Protein substitutes for PKU: what's new?

Transitioning of protein substitutes in patients with phenylketonuria: evaluation of current practice

Background

In children with phenylketonuria (PKU), transitioning protein substitutes at the appropriate developmental age is essential to help with their long-term acceptance and ease of administration. We assessed the parental experiences in transitioning from a second stage to third stage liquid or powdered protein substitute in patients with PKU.

Results

Sixteen interviews (23 open-ended questions) were carried out with parents/caregivers of children with PKU (8 females, 50%) with a median age of 8 years (range 5–11 years), continuously treated with diet, and on a third stage protein substitute. Parents/caregivers identified common facilitators and barriers during the third stage protein substitute transition process. The main facilitators were: child and parent motivation, parent knowledge of the transition process, a role model with PKU, low volume and easy preparation of the third stage protein substitute (liquid/powder), anticipation of increasing child independence, lower parent workload, attractive packaging, better taste and smell, school and teacher support, dietetic plans and guidance, PKU social events, child educational materials and written resources. The main barriers were child aversion to new protein substitutes, poor child behaviour, child aged > 5 years, parental fear of change, the necessity for  parental time and persistence, loss of parental control, high product volume, different taste, smell, and texture of new protein substitutes, and peer bullying.

Conclusion

A stepwise, supportive approach is necessary when transitioning from second to third stage protein substitutes in PKU. Future studies are needed to develop guidance to assist parents/caregivers, health professionals, and teachers during the transition process.

Tyrosine metabolism in health and disease: slow-release amino acids therapy improves tyrosine homeostasis in phenylketonuria

OBJECTIVES

Phenylalanine (Phe) hydroxylase (PAH) deficiency leads to hyperphenylalaninemia (HPA) and tyrosine (Tyr) depletion. We investigated Tyr homeostasis in patients with PAH deficiency and the effect of a slow-release amino acids therapy in phenylketonuria (PKU).

METHODS

We performed four complementary investigations: (1) Tyr concentrations were monitored in 114 patients (10.6 ± 11.9 years) with PKU on dietary treatment supplemented with traditional amino acid formulations (n=52, 1175 samples) or non-PKU HPA on a free diet (n=62, 430 samples); (2) Tyr metabolism in PKU was quantitatively evaluated in three patients by a simple Tyr oral loading test (100 mg/kg); (3) diurnal and (4) long-term Tyr concentrations were evaluated in 5 and 13 patients with PKU, respectively, who switched from traditional to slow-release amino acids therapy.

RESULTS

1) Tyr concentrations in the PKU population were subnormal and significantly lower than in non-PKU HPA (p<0.01); (2) the response to a Tyr loading test in PKU was normal, with basal Tyr concentrations reached within 12 h; (3) the diurnal metabolic profile in patients on slow-release amino acids therapy revealed higher morning fasting and nocturnal Tyr concentrations with respect to traditional therapy (p<0.01); (4) this picture was confirmed at follow-up, with normalization of morning fasting Tyr concentrations in patients on slow-release amino acids therapy (p<0.01) and unchanged Phe control (p=0.19).

CONCLUSIONS

Slow-release amino acids therapy can improve Tyr homeostasis in PKU. If associated to optimized Phe control, such a metabolic goal may allow long-term clinical benefits in patients with PKU.

Preliminary Investigation to Review If a Glycomacropeptide Compared to L-Amino Acid Protein Substitute Alters the Pre- and Postprandial Amino Acid Profile in Children with Phenylketonuria

In Phenylketonuria (PKU), the peptide structure of the protein substitute (PS), casein glycomacropeptide (CGMP), is supplemented with amino acids (CGMP-AA). CGMP may slow the rate of amino acid (AA) absorption compared with traditional phenylalanine-free amino acids (Phe-free AA), which may improve nitrogen utilization, decrease urea production, and alter insulin response.

