Carbohydrate derivatives reduced metabolic requirements

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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