurocognitive/psychosocial (N=60; 58 reporting suboptimal outcomes); quality of life (N=6; 4 reporting suboptimal outcomes); brain pathology (N=32; 30 reporting suboptimal outcomes); growth/nutrition (N=34; 29 reporting suboptimal outcomes); bone pathology (N=9; 9 reporting suboptimal outcomes); and/or maternal PKU (N=19; 19 reporting suboptimal outcomes).

CONCLUSIONS

Despite the remarkable success of public health programs that have instituted newborn screening and early introduction of dietary therapy for PKU, there is a growing body of evidence that suggests that neurocognitive, psychosocial, quality of life, growth, nutrition, bone pathology and maternal PKU outcomes are suboptimal. The time may be right for revisiting the 2000 NIH Consensus Statement in order to address a number of important issues related to PKU management, including treatment advancements for metabolic control in PKU, blood Phe variability, neurocognitive and psychological assessments, routine screening measures for nutritional biomarkers, and bone pathology.

Immobilization of phenylalanine dehydrogenase and its application in flow-injection analysis system for determination of plasma phenylalanine

Phenylalanine dehydrogenase (L-PheDH) from Sporosarcina ureae was immobilized on DEAE-cellulose, modified initially with 2-amino-4,6-dichloro-s-triazine followed by hexamethylenediamine and glutaraldehyde. The highest activity of immobilized PheDH was determined as 95.75 U/g support with 56% retained activity. The optimum pH value of immobilized L-PheDH was shifted from pH 10.4 to 11.0. The immobilized L-PheDH showed activity variations close to the maximum value in a wider temperature range of 45-55 °C, whereas it was 40 °C for the native enzyme. The pH and the thermal stability of the immobilized L-PheDH were also better than the native enzyme. At pH 10.4 and 25 °C, K (m) values of the native and the immobilized L-PheDH were determined as K(m Phe) = 0.118, 0.063 mM and K(m NAD)(+) = 0.234, 0.128 mM, respectively. Formed NADH at the exit of packed bed reactor column was detected by the flow-injection analysis system. The conversion efficiency of the reactor was found to be 100% in the range of 5-600 μM Phe at 9 mM NAD(+) with a total flow rate of 0.1 mL/min. The reactor was used for the analyses of 30 samples each for 3 h per day. The half-life period of the reactor was 15 days.

Nutritional Management of Phenylketonuria

Phenylketonuria (PKU) is caused by deficient activity of the enzyme phenylalanine hydroxylase, needed to convert the essential amino acid (AA) phenylalanine (phe) to tyrosine. In order to prevent neurological damage, lifelong adherence to a low-phe diet that is restricted in natural foods and requires ingestion of a phe-free AA formula to meet protein needs is required. The goal of nutritional management for those with PKU is to maintain plasma phe concentrations that support optimal growth, development, and mental functioning while providing a nutritionally complete diet. This paper reviews developing a lifelong dietary prescription for those with PKU, outcomes of nutritional management, compliance with the low-phe diet across the life cycle, and new options for nutritional management. An individualized dietary prescription is needed to meet nutrient requirements, and the adequacy of phe intake is monitored with assessment of blood phe levels. Elevated phe concentrations may occur due to illness, excessive or inadequate phe intake, or inadequate intake of AA formula. Although normal growth and development occurs with adherence to the low-phe diet, it is important to monitor vitamin, mineral and essential fatty acid status, especially in those who do not consume sufficient AA formula. Given the growing population of adults with PKU, further research is needed to understand the risks for developing osteoporosis and cardiovascular disease. There are promising new options to liberalize the diet and improve metabolic control such as tetrahydrobiopterin therapy or supplementation with large neutral AAs. Moreover, foods made with glycomacropeptide, an intact protein that contains minimal phe, improves the PKU diet by offering a palatable alternative to AA formula. In summary, continued efforts are needed to overcome the biggest challenge to living with PKU - lifelong adherence to the low-phe diet.

The truth of treating patients with phenylketonuria after childhood: the need for a new guideline

In recent years, an increasing number of national guidelines on the treatment of phenylketonuria (PKU) have emerged. Most of these guidelines are dedicated to the care of children, while less attention is paid to the care of adults, although all guidelines underline the importance of diet for life. This review aims to summarize issues that need to be addressed within a guideline on the treatment of PKU, especially when care for patients beyond childhood is concerned. In this respect, it is of importance that adult patients, both willing and unwilling to be treated, need a guideline for care and follow-up. In PKU there is certainly a need for an improved unified guideline, especially after childhood, although many of the considerations in this article also apply to recommendations for treatment of children. Such a guideline will be a tool to improve treatment in PKU patients but should also include recommendations for collecting data for clinical and research purposes. Guideline development should also focus on nutritional, neuropsychological and psychosocial issues and not only on target plasma phenylalanine concentrations. In addition, guidelines must address not only what has to be done but also how it can be done, thereby improving concordance with the recommendations for treatment and management.

Protein substitute for children and adults with phenylketonuria

BACKGROUND

Phenylketonuria is an inherited metabolic disorder characterised by an absence or deficiency of the enzyme phenylalanine hydroxylase. The aim of treatment is to lower blood phenylalanine concentrations to the recommended therapeutic range to prevent developmental delay and support normal growth. Current treatment consists of a low-phenylalanine diet in combination with a protein substitute which is free from or low in phenylalanine. Guidance regarding the use, dosage, and distribution of dosage of the protein substitute over a 24-hour period is unclear, and there is variation in recommendations among treatment centres.

OBJECTIVES

To assess the benefits and adverse effects of protein substitute, its dosage, and distribution of dose in children and adults with phenylketonuria who are adhering to a low-phenylalanine diet.

SEARCH STRATEGY

We searched the Cochrane Cystic Fibrosis and Genetic Disorders Group Trials Register which consists of references identified from comprehensive electronic database searches and hand searches of relevant journals and abstract books of conference proceedings. We also contacted manufacturers of the phenylalanine-free and low-phenylalanine protein substitutes for any data from published and unpublished randomised controlled trials.Date of the most recent search of the Group's Trials Register: April 2008.

SELECTION CRITERIA

All randomised or quasi-randomised controlled trials comparing: any dose of protein substitute with no protein substitute; an alternative dosage; or the same dose, but given as frequent small doses throughout the day compared with the same total daily dose given as larger boluses less frequently.

DATA COLLECTION AND ANALYSIS

Both authors independently extracted data and assessed trial quality.

MAIN RESULTS

Three trials (69 participants) are included in this review. One trial investigated the use of protein substitute in 16 participants, while a further two trials investigated the dosage of protein substitute in a total of 53 participants. Due to issues with data presentation in each trial, described in full in the review, formal statistical analyses of the data were impossible. Investigators are being contacted for further information.

AUTHORS' CONCLUSIONS

No conclusions could be drawn about the short- or long-term use of protein substitute in phenylketonuria due to the lack of adequate or analysable trial data. Additional data and randomised controlled trials are needed to investigate the use of protein substitute in phenylketonuria. Until further evidence is available, current practice in the use of protein substitute should continue to be monitored with care.