AIM

In children with PKU, to compare pre and postprandial AA concentrations when taking one of three PS's: Phe-free AA, CGMP-AA 1 or 2.

METHODS

43 children (24 boys, 19 girls), median age 9 years (range 5-16 years) were studied; 11 took CGMP-AA1, 18 CGMP-AA2, and 14 Phe-free AA. Early morning fasting pre and 2 h postprandial blood samples were collected for quantitative AA on one occasion. A breakfast with allocated 20 g protein equivalent from PS was given post fasting blood sample.

RESULTS

There was a significant increase in postprandial AA for all individual AAs with all three PS. Postprandial AA histidine (p < 0.001), leucine (p < 0.001), and tyrosine (p < 0.001) were higher in CGMP-AA2 than CGMP-AA1, and leucine (p < 0.001), threonine (p < 0.001), and tyrosine (p = 0.003) higher in GCMP-AA2 than Phe-free AA. This was reflective of the AA composition of the three different PS's.

CONCLUSIONS

In PKU, the AA composition of CGMP-AA influences 2 h postprandial AA composition, suggesting that a PS derived from CGMP-AA may be absorbed similarly to Phe-free AA, but this requires further investigation.

Nutritional and Metabolic Characteristics of UK Adult Phenylketonuria Patients with Varying Dietary Adherence

The nutritional and metabolic characteristics of adult phenylketonuria (PKU) patients in the UK with varying dietary adherence is unknown. In other countries, nutritional and metabolic abnormalities have been reported in nonadherent patients compared to adherent counterparts. A pooled analysis of primary baseline data from two UK multi-centre studies was therefore performed to establish whether this is true from a UK perspective. Adult PKU patients who had provided 3-day food records and amino acid blood samples were included and grouped according to dietary adherence (adherent; n = 16 vs. nonadherent; n = 14). Nonadherent patients consumed greater amounts of natural protein compared to adherent patients (61.6 ± 30.7 vs. 18.3 ± 7.7 g/day; q < 0.001). In contrast, the contribution of protein substitutes to total protein intake was lower in nonadherent compared to adherent patients (3.9 ± 9.2 g/day vs. 58.6 ± 10.2 g/day; q < 0.001). Intakes of iron, zinc, vitamin D₃, magnesium, calcium, selenium, iodine, vitamin C, vitamin A and copper were significantly lower in nonadherent compared to adherent patients and were below UK Reference Nutrient Intakes. Similarly, intakes of thiamin, riboflavin, niacin, vitamin B₆ and phosphorus were significantly lower in nonadherent compared to adherent patients but met the UK Reference Nutrient Intakes. Phenylalanine concentrations in nonadherent patients were significantly higher than adherent patients (861 ± 348 vs. 464 ± 196 µmol/L; q=0.040) and fell outside of European treatment target ranges. This study shows the nutritional and metabolic consequences of deviation from phenylalanine restriction and intake of PKU protein substitutes in nonadherent adult PKU patients. Collectively, these data further underlie the importance of life-long adherence to the PKU diet.

Long-Term Growth in Phenylketonuria: A Systematic Review and Meta-Analysis

There is an ongoing debate regarding the impact of phenylketonuria (PKU) and its treatment on growth. To date, evidence from studies is inconsistent, and data on the whole developmental period is limited. The primary aim of this systematic review was to investigate the effects of a phenylalanine (Phe)-restricted diet on long-term growth in patients with PKU. Four electronic databases were searched for articles published until September 2018. A total of 887 results were found, but only 13 articles met eligibility criteria. Only three studies had an adequate methodology for meta-analysis. Although the results indicate normal growth at birth and during infancy, children with PKU were significantly shorter and had lower weight for age than reference populations during the first four years of life. Impaired linear growth was observed until the end of adolescence in PKU. In contrast, growth impairment was not reported in patients with mild hyperphenylalaninemia, not requiring dietary restriction. Current evidence indicates that even with advances in dietary treatments, “optimal” growth outcomes are not attained in PKU. The majority of studies include children born before 1990s, so further research is needed to show the effects of recent dietary practices on growth in PKU.

