CLINICAL
THE USE OF GLYCOMACROPEPTIDE AS AN ALTERNATIVE PROTEIN SOURCE FOR PKU Paula Hallam Registered Dietitian Paula is a Clinical Dietitian at Great Ormond Street Hospital for Children in the Metabolic Team, working predominantly with children with PKU and their families. She is also a Freelance Paediatric Dietitian, director of ‘Tiny Tots Nutrition Ltd’ and mum to two girls.
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Over the past decade, there has been a growing interest in the use of glycomacropeptide (GMP) as an alternative protein source for Phenylketonuria (PKU), as it is naturally low in phenylalanine (Phe) and rich in other amino acids that are potentially beneficial.1,2,3 However, amino acid based protein substitutes have been used successfully in the treatment of PKU,4,5 so is there any need to change? A recent survey shows that patients are keen for new treatments to be developed.6 PKU is an inborn error of amino acid (AA) metabolism caused by deficient activity in the phenylalanine hydroxylase enzyme, which is needed to convert the essential AA phenylalanine (Phe) to tyrosine.1,7,8 The resulting elevated plasma Phe concentrations adversely affect the developing central nervous system, which causes profound neurological impairment and mental retardation.1 Lifelong treatment with a low phenylalanine diet results in reversal of this devastating phenotype9,10 and a ‘diet for life’ approach is recommended for all patients with PKU.11 The PKU diet consists of three main parts: 1. Restriction of natural protein to limit the amount of phenylalanine ingested to essential amounts for growth and tissue repair. The amount of natural protein allowed depends on the phenylalanine tolerance of each individual child, which is determined by the residual enzyme activity. 2. Phenylalanine-free protein substitute to provide all other essential and non-essential amino acids, as well as vitamins, minerals and trace elements. 3. Foods naturally low in phenylalanine (mainly fruits and some vegetables) and low protein foods available on prescription, to provide calories and variety in the diet. AMINO ACID BASED PROTEIN SUBSTITUTES
In the UK, it is standard practice for all children with PKU to be prescribed an
amino acid based protein substitute to provide all other essential and nonessential amino acids, apart from phenylalanine.7 These protein substitutes are made from synthetic, individual amino acids and are presented as powders, liquids, pastes/gels or tablets. The specialist metabolic dietitian calculates the amount of a particular protein substitute required by a child, depending on their age, weight, metabolic control and phenylalanine tolerance.7 What is glycomacropeptide (GMP)? GMP is a glycophosphopeptide comprised of 64 amino acids (AA) whose unique AA profile includes an absence of aromatic amino acids phenylalanine, tryptophan and tyrosine and higher concentrations of isoleucine and threonine, than those found in other dietary proteins.2 Commercial GMP occurs as a byproduct of cheese production and contains 2.5-5mg phenylalanine per gram of protein.3,12 If GMP is to be used as a primary source of protein in PKU, it must be supplemented with arginine, leucine, histidine, tryptophan and tyrosine.2,3 Adherence issues with protein substitutes Historically, patient compliance with protein substitutes has been poor, mainly due to palatability issues, often described as having a bitter taste and strong odour.13,14 MacDonald et al15 reported that only 38% of young www.NHDmag.com April 2017 - Issue 123
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