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Inborn Errors of Metabolism

Inborn errors of metabolism (IEM) are rare genetic disorders which result in the body not being able to properly metabolise food into energy. These disorders are usually caused by defects in the specific enzymes that help to break down (metabolise) parts of food.

Nutrients that are not broken down can build up in the body and this can lead to a variety of symptoms. Several IEM can cause developmental delay if they are not controlled.

IEM are typically categorised into three main types. This depends on the nutrient that is affected. These are:
  • Disorders of amino acid (protein) metabolism
  • Disorders of carbohydrate metabolism
  • Disorders of fat metabolism

These are primarily managed with either lifelong dietary intervention alone or a combination of diet and drug therapy.
Newborn screening programs help to identify infants with IEM shortly after birth. Many, but not all IEM are tested for in newborn screening; this will vary in each country.

An example of a disorder of amino acid (protein) metabolism is Phenylketonuria (PKU). PKU is one of the more common disorders although it is still rare, it occurs in approximately 1 in every 12,000 newborns worldwide.1

The ‘building blocks’ of protein are called amino acids. In PKU, due to the lack of an enzyme known as phenylalanine hydroxylase, the body cannot properly deal with one of the amino acids known as phenylalanine (phe) and phe can then build up to toxic levels in the blood. This means that people with PKU need to severely restrict the amount of protein they eat.

A key element in the dietary management of PKU is the use of a phenylalanine-free protein substitutes. These provide all of the amino acids found in protein, but no phe. In combination with small, measured amounts of dietary protein, protein substitutes will ensure that an individual with PKU has an adequate intake of protein for normal growth and development. The primary role of a protein substitute is to provide the protein (minus phe) that cannot be consumed by eating protein-containing foods. Modern protein substitutes may contain vitamins, minerals and omega fats which are limited within a restricted low protein diet.

Protein substitutes developed for PKU have improved dramatically over the years. Compliance to the strict diet can be difficult; therefore protein substitutes are specifically designed to complement modern living. This is achieved through a combination of easily transported ‘single use’ formats, available in a range of flavours that do not require mixing, weighing or measuring.

In combination, all of the above attributes help make taking the protein substitute easier, therefore encouraging good compliance. Anyone affected by IEM wanting more information about the disorder, management and outcomes, should seek medical advice from their clinician / dietitian.


Walter J, Lachmann RJ, Burgard P Hyperphenylalanineamia. In: Saudubray JM, van den Berghe G, Walter JH (eds) Inborn Metabolic Diseases Diagnosis and Treatment, 5th edn. Berlin: Springer-Verlag, 2012, pp. 252−64.

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