Improved Eating Behaviour and Nutrient Intake in Noncompliant Patients with Phenylketonuria after Reintroducing a Protein Substitute: Observations from a Multicentre Study

Noncompliance is widespread in adults with PKU and is associated with adverse metabolic, nutritional and cognitive abnormalities. Returning to the PKU diet is important for this at-risk population, yet for many this is challenging to achieve. Strategies that ease the return to the PKU diet, while offering nutritional and cognitive advantages, are needed. Twelve PKU adults (33.7 ± 2.6 years), who had been noncompliant for 4.5 years (range: 1 to 11 years), took 33 g of a low-volume, nutrient-enriched, protein substitute daily for 28 days. Outcomes of eating behaviour, nutrient intake and mood were assessed at entry (baseline, days 1-3) and after the intervention period (days 29-31). At baseline, intakes of natural protein and estimated phenylalanine were high (66.4 g and 3318.5 mg, respectively) and intakes of calcium, magnesium, iron, zinc, iodine and vitamin D were below country-specific recommendations. With use of the experimental protein substitute, natural protein and estimated phenylalanine intake declined (p = 0.043 for both). Fat and saturated fat intakes also decreased (p = 0.019 and p = 0.041, respectively), while energy and carbohydrate intake remained unchanged. Micronutrient intake increased (p ≤ 0.05 for all aforementioned) to levels well within reference nutrient intake recommendations. Blood vitamin B₁₂ and vitamin D increased by 19.8% and 10.4%, respectively. Reductions in anxiety and confusion were also observed during the course of the study yet should be handled as preliminary data. This study demonstrates that reintroducing a low-volume, nutrient-enriched protein substitute delivers favourable nutritional and possible mood benefits in noncompliant PKU patients, yet longer-term studies are needed to further confirm this. This preliminary knowledge should be used in the design of new strategies to better facilitate patients' return to the PKU diet, with the approach described here as a foundation.

How Does Feeding Development and Progression onto Solid Foods in PKU Compare with Non-PKU Children During Weaning?

Weaning is complex for children with phenylketonuria (PKU). Breastmilk/infant formula and phenylalanine (Phe)-free infant protein-substitute (PS) are gradually replaced with equivalent amounts of Phe-containing food, a semi-solid/spoonable weaning PS and special low-protein foods. In PKU, feeding patterns/practices during weaning in PKU have not been formally evaluated. In this longitudinal, prospective, case-control study (n = 20) infants with PKU transitioning to a second-stage PS, were recruited at weaning (4–6 months) for a comparison of feeding practices and development with non-PKU infants. Subjects were monitored monthly to 12 months and at age 15 months, 18 months and 24 months for: feeding progression; food textures; motor skill development and self-feeding; feeding environment; gastrointestinal symptoms; and negative feeding behaviours. Children with PKU had comparable weaning progression to non-PKU infants including texture acceptance, infant formula volume and self-feeding skills. However, children with PKU had more prolonged Phe-free infant formula bottle-feeding and parental spoon feeding than controls; fewer meals/snacks per day; and experienced more flatulence (p = 0.0005), burping (p = 0.001), retching (p = 0.03); and less regurgitation (p = 0.003). Negative behaviours associated with PS at age 10–18 months, coincided with the age of teething. Use of semi-solid PS in PKU supports normal weaning development/progression but parents require support to manage the complexity of feeding and to normalise the social inclusivity of their child’s family food environment. Further study regarding parental anxiety associated with mealtimes is required.

Fifteen years of using a second stage protein substitute for weaning in phenylketonuria: a retrospective study

BACKGROUND

In phenylketonuria (PKU), during weaning, it is necessary to introduce a second stage phenylalanine (Phe)-free protein substitute (PS) to help meet non-Phe protein requirements. Semi-solid weaning Phe-free PS have been available for >15 years, although no long-term studies have reported their efficacy.

