Impact Of A Phe-Restricted Diet On Gut Health In Children With PKU

Impact Of A Phenylalanine-Restricted Diet On The Microbiota Composition And Metabolome Of Children With PKU

Phenylketonuria (PKU) is an inherited disorder of phenylalanine (Phe) metabolism. The mainstay of treatment is a Phe-restricted diet, which aims to maintain blood Phe concentrations within the recommended range and prevent neurological complications. Some individuals with PKU respond to pharmacological treatments, including sapropterin, a synthetic form of tetrahydrobiopterin (BH4), or sepiapterin. These treatments may increase Phe tolerance and allow a less restrictive diet.

Diet is an important determinant of gut microbiota composition and function. However, the effects of the Phe-restricted diet and pharmacologically enabled dietary relaxation on the gut microbiota in PKU remain poorly understood.

This observational study includes children and adolescents with PKU aged 3-17 years attending Birmingham Children's Hospital. Participants include those managed exclusively with a Phe-restricted diet, those receiving sapropterin, and those receiving sepiapterin. One healthy household control is recruited for each participant with PKU.

Faecal samples are collected for shotgun metagenomic sequencing and metabolite profiling. Dietary intake, gastrointestinal symptoms, stool characteristics, clinical information, and PKU treatment are also assessed.

The study investigates whether gut microbiota composition, microbial functional potential, and faecal metabolite profiles differ between participants managed with a Phe-restricted diet, those receiving pharmacological treatment, and healthy household controls. The findings may improve understanding of the relationships between PKU treatment, dietary restriction, gastrointestinal health, and the gut microbiome and may inform future nutritional strategies for individuals with PKU.

Study Overview

• Status: Recruiting
• Conditions: Phenylketonuria
• Intervention / Treatment: Other: Phenylalanine-restricted diet alone; Drug: Sapropterin Dihydrochloride; Drug: Sepiapterin

Detailed Description

This is a single-centre, cross-sectional observational study conducted at Birmingham Children's Hospital. The study investigates the association between phenylketonuria (PKU) treatment, degree of dietary restriction, gastrointestinal health, and the composition and functional potential of the gut microbiota.

Children and adolescents aged 3-17 years with early-treated PKU are recruited into one of three cohorts according to their existing clinical management: treatment with a phenylalanine-restricted diet alone (anticipated n=30), treatment with sapropterin alongside an individualised phenylalanine-restricted diet (anticipated n=30), or treatment with sepiapterin alongside an individualised phenylalanine-restricted diet (anticipated n=14). Participants receiving sapropterin or sepiapterin have been receiving treatment for at least three months and have achieved at least a 100% increase in natural protein tolerance compared with their pre-treatment prescription. Treatment allocation, medication dosage, and dietary management are determined as part of routine clinical care and are not assigned or modified by the study.

One healthy household control is recruited for each participant with PKU. Household controls are aged 3 years or older and do not have PKU or another inherited metabolic disorder. Recruiting controls from the same household aims to reduce the influence of shared environmental factors, including living conditions and food availability, when comparing gut microbiota profiles. The anticipated total enrolment is 148 participants, comprising 74 participants with PKU and 74 household controls.

Each participant provides a single faecal sample. Samples are stored at -80°C and analysed using shotgun metagenomic sequencing to characterise microbial taxonomic composition, within-sample diversity, between-sample community differences, and microbial functional potential. Taxonomic profiles are assessed at multiple levels, including phylum, genus, and species. Functional annotation is used to characterise microbial genes, enzymes, and metabolic pathways.

Dietary intake is assessed using a 24-hour dietary recall, including natural protein intake, protein substitute intake, dietary fibre intake, and consumption of special low-protein foods. Gastrointestinal symptoms are assessed using the age-appropriate self-report or parent-proxy version of the PedsQL Gastrointestinal Symptoms Module, and stool form is assessed using the Bristol Stool Form Scale. Relevant demographic and clinical information, including age, sex, anthropometric measurements, PKU treatment, and metabolic control, is also collected.

The findings from this study may support the development of nutritional strategies and protein substitutes that more effectively promote gastrointestinal and metabolic health in individuals with PKU.

