Phenylketonuria protein intake

Recommended protein intake from elemental-protein medical food is greater than the RDA and necessary to support growth in individuals with phenylketonuria... 

Acosta PB, Yamricelli S. Protein intake affects phenylalanine requirements and growth of infants with phenylketonuria. Acta Paediatr Suppl. 1994 407 66-7. 

Regulation of the dietary intake of amino acids can also be important when considering the treatment of certain defects in amino add biosynthesis. Phenylalanine is an essential amino acid that is also used to generate the nonessential amino add tyrosine. The enzyme that carries out this readion is the mixed function oxidase phenylalanine hydroxylase (PAH). Inherited deficiencies in PAH are associated with a condition known as phenylketonuria (PKU see Case 38). The absence of PAH results in elevations of phenylalanine and various phenylketones, the accumulation of which is associated with the neurologic defects seen in this disorder. PKU can be treated by controlling the dietary intake of phenylalanine. Diets low in phenylalanine will help prevent excessive elevations in phenylalanine. Phenylalanine can not be completely eliminated from the diet because it is an essential amino acid needed for protein synthesis. In the absence of PAH activity, tyrosine becomes an essential amino add because it cannot be generated from phenylalanine. 

Hoeksma M et al. The intake of total protein, natural protein and protein substitute and growth of height and head circumference in Dutch infants with phenylketonuria. J Inherit Metab Dis. 2005 28(6) 845-54. 

Phenylalanine (P) is an essential AA that participates in protein synthesis. It is converted to tyrosine via hydroxylation (see Section 15.6.7). Phenylalanine is both glncogenic and ketogenic." Phenylketonuria (PKU) is a rare disease (generally diagnosed at birth) caused by an inborn error in the abihty to metabolize P (lacking the enzyme phenylalanine hydroxylase). In affected people, if the diet is not controlled by severe restriction of P intake, PKU can lead to serious irreversible neurological disorders, such as mental retardation. 

The most widely known metabolic disorders are those which result in impairment of the intermediary metabolism of nutrients such as proteins, carbohydrates and lipids. For example, phenylketonuria is due to a genetic deficiency of phenylalanine hydroxylase, an enzyme involved in the conversion of phenylalanine to tyrosine. As a result, when ingested in amounts normally encountered in the diet, phenylalanine accumulates in blood and cerebrospinal fluid along with its pyruvate, lactate and acetate derivatives. (See review by McBean and Stephenson. ) The toxic response takes the form of severe mental retardation, neural and dermal lesions and premature death. But phenylalanine is an essential dietary amino acid and cannot be rigorously excluded from the diet, even of sufferers from phenylketonuria, though fortunately they do respond to reduced dietary intakes. Clearly, phenylalanine hydroxylase deficiency narrows the gap between the required intake and that which elicits a toxic response because this pathway is more readily overloaded . 

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