Justine R. Horne

Background/Objectives: Biological factors such as genetics contribute to nutrition-related outcomes, but nutritional epidemiological studies often lack consideration of genetics despite evidence of their functional impacts on health and cognition. Phenylalanine hydroxylase (PAH) genetic variation has been hypothesized to influence health and cognitive outcomes due to evidence of metabolic perturbations in L-phenylalanine to L-tyrosine hydroxylation, including plausible downstream effects on catecholamine neurotransmitters among not only individuals with phenylketonuria (PKU) [homozygotes for PAH mutations] but also PKU carriers [heterozygotes]. Related to these metabolic perturbations, diminished executive functioning has been observed in individuals with PKU, even when treated, but research is lacking exploring this outcome in PKU carriers. The present study aims to detail methods for stratifying populations based on genetic variation, for use in epidemiological precision nutrition research. It further provides an exploratory exemplar of such research through population stratification by PAH genetic variation (i.e., PKU carriers vs. non-carriers), while providing the first descriptive data on executive functioning skills using the validated Executive Skills Questionnaire—Revised (ESQ-R) tool with PAH-genetically stratified groups (PKU carriers and non-carriers). Methods: Participants were ≥18 years of age and PAH heterozygotes (PKU carriers) or non-carriers. Levels of executive functioning were self-reported anonymously online and included the validated Executive Skills Questionnaire—Revised (ESQ-R) tool. Data were analyzed using t-tests, chi-square tests, ANOVAs, and ANCOVAs. Results: Respondents (n = 99, n = 79 carriers and n = 20 non-carriers) consisted of males (22.2%) and females (77.8%), primarily of European ancestry. There were no significant differences between groups (carriers vs. non-carriers) for total scores (mean ± SD ESQ-R score carriers = 17.41 ± 14.01; non-carriers = 14.95 ± 10.00), but carriers scored significantly worse than non-carriers for the ESQ-R item “I have trouble making a plan” in the adjusted model. Conclusions: This study provides a methodological exemplar for exploring genetically stratified subpopulations in epidemiological precision nutrition research.

Background: Phenylketonuria (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.” Methods: This 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. Results: The 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. Conclusions: Overall, 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 no: Phe For Me? ClinicalTrials.gov. NCT06119048. Registered 31 October 2023 - https://clinicaltrials.gov/study/NCT06119048

The phenylalanine hydroxylase (PAH) pathway is an enzymatic pathway focused primarily on the hydroxylation of the amino acid L-phenylalanine (Phe) to L-tyrosine (Tyr). However, the functioning of this pathway can also have significant impacts on other downstream metabolites. In addition, both common and rare genetic variants have been demonstrated to significantly impair PAH pathway function. However, to our knowledge, the comprehensive PAH pathway including influences on related downstream metabolic pathways has not been mapped in an integrative manner. As such, we conducted a narrative review of the literature to describe the comprehensive PAH pathway, including influences on downstream related metabolic pathways, while mapping a visual depiction of these integrative processes. In addition to the hydroxylation of Phe to Tyr, the comprehensive pathway narrative describes Phe metabolism and transport across the blood brain barrier, including competition with other large neutral amino acids, Tyr catabolism, and others. Discussion of the importance of these metabolic processes in the context of PAH genetic variation are also provided. With an increased focus on conducting health research from a systems biology perspective, this work is critical to improving our understanding of the downstream effects of amino acid metabolism and their possible impacts on health outcomes.

Background: Phenylketonuria (PKU) is an autosomal recessive genetic condition that results in reduced enzymatic functioning within the phenylalanine hydroxylase (PAH) pathway, which is involved in the metabolism of Phenylalanine (Phe) into Tyrosine (Tyr). Without dietary intervention, individuals with PKU exhibit significantly elevated levels of Phe, which is presumed to cause severe neurological dysfunction and other associated health risks. Carriers of PKU are heterozygotes for a PAH gene mutation and are typically described in the literature as “unaffected.” However, decades of existing research challenges this classical thinking and it is plausible that these individuals currently classified as carriers may present with an intermediate phenotype or may be “moderately affected.” Summary: The purpose of this scoping review was to explore this hypothesis further, by searching for and summarizing existing literature on metabolism and health outcomes among PKU carriers. Preliminary research has suggested that some PKU carriers exhibit reduced PAH enzyme function, and related, elevated circulating Phe levels compared to non-carriers. In addition, Phe dosing trials have further demonstrated that carriers have increased Phe levels and decreased Tyr levels compared to non-carriers. Because of these metabolic perturbations, it is biologically plausible for carriers to experience an intermediate phenotype in terms of metabolic consequences and clinical outcomes. While these outcomes have yet to be thoroughly explored, early research has found associations between PKU carrier status and lower IQs as well as decreased executive functioning, memory, processing speed, and inhibitory control. The PAH pathway is also involved in melanogenesis and research has demonstrated increased melanoma risk among PKU carriers. However, there are many limitations to this research and thus whether or not carriers are clinically impacted cannot yet be conclusively determined. Key message: Overall, while preliminary research suggests a possible intermediate phenotype among PKU carriers, the current available research is limited and PKU carriers are still clinically considered “unaffected.” This review outlines the current literature while discussing future research endeavours related to the metabolism and health of PKU carriers.