• Study Type: Observational
• Enrollment (Estimated): 148

Contacts and Locations

• Study Contact: Name: Anita MacDonald, PhD; Phone Number: +44 121 333 9999; Email: anita.macdonald@nhs.net
• Study Contact Backup: Name: Catarina Rodrigues, MSc; Phone Number: +44 121 472 1377; Email: catarina.rodrigues1@nhs.net

Study Locations

• United Kingdom
  • Birmingham
    • Birmingham, Birmingham, United Kingdom, B4 6NH
      • Recruiting
      • Birmingham Children's Hospital
      • Contact: Anita MacDonald, PhD; Phone Number: +44 121 333 9999; Email: anita.macdonald@nhs.net

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Child
• Accepts Healthy Volunteers: Yes

• Sampling Method: Non-Probability Sample

Study Population

Children and adolescents aged 3-17 years with early-treated phenylketonuria receiving clinical care at Birmingham Children's Hospital are recruited into one of three cohorts according to their current treatment: a phenylalanine-restricted diet alone, sapropterin, or sepiapterin.

One healthy household control aged 3 years or older, without phenylketonuria or another inherited metabolic disorder, is recruited for each participant with phenylketonuria. All participants are enrolled according to predefined eligibility criteria.

Description

Inclusion Criteria:

Participants with PKU:

• Aged 3-17 years.
• Confirmed diagnosis of phenylketonuria following newborn screening.
• Receiving ongoing clinical management for PKU.
• For the diet-only cohort: managed with standard phenylalanine-restricted dietary treatment and not receiving sapropterin or sepiapterin.
• For the sapropterin cohort: receiving sapropterin for at least three consecutive months and having achieved at least a 100% increase in natural protein tolerance compared with the pre-treatment prescription.
• For the sepiapterin cohort: receiving sepiapterin for at least three consecutive months and having achieved at least a 100% increase in natural protein tolerance compared with the pre-treatment prescription.

Healthy household controls:

• Aged 3 years or older.
• Living in the same household as a participating child or adolescent with PKU.
• No known diagnosis of PKU or another inherited metabolic disorder.

Exclusion Criteria:

• Congenital malformations.
• Chronic gastrointestinal disease.
• Endocrine, liver, or kidney disease.
• Other chronic medical conditions likely to affect gut microbiota composition.
• Following a therapeutic diet for a medical condition other than PKU within the six months before stool sample collection.

Study Plan

How is the study designed?

Number of groups / cohorts

4

Cohorts and Interventions

Group / Cohort	Intervention / Treatment
PKU - Diet Only; Children and adolescents aged 3-17 years with early-treated phenylketonuria who are managed exclusively with a phenylalanine-restricted diet and are not receiving sapropterin or sepiapterin. Participants provide a faecal sample and undergo dietary, clinical, gastrointestinal symptom, and stool assessments.	Other: Phenylalanine-restricted diet alone; Participants are managed with a phenylalanine-restricted diet and prescribed protein substitutes as part of their usual clinical care. They are not receiving sapropterin or sepiapterin. Dietary treatment is not assigned or modified by this observational study.
PKU - Sapropterin; Children and adolescents aged 3-17 years with early-treated phenylketonuria who have been receiving sapropterin for at least 3 months and have achieved at least a 100% increase in natural protein tolerance compared with before treatment. Sapropterin treatment is not assigned as part of this observational study. Participants provide a faecal sample and undergo dietary, clinical, gastrointestinal symptom, and stool assessments.	Drug: Sapropterin Dihydrochloride; Participants receive sapropterin as part of their usual clinical care, alongside an individualised phenylalanine-restricted diet. Sapropterin treatment and dosage are prescribed independently of this observational study and are not assigned or modified by the investigators.; Other Names: Kuvan
PKU - Sepiapterin; Children and adolescents aged 3-17 years with early-treated phenylketonuria who have been receiving sepiapterin for at least 3 months and have achieved at least a 100% increase in natural protein tolerance compared with before treatment. Sepiapterin treatment is not assigned as part of this observational study. Participants provide a faecal sample and undergo dietary, clinical, gastrointestinal symptom, and stool assessments.	Drug: Sepiapterin; Participants receive sepiapterin as part of their existing clinical management, alongside an individualised phenylalanine-restricted diet. Sepiapterin treatment and dosage are determined independently of this observational study and are not assigned or modified by the investigators.; Other Names: Sephience
Healthy Household Controls; Healthy household members without phenylketonuria who are recruited at a ratio of one control for each participant with phenylketonuria. Controls provide a faecal sample and complete the relevant dietary, gastrointestinal symptom, and stool assessments.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Faecal gut microbiota taxonomic composition assessed by shotgun metagenomic sequencing	Taxonomic profiles, expressed as the relative abundance of microbial taxa at phylum, genus, and species levels, are derived from shotgun metagenomic sequencing of one faecal sample per participant. Profiles are compared among participants with PKU managed by a phenylalanine-restricted diet alone, sapropterin, or sepiapterin and healthy household controls.	At enrolment (single faecal sample collection)

