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Overview of Hereditary Metabolic Disorders

by Lee M. Sanders, MD, MPH

Most of the foods and drinks people ingest are complex materials that the body must break down into simpler substances. This process may involve several steps. The simpler substances are then used as building blocks, which are assembled into the materials the body needs to sustain life. The process of creating these materials may also require several steps. The major building blocks are

  • Carbohydrates

  • Amino acids

  • Fats (lipids)

This complicated process of breaking down and converting the substances ingested is called metabolism.

Metabolism is carried out by chemical substances called enzymes, which are made by the body. If a genetic abnormality affects the function of an enzyme or causes it to be deficient or missing altogether, various disorders can occur. The disorders usually result from an inability to break down some substance that should be broken down, allowing some intermediate substance that is often toxic to build up, or from an inability to produce some essential substance. Metabolic disorders are classified by the particular building block that is affected.

Some hereditary disorders of metabolism (such as phenylketonuria and lipidoses) can be diagnosed in the fetus by using amniocentesis or chorionic villus sampling (see see Procedures : Chorionic Villus Sampling). Usually, a hereditary metabolic disorder is diagnosed by using a blood test or examination of a tissue sample to determine whether a specific enzyme is deficient or missing.

Many of these disorders are now detected by routine screening tests done at birth. For a complete list of routine newborn screening tests by state, see the National Newborn Screening and Genetics Resource Center web site ( http://genes-r-us.uthscsa.edu/ ).