METHODS

Retrospective data from 31 children with PKU who commenced a weaning PS were collected from clinical records from age of weaning to 2 years, on: gender; birth order; weaning age; anthropometry; blood Phe levels; age commenced and dosage of weaning PS and Phe-free infant L-amino acid formula; natural protein intake; and issues with administration of PS or food.

RESULTS

Median commencement age for weaning was 17 weeks (range 12-25 weeks) and, for weaning PS, 20 weeks (range 13-37 weeks). Median natural protein was 4 g day^-1 (range 3-11 g day^-1 ) and total protein intake was >2 g kg^-1 day^-1 from weaning to 2 years of age. Children started on 2-4 g day^-1 protein equivalent (5-10 g day^-1 of powder) from weaning PS, increasing by 0.2 g kg^-1 day^-1 (2 g day^-1 ) monthly to 12 months of age. Teething and illness adversely affected the administration of weaning PS and the acceptance of solid foods. Altogether, 32% of children had delayed introduction of more textured foods, associated with birth order (firstborn 80% versus 38%; P = 0.05) and food refusal when teething (80% versus 29%; P = 0.02).

CONCLUSIONS

Timing of introduction of solid foods and weaning PS, progression onto more textured foods and consistent feeding routines were important in aiding their acceptance. Any negative behaviour with weaning PS was mainly associated with food refusal, teething and illness. Parental approach influenced the acceptance of weaning PS.

Longitudinal modelling of the exposure of young UK patients with PKU to acesulfame K and sucralose

Artificial sweeteners are used in protein substitutes intended for the dietary management of inborn errors of metabolism (phenylketonuria, PKU) to improve the variety of medical foods available to patients and ensure dietary adherence to the prescribed course of dietary management. These patients can be exposed to artificial sweeteners from the combination of free and prescribed foods. Young children have a higher risk of exceeding acceptable daily intakes (ADI) for additives than adults, due to higher food intakes per kg body weight. Young patients with PKU aged 1-3 years can be exposed to higher levels of artificial sweeteners from these dual sources than normal healthy children and are at a higher risk of exceeding the ADI. Standard intake assessment methods are not adequate to assess the additive exposure of young patients with PKU. The aim of this study was to estimate the combination effect on the intake of artificial sweeteners and the impact of the introduction of new provisions for an artificial sweetener (sucralose, E955) on exposure of PKU patients using a validated probabilistic model. Food consumption data were derived from the food consumption survey data of healthy young children in the United Kingdom from the National Diet and Nutrition Survey (NDNS, 1992-2012). Specially formulated protein substitutes as foods for special medical purposes (FSMPs) were included in the exposure model to replace restricted foods. Inclusion of these protein substitutes is based on recommendations to ensure adequate protein intake in these patients. Exposure assessment results indicated the availability of sucralose for use in FSMPs for PKU leads to changes in intakes in young patients. These data further support the viability of probabilistic modelling as a means to estimate food additive exposure in patients consuming medical nutrition products.

Probabilistic modelling to assess exposure to three artificial sweeteners of young Irish patients aged 1-3 years with PKU and CMPA

