BackgroundPhenylketonuria (PKU) is an autosomal recessive genetic condition caused by a PAH gene mutation that results in impaired function of the phenylalanine hydroxylase (PAH) pathway. Thus, L-phenylalanine (Phe) cannot be effectively hydroxylated into L-tyrosine (Tyr), so without treatment, Phe levels accumulate while Tyr levels remain low. While PKU is relatively well understood, there is currently sparse literature regarding the putative health and metabolic impacts among PKU carriers, who have been historically described as “unaffected.”MethodsThis article provides a detailed overview of the methods for a single-arm clinical trial of PKU carriers and non-carriers, as well as results from a pilot feasibility study. This trial aims to determine whether PKU carriers, following an oral Phe challenge, exhibit an intermediate phenotype of PKU that may perturb metabolic, physiological and/or cognitive function. In this study, PKU carriers and non-carriers were recruited and following baseline measurements of chronic mental health, cognition, blood pressure, acute mood, and minimally invasive biospecimen collection, participants ingested a 100 mg/kg dose of Phe. Two hours post-Phe consumption, baseline measurements were repeated (except chronic mental health measures) to determine potential treatment response differences between carriers and non-carriers of PKU. The pilot study aimed to evaluate feasibility of the methods, and the small sample size limited statistical power; as such, no statistical analyses were conducted.ResultsThe pilot study demonstrated protocol feasibility and several intriguing but highly preliminary data trends. Among genetic carriers of PKU, we observed higher chronic impulsivity scores, and following the oral Phe challenge, greater increases in Phe/Tyr ratio in dried blood spot extracts, both systolic and diastolic blood pressure, as well as worsening acute mood scores compared to non-carriers.ConclusionsOverall, this research aims to enhance our understanding of the impact of nutrition on carriers of autosomal recessive inborn errors of metabolism who are presumed to be unaffected, starting with a focus on PKU. Pilot study results demonstrated feasibility of the protocol and some interesting preliminary data trends, however further studies that are adequately powered with larger sample sizes are needed to corroborate these findings.Clinical trail noPhe For Me? ClinicalTrials.gov. NCT06119048. Registered 31 October 2023 - https://clinicaltrials.gov/study/NCT06119048