Mendel’s Law of Dominance suggests that recessive disease expression requires the inheritance of two mutated alleles as the dominant, wildtype allele suppresses disease presentation leading to the expression of physiological normal phenotypes. However, there is existing evidence that challenges this school of thought. Here, we summarise existing literature evaluating metabolic and health impacts among carriers of autosomal recessive conditions, focusing on phenylketonuria (PKU), classical homocystinuria, galactosemia and Usher syndrome as examples. Our findings suggest that carriers, often described as ‘unaffected’, may actually display attenuated symptoms for the recessive disease they are carrying. For instance, PKU is an inborn error of metabolism characterised by the build-up of plasma phenylalanine attributed to the deficiency of the phenylalanine hydroxylase (PAH) enzyme. While less severe, PKU carriers also exhibit this impaired enzymatic activity, leading to elevated plasma phenylalanine levels, especially after phenylalanine consumption. Related to these metabolic alterations in the PAH pathway, there is early evidence to suggest that PKU carriers may have compromised cognitive and mental health outcomes. Overall, research on the health and metabolic impacts of PKU carriers is sparse, with most studies conducted several decades ago. However, early evidence suggests that intermediate phenotypes among carriers of autosomal recessive conditions are plausible. The illustrated possible intermediate phenotypes observed among carriers necessitates future research to determine possible clinical implications among this population.

The recognition that microbes are integral to human life has led to studies on how to manipulate them in favor of health outcomes. To date, there has been no conjoint recommendation for the intake of dietary compounds that can complement the ingested organisms in terms of promoting an improved health outcome. The aim of this review is to discuss how beneficial microbes in the form of probiotics, fermented foods, and donor feces are being used to manage health. In addition, we explore the rationale for selecting beneficial microbial strains and aligning diets to accommodate their propagation in the gut. A pilot clinical trial design is presented to examine the effects of probiotics and exercise in patients with phenylketonuria (PKU); it is the most common inborn error of amino acid metabolism, and it is a complication that requires lifelong dietary intervention. The example design is provided to illustrate the importance of using omics technology to see if the intervention elevates neuroactive biogenic amines in the plasma; increases the abundance of Eubacterium rectale, Coprococcus eutactus, Akkermansia muciniphila, or Butyricicoccus; and increases Escherichia/Shigella in the gut, all as markers of improved health. By emphasizing the combined importance of diet, microbial supplements, and the gut microbiome, we hope that future studies will better align these components, not only to improve outcomes, but also to enhance our understanding of the mechanisms.

Previous research suggests that effective lifestyle interventions for solid organ transplant (SOT) recipients must be tailored to address the unique life circumstances of this population. As few studies have investigated this design consideration, this study aimed to explore the perspectives and experiences of SOT recipients after completing a Group Lifestyle Balance™ [GLB]-based intervention incorporating either (a) standard population-based nutrition guidance or (b) nutrigenomics-based nutrition guidance. All active participants in the Nutrigenomics, Overweight/Obesity, and Weight Management-Transplant (NOW-Tx) pilot study were invited to participate. Data were collected through focus groups and individual interviews. Ninety-five percent (n = 18) of the NOW-Tx pilot study participants enrolled in the current study: 15 participated in 3 focus groups; 3 were interviewed individually. Three themes were common to both intervention groups: (1) the post-transplant experience; (2) beneficial program components; (3) suggestions for improvement. A unique theme was identified for the nutrigenomics-based intervention, comprising the sub-themes of intervention-specific advantages, challenges, and problem-solving. The readily available and adaptable GLB curriculum demonstrated both feasibility and acceptability and was aligned with participants’ needs and existing health self-management skills. The addition of nutrigenomics-based guidance to the GLB curriculum may enhance motivation for behaviour change in this patient population.

Background: Several studies demonstrate that the provision of personalized lifestyle advice, based on genetics, can help motivate individuals to engage in greater nutrition and physical activity changes compared to the provision of population-based advice. The theoretical mechanism behind this phenomenon is poorly understood. The objective of this study was to determine the impact of providing genetically tailored and population-based lifestyle advice on key constructs of the Theory of Planned Behaviour (TPB). Materials and Methods: A pragmatic, cluster randomized controlled trial (n = 140) took place at the East Elgin Family Health Team, in Aylmer, Ontario, Canada. Participants were primarily Caucasian females enrolled in a weight management program (BMI ≥ 25.0 kg/m²). Weight management program groups were randomized (1:1) to receive a population-based lifestyle intervention for weight management (Group Lifestyle Balance™ (GLB)) or a lifestyle genomics (LGx)-based lifestyle intervention for weight management (GLB+LGx). Attitudes, subjective norms and perceived behavioural control were measured at baseline, immediately after receiving a report of population-based or genetic-based recommendations and after 3-, 6- and 12-month follow-ups. Linear mixed models were conducted, controlling for measures of actual behavioural control. All analyses were intention-to-treat by originally assigned groups. Results: Significant changes (p < 0.05) in attitudes, subjective norms, and perceived behavioural control tended to be short-term in the GLB group and long-term for the GLB+LGx group. Short-term and long-term between-group differences in measures of subjective norms were discovered, favouring the GLB+LGx group. Conclusions: The TPB can help provide a theoretical explanation for studies demonstrating enhanced behaviour change with genetic-based lifestyle interventions. Clinical Trial Registration: NCT03015012.