The choice of suitable normal foods is limited for individuals with particular medical conditions, e.g., inborn errors of metabolism (phenylketonuria - PKU) or severe cow's milk protein allergy (CMPA). Patients may have dietary restrictions and exclusive or partial replacement of specific food groups with specially formulated products to meet particular nutrition requirements. Artificial sweeteners are used to improve the appearance and palatability of such food products to avoid food refusal and ensure dietary adherence. Young children have a higher risk of exceeding acceptable daily intakes for additives than adults due to higher food intakes kg^-1 body weight. The Budget Method and EFSA's Food Additives Intake Model (FAIM) are not equipped to assess partial dietary replacement with special formulations as they are built on data from dietary surveys of consumers without special medical requirements impacting the diet. The aim of this study was to explore dietary exposure modelling as a means of estimating the intake of artificial sweeteners by young PKU and CMPA patients aged 1-3 years. An adapted validated probabilistic model (FACET) was used to assess patients' exposure to artificial sweeteners. Food consumption data were derived from the food consumption survey data of healthy young children in Ireland from the National Preschool and Nutrition Survey (NPNS, 2010-11). Specially formulated foods for special medical purposes were included in the exposure model to replace restricted foods. Inclusion was based on recommendations for adequate protein intake and dietary adherence data. Exposure assessment results indicated that young children with PKU and CMPA have higher relative average intakes of artificial sweeteners than healthy young children. The reliability and robustness of the model in the estimation of patient additive exposures was further investigated and provides the first exposure estimates for these special populations.

The Use of Glycomacropeptide in Dietary Management of Phenylketonuria

Dietary therapy is the most common therapy applied in treatment of Phenylketonuria (PKU) with restriction of intake of most natural proteins that are rich in Phenylalanine (Phe). Recently, it has been claimed that caseinoglycomacropeptide (GMP), derived of whey, may be used to replace the amino acid formulae (AAF). The Aim of Work. To study the feasibility of use of GMP for partial replacement of artificial formula in treatment of children with PKU. Methods. Ten patients with PKU were included in the study. They received the recommended daily allowances of protein in the form of AAF or a combination of AAF and GMP. The percent of intake of GMP in phases 1 and 2 was 50% and zero%, respectively. Results. The median and interquartiles of phenyl alanine Phe levels phase were not significantly different in phases I and II, 376 (167-551) μmol/L versus 490 (289-597) μmol/L, respectively. Phenylalanine/tyrosine ratio, amino acids, and other laboratory data showed no significant difference between the two phases. Conclusion. GMP may be used to replace 50% of the protein intake to improve the nutritive value and palatability of diet and to provide a more satisfactory diet. No toxicity or side effects were reported in patients on that regimen.

New ways of defining protein and energy relationships in inborn errors of metabolism

Dietary restrictions required to manage individuals with inborn errors of metabolism (IEM) are essential for metabolic control, however may result in an increased risk to both short and long-term nutritional status. Dietary factors most likely to influence nutritional status include energy intake, protein quality and quantity, micronutrient intake and the frequency and extent to which the diet must be altered during periods of increased physical or metabolic stress. Patients on the most restrictive diets, including those with intakes consisting of low levels of natural protein or those with recurrent illness or frequent metabolic decompensation carry the most nutritional risk. Due to the difficulties in determining condition specific requirements, dietary intake recommendations and nutritional monitoring tools used in patients with IEM are the same as, or extrapolated from, those used in healthy populations. As a consequence, evidence is lacking for the safest dietary prescriptions required to manage these patients long term, as tolerance to dietary therapy is generally described in terms of metabolic stability rather than long term nutritional and health outcomes. As the most frequent therapeutic dietary manipulation in IEM is alteration in dietary protein, and as protein status is critically dependent on adequate energy provision, the use of a Protein to Energy ratio (P:E ratio) as an additional tool will better define the relationship between these critical components. This could accurately define dietary quality and ensure that not only an adequate, but also a safe and balanced intake is provided.

