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Urea Cycle Disorders (UCDs)

How does the urea cycle work?

The urea cycle is the part of the process in the body where food turns to energy and waste is eliminated.

When you eat foods with protein in them – such as meat, fish, eggs, dairy products, nuts, beans, or soy – your body breaks the protein down. It turns the protein into amino acids. Those amino acids are used to build muscle, skin, blood cells, and more. This is what helps you grow and remain healthy.

One of the waste products of protein metabolism is ammonia. Too much ammonia can be toxic and harmful to the body. Enzymes in the urea cycle convert this toxic ammonia into a nontoxic substance called urea, which can be eliminated from the body.1, 2

What are Urea Cycle Disorders?

Urea Cycle Disorders or UCDs are inherited metabolic diseases caused by a deficiency of one of the enzymes or transporters that constitute the urea cycle. The urea cycle involves a series of biochemical steps in which ammonia, a potent neurotoxin, is converted to urea, which is excreted in the urine.1,2 UCD patients may experience episodes where they get symptoms from the ammonia in their blood being excessively high – called hyperammonemic crises – which may result in irreversible brain damage, coma or death.3 UCD symptoms may first occur at any age depending on the severity of the disorder, with more severe defects presenting earlier in life.3


  1. Gropman, AL, et al. Neurological implications of urea cycle disorders. J Inherit Metab Dis 2007;30:865–869
  2. Lanpher, BC, et al. Urea Cycle Disorders Overview. GeneReviews [Internet] 2011
  3. Häberle, J, et al. Suggested guidelines for the diagnosis and management of urea cycle disorders. Orphanet Journal of Rare Diseases 2012;7:32

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