Medium-chain acyl-CoA dehydrogenase deficiency (MCADD) is a rare genetic condition where a person has problems breaking down fatty acids for energy.

Symptoms can occur:

  • After not eating (fasting) for a long period. In babies, symptoms can be triggered after they have stopped receiving regular nightly feeds or shortly after birth (when their feeding is interrupted).
  • During or after an infection, such as the common cold (being ill increases the body’s demand for energy and can also cause loss of appetite).

What are the symptoms of MCADD?

Symptoms of MCADD include:

  • your baby or child appearing unusually tired, sluggish and lazy
    being sick
  • excessive sweating
  • rapid breathing
    swelling of their liver, which may be noticeable by touch
  • seizures (fits)

Older children and adults may appear unusually nervous and excitable.

How serious is it?

Left untreated, MCADD can lead to more serious complications  such as coma, brain damage and liver failure  and can sometimes be fatal.

You will be given an emergency contact number to call immediately should your baby or child start experiencing the symptoms of MCADD.

If this information is not easily available, call 999 for an ambulance or go to your nearest accident and emergency (A&E) department and tell them your child is experiencing symptoms of MCADD.

What happens in MCADD?

People with MCADD are born with a faulty or missing enzyme that the body normally uses to break down fatty acids (an enzyme is a protein that can cause a chemical reaction inside cells).

During long periods between eating, the body breaks down its fat stores to produce energy. Fats are broken down into fatty acids, which are then broken down into shorter and shorter lengths (by breaking the chains holding them together). At each step, energy is released.

When people with MCADD go for a long period of time without eating, two harmful effects occur:

  • there is a build-up of medium-chain fats inside the body, which can have a toxic (poisonous) effect
  • the body then uses its glucose resources as an alternative energy source; this can lead to a sudden and severe drop in blood sugar levels (hypoglycaemia)

The faulty or missing enzyme is the result of genetic mutations a child inherits from their parents (the parents themselves are usually unaffected by MCADD).

A genetic mutation is when the instructions found in all living cells (DNA) becomes scrambled so that one or more of the body’s processes does not work properly.

Read more about the causes of medium-chain acyl-CoA dehydrogenase deficiency.

How is MCADD treated?

There is no cure for MCADD. Treatment involves making sure you don't go for long periods without eating.

The maximum "safe fasting time" for children who are not ill can vary, depending on their age.

Speak to your doctor for advice on the maximum time your child should fast for. Children who are ill need to be fed more frequently.

If your child experiences any symptoms of MCADD, there are many high-glucose drinks that can be used to boost blood sugar levels. If your child is not drinking, or does not improve quickly, hospital treated will be needed.

Read more about the treatment of medium-chain acyl-CoA dehydrogenase deficiency.

Screening for MCADD

In the past, babies with MCADD went undiagnosed until they experienced symptoms – often known as an acute attack or metabolic crisis.

This could be life-threatening, as the parents usually had no idea what was happening or how to treat the symptoms.

Because of this, the NHS introduced a screening programme for MCADD in 2009. All newborn babies in England are given a blood test that can be used to screen for MCADD.

Read more about diagnosing medium-chain acyl-CoA dehydrogenase deficiency.

Who is affected

MCADD is a rare condition. It is estimated that around 1 in 8,000 babies is born with the condition. Both sexes are equally affected.


The outlook for MCADD is usually very good, as the condition is relatively straightforward to manage. Providing that the condition is diagnosed and that dietary recommendations are met, there is no reason why a child with MCADD cannot live a normal healthy life.

MCADD is caused by an inherited faulty or missing enzyme that the body usually uses to break down fatty acids  

Support for children with metabolic diseases

Climb Group (Children Living with Inherited Metabolic Diseases) is a great source of advice for parents who have children with MCADD.

Page last reviewed: 06/05/2014

Next review due: 06/05/2016