Differential effects of low-phenylalanine protein sources on brain neurotransmitters and behavior in C57Bl/6-Pah(enu2) mice

Phenylketonuria (PKU) is an inborn error of metabolism caused by a deficiency of the enzyme phenylalanine hydroxylase, which metabolizes phenylalanine (phe) to tyrosine. A low-phe diet plus amino acid (AA) formula is necessary to prevent cognitive impairment; glycomacropeptide (GMP) contains minimal phe and provides a palatable alternative to the AA formula. Our objective was to assess neurotransmitter concentrations in the brain and the behavioral phenotype of PKU mice (Pah(enu2) on the C57Bl/6 background) and how this is affected by low-phe protein sources. Wild type (WT) and PKU mice, both male and female, were fed high-phe casein, low-phe AA, or low-phe GMP diets between 3 and 18 weeks of age. Behavioral phenotype was assessed using the open field and marble burying tests, and brain neurotransmitter concentrations were measured using HPLC with electrochemical detection system. Data were analyzed by 3-way ANOVA with genotype, sex, and diet as the main treatment effects. Brain mass and the concentrations of catecholamines and serotonin were reduced in PKU mice compared to WT mice; the low-phe AA and GMP diets improved these parameters in PKU mice. Relative brain mass was increased in female PKU mice fed the GMP diet compared to the AA diet. PKU mice exhibited hyperactivity and impaired vertical exploration compared to their WT littermates during the open field test. Regardless of genotype or diet, female mice demonstrated increased vertical activity time and increased total ambulatory and horizontal activity counts compared with male mice. PKU mice fed the high-phe casein diet buried significantly fewer marbles than WT control mice fed casein; this was normalized in PKU mice fed the low-phe AA and GMP diets. In summary, C57Bl/6-Pah(enu2) mice showed an impaired behavioral phenotype and reduced brain neurotransmitter concentrations that were improved by the low-phe AA or GMP diets. These data support lifelong adherence to a low-phe diet for PKU.

Low bone strength is a manifestation of phenylketonuria in mice and is attenuated by a glycomacropeptide diet

Purpose

Phenylketonuria (PKU), caused by phenylalanine (phe) hydroxylase loss of function mutations, requires a low-phe diet plus amino acid (AA) formula to prevent cognitive impairment. Glycomacropeptide (GMP), a low-phe whey protein, provides a palatable alternative to AA formula. Skeletal fragility is a poorly understood chronic complication of PKU. We sought to characterize the impact of the PKU genotype and dietary protein source on bone biomechanics.

Procedures

Wild type (WT; Pah^+/+) and PKU (Pah^enu2/enu2) mice on a C57BL/6J background were fed high-phe casein, low-phe AA, and low-phe GMP diets between 3 to 23 weeks of age. Following euthanasia, femur biomechanics were assessed by 3-point bending and femoral diaphyseal structure was determined. Femoral ex vivo bone mineral density (BMD) was assessed by dual-enengy x-ray absorptiometry. Whole bone parameters were used in prinicipal component analysis. Data were analyzed by 3-way ANCOVA with genotype, sex, and diet as the main factors.

Findings

Regardless of diet and sex, PKU femora were more brittle, as manifested by lower post-yield displacement, weaker, as manifested by lower energy and yield and maximal loads, and showed reduced BMD compared with WT femora. Four principal components accounted for 87% of the variance and all differed significantly by genotype. Regardless of genotype and sex, the AA diet reduced femoral cross-sectional area and consequent maximal load compared with the GMP diet.

Conclusions

Skeletal fragility, as reflected in brittle and weak femora, is an inherent feature of PKU. This PKU bone phenotype is attenuated by a GMP diet compared with an AA diet.

Dietary protein counting as an alternative way of maintaining metabolic control in phenylketonuria

OBJECTIVE

To compare a gram protein exchange system (1g=50-mg Phenylalanine) with a unit exchange system (1unit=15-mg Phenylalanine) and its effect on the blood Phenylalanine (Phe) levels and acceptance in the dietary management for children and adolescents with Phenylketonuria.

METHODS

In Phase One, participants were randomised to continue counting Phe unit exchanges (n=8) or changed to counting gram protein exchanges (n=10), using a new diet chart developed in-house. Foods containing less than 20mg Phe per serve were now considered "free." Interim data analysis confirmed no significant deterioration in Phe levels of the study group and the control group was changed to protein counting.In Phase Two, 18 participants were educated to use an updated version of the in-house diet chart - in this version foods containing less than 50mg Phe per serve were considered "free."In both phases, attitudes to PKU and its management were evaluated at baseline and 6months. Phenylalanine and tyrosine levels were measured from filter paper blood spots by tandem mass spectrometry.

