Introduction

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, and 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--so that some intermediate substance that is toxic builds 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 the lipidoses) can be diagnosed in the fetus using amniocentesis or chorionic villus sampling (see Section 22, Chapter 256). Usually, the diagnosis of a hereditary disorder of metabolism is made using a blood test or an examination of a tissue sample to determine whether a specific enzyme is deficient or missing.