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Faecal gut microbiota alpha diversity assessed by shotgun metagenomic sequencing	Within-sample microbial diversity is assessed from shotgun metagenomic sequencing data using alpha-diversity indices, including Shannon, Simpson, Chao1, ACE, and observed species. Alpha-diversity measures are compared across the study cohorts.	At enrolment, based on a single faecal sample
Faecal gut microbiota beta diversity assessed by shotgun metagenomic sequencing	Differences in overall microbial community composition between participants are assessed using beta-diversity measures derived from shotgun metagenomic sequencing data, including Bray-Curtis dissimilarity. Microbial community profiles are compared across the study cohorts.	At enrolment, based on a single faecal sample
Microbial functional potential assessed by shotgun metagenomic sequencing	Microbial genes and functional pathways are characterised from shotgun metagenomic sequencing data using functional annotation databases. The relative abundance of microbial genes, enzymes, and metabolic pathways is compared across the study cohorts.	At enrolment, based on a single faecal sample
Gastrointestinal symptom burden assessed using the PedsQL Gastrointestinal Symptoms Module	Gastrointestinal symptoms are assessed using the age-appropriate self-report or parent-proxy version of the PedsQL Gastrointestinal Symptoms Module. Total and individual symptom-domain scores are calculated, with lower scores indicating a greater gastrointestinal symptom burden, and compared across the study cohorts.	At enrolment, based on a single faecal sample
Stool form assessed using the Bristol Stool Form Scale	Usual stool form is classified using the Bristol Stool Form Scale, ranging from type 1, representing separate hard lumps, to type 7, representing entirely liquid stool. Stool-form distributions are compared across the study cohorts.	At enrolment, based on a single faecal sample
Dietary intake assessed using a 24-hour dietary recall	Dietary intake is assessed using a 24-hour dietary recall. Nutrient and food-group measures, including natural protein intake, protein substitute intake, dietary fibre intake, and consumption of special low-protein foods, are estimated and compared across the study cohorts.	At enrolment, based on a single faecal sample

Collaborators and Investigators

• Sponsor: Birmingham Women's and Children's NHS Foundation Trust
• Collaborators: Universidade Nova de Lisboa
• Investigators: Principal Investigator: Anita MacDonald, PhD, Birmingham Children's Hospital

Study record dates

Study Major Dates

• Study Start (Actual): October 1, 2025
• Primary Completion (Estimated): December 1, 2026
• Study Completion (Estimated): December 1, 2026

Study Registration Dates

• First Submitted: July 21, 2026
• First Submitted That Met QC Criteria: July 21, 2026
• First Posted (Actual): July 27, 2026

Study Record Updates

• Last Update Posted (Actual): July 27, 2026
• Last Update Submitted That Met QC Criteria: July 21, 2026
• Last Verified: July 1, 2026

More Information

Terms related to this study

• Additional Relevant MeSH Terms: Brain Diseases; Central Nervous System Diseases; Nervous System Diseases; Metabolism, Inborn Errors; Genetic Diseases, Inborn; Metabolic Diseases; Brain Diseases, Metabolic, Inborn; Brain Diseases, Metabolic; Amino Acid Metabolism, Inborn Errors; Congenital, Hereditary, and Neonatal Diseases and Abnormalities; Nutritional and Metabolic Diseases; Phenylketonurias; sepiapterin; sapropterin
• Other Study ID Numbers: IRAS project ID: 353831

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: NO
• IPD Plan Description: There is no current plan to share individual participant data because the study involves a small population with a rare condition, which may increase the risk of participant identification. Any future data sharing would be subject to participant consent, ethical and institutional approvals, and an appropriate data-sharing agreement. Aggregated study results will be disseminated through scientific publications and presentations.

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No