RESULTS

Phase One: Phe levels over 6months were comparable to pre-study levels (mean Phe pre 366μmol/L+/- 169, mean Phe post change=388μmol/L+/- 160).Phase Two: Four participants had a significant improvement in blood Phe levels, nine showed no significant change and one participant's levels were significantly higher. There was incomplete data on four participants. All participants preferred the freer diet chart.

CONCLUSION

Protein exchanges (foods containing less than 50mg Phe/serve uncounted) are an alternative method of measuring Phe intake in the dietary management of Phenylketonuria.

Adjusting diet with sapropterin in phenylketonuria: what factors should be considered?

The usual treatment for phenylketonuria (PKU) is a phenylalanine-restricted diet. Following this diet is challenging, and long-term adherence (and hence metabolic control) is commonly poor. Patients with PKU (usually, but not exclusively, with a relatively mild form of the disorder) who are responsive to treatment with pharmacological doses of tetrahydrobiopterin (BH4) have either lower concentrations of blood phenylalanine or improved dietary phenylalanine tolerance. The availability of a registered formulation of BH4 (sapropterin dihydrochloride, Kuvan®) has raised many practical issues and new questions in the dietary management of these patients. Initially, patients and carers must understand clearly the likely benefits (and limitations) of sapropterin therapy. A minority of patients who respond to sapropterin are able to discontinue the phenylalanine-restricted diet completely, while others are able to relax the diet to some extent. Care is required when altering the phenylalanine-restricted diet, as this may have unintended nutritional consequences and must be undertaken with caution. New clinical protocols are required for managing any dietary change while maintaining control of blood phenylalanine, ensuring adequate nutrition and preventing nutritional deficiencies, overweight or obesity. An accurate initial evaluation of pre-sapropterin phenylalanine tolerance is essential, and the desired outcome from treatment with sapropterin (e.g. reduction in blood phenylalanine or relaxation in diet) must also be understood by the patient and carers from the outset. Continuing education and support will be required thereafter, with further adjustment of diet and sapropterin dosage as a young patient grows.

Sapropterin hydrochloride: enzyme enhancement therapy for phenylketonuria

Phenylketonuria (PKU) is an inherited disorder of amino acid metabolism caused by deficiency of the enzyme phenylalanine hydroxylase (PAH). Historically PKU was a common genetic cause of severe learning difficulties and developmental delay, but with the introduction of newborn screening and early dietary management, it has become a treatable disease and people born with PKU should now have IQs and achievements similar to their peers. Dietary treatment, however, involves lifestyle changes that pervade most aspects of daily life for an individual and their family. A simple pharmacological treatment for PKU would have a great appeal. Sapropterin hydrochloride is a synthetic form of tetrahydrobiopterin, the cofactor for PAH. A proportion of mutant PAH enzymes show enhanced activity in the presence of pharmacological doses of sapropterin and, for some patients with milder forms of PKU, sapropterin can effectively lower plasma phenylalanine levels. This article discusses the potential place for sapropterin in the management of PKU and how this expensive orphan drug is being integrated into patient care in different healthcare systems.

Dietary management practices in phenylketonuria across European centres

BACKGROUND

Dietary phenylalanine restriction is the cornerstone of phenylketonuria (PKU) management. However, there are no European consensus guidelines for its optimal dietary care.

METHODS

Detailed information on the routine dietary management of PKU was obtained from 10 European centres using structured questionnaires. Each centre was represented by one dietitian/nutritionist or physician (European Nutritionist Expert Panel).

