Amino acid metabolism disorders

Your baby gets newborn screening tests before he leaves the hospital after birth. These tests look for rare but serious and mostly treatable health disorders that affect how the body works. 

Your baby’s newborn screening may include testing for certain amino acid metabolism disorders. These are rare health conditions that affect a body’s metabolism. Metabolism is the way your body changes food into the energy it needs to breathe, digest food and grow.

Amino acids help build protein in your body. People with these disorders can’t break down (change) certain amino acids. This can cause harmful substances to build up in the body. For these people, eating foods that are high in protein can cause serious health problems and, sometimes, death. People with these kinds of disorders may need to limit or avoid certain foods because their bodies can’t process them properly. Illness or infection, eating the wrong kinds of foods, or going for a long time without eating can bring about signs and symptoms of each disorder.

These conditions are inherited. This means they’re passed from parent to child through genes. Genes are parts of your body's cells that store instructions for the way your body grows and works. In the United States, all babies have newborn screening tests to see if they may have certain inherited conditions, like amino acid metabolism disorders.

Many babies with these disorders look and seem healthy at birth. Without treatment, these conditions can lead to serious health problems and even death. With early diagnosis and treatment, most babies with these disorders can lead healthy lives. This is why newborn screening right after birth is so important.

The March of Dimes recommends that all babies be screened for these amino acid metabolism disorders:

  • Argininosuccinic acidemia (also called ASA)
  • Citrullinemia (also called CIT)
  • Homocystinuria (also called HCY)
  • Maple syrup urine disease (also called MSUD)
  • Phenylketonuria (also called PKU)Tyrosinemia type I (also called TYR I)

For more information on these disorders, go to

What is argininosuccinic acidemia?

In ASA, the body can’t remove ammonia or a substance called argininosuccinic acid from the blood. Fewer than 1 in 100,000 babies in the United States is born with ASA.

Babies with ASA who don’t get treatment often die within the first few weeks of life. Early treatment can help prevent serious problems, including brain damage, liver problems and intellectual and developmental disabilities. These are problems with how the brain works that can cause a person to have trouble or delays in physical development, learning, communicating, taking care of himself or getting along with others.

Signs and symptoms of ASA in newborns include:

  • Drowsiness
  • Fussiness
  • Little appetite
  • Throwing up

Signs and symptoms of ASA in older babies and young children can be mild or serious. Physical and behavioral signs and symptoms include:

  • Being restless or having trouble being still or quiet
  • Changes in muscle tone
  • Coordination or balance problems
  • Drowsiness
  • Dry hair that breaks easily
  • Learning problems
  • Slow growth
  • Small head
  • Unclear speech

Medical signs and symptoms of ASA in older babies and children include:

  • Breathing problems
  • High levels of ammonia in the bloodIntense headache, especially after a high-protein meal (for example, a large meal of meat, fish or eggs)
  • Little appetite and disliking meat and other high-protein foods
  • Throwing up

What is citrullinemia?

In CIT, the body has trouble breaking down amino acids and getting rid of ammonia in the blood. For babies with CIT, the liver may not work properly to help get rid of ammonia in the body. Fewer than 1 in 100,000 babies in the United States is born with CIT each year.

The form of CIT called classic CIT usually appears in the first few days after birth. If untreated, a baby with classic CIT can die within the first week of life. Health problems caused by CIT can include brain damage, intellectual and developmental disabilities, liver problems and coma. Early treatment can help prevent these problems. Milder forms of CIT may not start until childhood or later.

Signs and symptoms of classic CIT in babies include: 

  • Breathing problems
  • Changes in muscle tone
  • Drowsiness
  • Fussiness
  • Little appetite or trouble eating
  • Loss of consciousness (passing out)
  • Low body temperature 
  • Throwing up
  • Weak muscles

Signs and symptoms of milder forms of CIT in older babies and young children are the same as for those for older babies and children with ASA.

What is homocystinuria?

In HCY, the body can’t break down the amino acid homocysteine. Fewer than 1 in 100,000 babies is born with HCY in the United States each year.

If untreated, HCY can cause problems with the blood, bones, eyes, heart and pancreas. Babies with HCY may look healthy at birth, but symptoms usually appear within the first year of life.

Physical signs and symptoms include:

  • Being tall and thin with long legs and arms, or long, curved fingers
  • Chest deformities (abnormal shape) 
  • Dislocation of the eye lens
  • Failure to thrive (slow weight gain and growth) 
  • Knock knees
  • Pale hair and skin
  • Problems with movement
  • Redness across the cheeks

Behavioral signs and symptoms include:

  • Behavior and emotional problems
  • Intellectual and developmental disabilities

What is maple syrup urine disease?

This disorder gets its name because people with the condition may have urine that smells like maple syrup. In this disorder, the body can’t break down the amino acids leucine, isoleucine and valine. These amino acids are found in foods high in protein, like meat, fish and eggs. Fewer than 1 in 100,000 babies is born with MSUD each year in the United States.

If not treated early, babies who are very sick with MSUD often die within the first month of life. Early treatment can help prevent serious health problems, including brain and spinal cord problems, coma and seizures.  If treated early, most babies with MSUD can live healthy lives.

Babies with MSUD may seem normal at birth but show signs and symptoms when they start eating protein in food. Signs and symptoms include:

  • Drowsiness
  • Fussiness
  • High levels of acids and ammonia in the blood
  • High-pitched cry
  • Little appetite or trouble feeding
  • Throwing up
  • Urine that smells like maple syrup
  • Weight loss

What is phenylketonuria?

In PKU, the body can’t break down the amino acid phenylalanine. Signs and symptoms of PKU do not appear in the first month of life. At least 1 in 25,000 babies in the United States is born with PKU each year.

What is tyrosinemia type I?

In TYR I, the body can’t break down the amino acid tyrosine. Fewer than 1 in 100,000 babies is born with TYR I each year in the United States.

If untreated, TYR 1 can cause liver and kidney problems, seizures, rickets (a disease that weakens bones), coma and death. Signs and symptoms of TYR 1 include:

  • Breathing problems
  • Bruising or bleeding, especially nosebleeds 
  • Diarrhea, bloody stools, throwing up
  • Drowsiness
  • Fast heartbeat
  • Fussiness
  • Jaundice. This is a common condition caused by the build-up of a substance called bilirubin in the blood. It makes a baby’s skin and the white parts of his eyes look yellow.
  • Skin or urine that smells like cabbage
  • Slow weight gain and growth
  • Swollen belly or legs
  • Trouble walking

Last reviewed: January, 2014