About Metabolic Diseases

Inborn errors of metabolism are a group of genetic disorders in which the body cannot break down (metabolize) parts of foods such as protein, carbohydrates, fat, or vitamins. These disorders are usually caused by defects in specific proteins (enzymes), resulting in a build-up that can cause a wide array of symptoms. There are a large number of inborn errors of metabolism.

What is an Inborn Error of Protein Metabolism?

To understand protein-related metabolic disorders, it is helpful to learn about dietary proteins. All proteins are composed of a specific chain of amino acids, linked together like pearls in a necklace. There are twenty different amino acids that can be arranged in countless orders; much like the letters of the alphabet can be arranged in varying combinations to make words. A single protein can be made of thousands of amino acids, the order of which is specific to that type of protein. Just as individual words do not contain every letter of the alphabet, single proteins do not necessarily contain all twenty amino acids.

Understanding protein-related metabolic disorders also requires an awareness of the purpose of enzymes in the body. Enzymes are substances produced inside the body that complete very specific functions. There are many different kinds of enzymes, and each is responsible for only one or a few related functions. For each amino acid, there is a specific enzyme assigned to aid in its breakdown. Once amino acids are broken down, they can be converted into other substances that the body utilizes in order to work properly.

The amount of each enzyme in the body is controlled by our genes. When someone is born with a genetic inability to produce a certain enzyme, the amino acid on which that enzyme should work cannot be broken down. While a patient with an inborn error of protein metabolism may be able to break down most amino acids, he or she may be lacking the enzyme to break down one or more specific amino acids. This can lead to a dangerous build-up of certain amino acids, which can be toxic. Unfortunately, in most cases there is no cure for the lack of a specific enzyme. Instead, the condition must be managed with a proper low protein diet and special nutritional products.

Types of diseases that cause people to require a low protein diet

Inborn errors of metabolism that may require protein-restricted diets include, but are not limited to, disorders of amino acid metabolism, organic acidemias, and urea cycle defects. A partial list of these disorders is included below:

Amino Acid Disorders and Related Disorders

Organic Acidemias 

Urea Cycle Defects


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