RESULTS

All centres screened for PKU within the first 10 days of life. PKU prevalence was highest in Turkey. The training, roles and responsibilities of dietitians and nutritionists varied widely; in some centres dietitians were responsible for managing the diet, while in others this was performed by a physician. There were marked differences in target blood phenylalanine concentrations, the dosages of protein substitutes, systems for allocating daily phenylalanine allowance, and the definition of foods that could be eaten without restriction ('free foods'). Eighty percent (n=8/10) of centres encouraged breastfeeding together with protein substitute in infants with PKU.

CONCLUSIONS

Important differences exist among centres across Europe in the dietary management of PKU, and in support systems designed to assist patients in managing their diets. Further studies are needed to compare different dietary treatments with the aim of identifying best practice to optimise phenylalanine control and dietary adherence.

Improved nutritional management of phenylketonuria by using a diet containing glycomacropeptide compared with amino acids

BACKGROUND

Phenylketonuria (PKU) requires a lifelong low-phenylalanine diet that provides the majority of protein from a phenylalanine-free amino acid (AA) formula. Glycomacropeptide (GMP), an intact protein formed during cheese production, contains minimal phenylalanine.

OBJECTIVE

The objective was to investigate the effects of substituting GMP food products for the AA formula on acceptability, safety, plasma AA concentrations, and measures of protein utilization in subjects with PKU.

DESIGN

Eleven subjects participated in an inpatient metabolic study with two 4-d treatments: a current AA diet (AA diet) followed by a diet that replaced the AA formula with GMP (GMP diet) supplemented with limiting AAs. Plasma concentrations of AAs, blood chemistries, and insulin were measured and compared in AA (day 4) and GMP diets (day 8).

RESULTS

The GMP diet was preferred to the AA diet in 10 of 11 subjects with PKU, and there were no adverse reactions to GMP. There was no significant difference in phenylalanine concentration in postprandial plasma with the GMP diet compared with the AA diet. When comparing fasting with postprandial plasma, plasma phenalyalanine concentration increased significantly with the AA but not with the GMP diet. Blood urea nitrogen was significantly lower, which suggests decreased ureagenesis, and plasma insulin was higher with the GMP diet than with the AA diet.

CONCLUSIONS

GMP, when supplemented with limiting AAs, is a safe and highly acceptable alternative to synthetic AAs as the primary protein source in the nutritional management of PKU. As an intact protein source, GMP improves protein retention and phenylalanine utilization compared with AAs.

Nutritional management of PKU with glycomacropeptide from cheese whey

Individuals with phenylketonuria (PKU) must follow a lifelong low-phenylalanine (Phe) diet to prevent neurological impairment. Compliance with the low-Phe diet is often poor owing to restriction in natural foods and the requirement for consumption of a Phe-free amino acid formula or medical food. Glycomacropeptide (GMP), a natural protein produced during cheese-making, is uniquely suited to a low-Phe diet because when isolated from cheese whey it contains minimal Phe (2.5-5 mg Phe/g protein). This paper reviews progress in evaluating the safety, acceptability and efficacy of GMP in the nutritional management of PKU. A variety of foods and beverages can be made with GMP to improve the taste, variety and convenience of the PKU diet. Sensory studies in individuals with PKU demonstrate that GMP foods are acceptable alternatives to amino acid medical foods. Studies in the PKU mouse model demonstrate that GMP supplemented with limiting indispensable amino acids provides a nutritionally adequate source of protein and improves the metabolic phenotype by reducing concentrations of Phe in plasma and brain. A case report in an adult with classical PKU who followed the GMP diet for 10 weeks at home indicates safety, acceptability of GMP food products, a 13-14% reduction in blood Phe levels (p<0.05) and improved distribution of dietary protein throughout the day compared with the amino acid diet. In summary, food products made with GMP that is supplemented with limiting indispensable amino acids provide a palatable alternative source of protein that may improve dietary compliance and metabolic control of PKU.