Medium chain acyl dehydrogenase deficiency (MCADD) is a rare inherited disorder where your body cannot metabolise (break down) fat properly. 

MCADD can cause drowsiness, lack of energy and diarrhoea, and can put you at risk of complications such as seizures, breathing difficulties and even coma or sudden death (see Symptoms for more information).

Symptoms usually appear during infancy or early childhood, but in rare cases they may not appear until adulthood.

What happens?

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. At each step, energy is released.

People with MCADD cannot break down fatty acids properly, because they lack one of the enzymes needed. This leads to a build-up of medium-chain fats, which can become toxic (see Causes for more information). Because they cannot get energy from fat, they must rely on glucose for their energy.

How common is it?

Around one in 10,000 babies born in the UK has MCADD. Newborn screening throughout England identifies around 60 cases each year.

Around one in 80 healthy people is a carrier of MCADD, but will not have any symptoms. However, if both parents are MCADD carriers, there is a one-in-four chance of their child being born with MCADD.

Outlook

Once diagnosed, MCADD is usually quite straightforward to manage and children can lead healthy, normal lives.

It is managed by eating regular meals and having frequent glucose drinks if the child falls ill, so that their blood glucose level does not fall (see Treatment for more information).

However, if MCADD is not identified and treated early, children may fall into repeated comas with complications including breathing problems, seizures, brain damage or sudden death.

Those affected by medium chain acyl dehydrogenase deficiency (MCADD) usually do not show any symptoms until they are between three months and three years old, usually after a period of fasting, infection or stress.

Symptoms vary, but may include:

• drowsiness
• lack of energy
• diarrhoea
• vomiting
• low blood sugar level

Complications

In rare cases, the child may experience serious complications, such as:

• seizures
• breathing difficulties
• liver problems
• brain damage
• coma
• sudden death

Medium chain acyl dehydrogenase deficiency (MCADD) occurs when an enzyme called medium chain acyl-CoA dehydrogenase (MCAD) is either missing or does not work properly because of a fault in the gene that provides instructions for making this enzyme. This means that your body cannot break down stored fat to release energy.

Fats are normally broken down into fatty acids, which are then broken down into shorter and shorter lengths, with each step producing energy. If you have MCADD, there is a build-up of medium-length chains due to the lack of the MCAD enzyme.

If you need to break down fats quickly (for example, when you have not eaten for a while or when you have an infection), this build-up of medium-chain fats can become toxic, leading to serious symptoms and even death. 

If you have MCADD, you must rely on glucose (a type of sugar in the blood) to produce energy. If you are not eating properly or are ill, your glucose may be all used up and your blood sugar levels may drop to a dangerously low level, with a build-up of toxins in your blood.

Medium chain acyl dehydrogenase deficiency (MCADD) is diagnosed by a screening test (blood test) that is offered to all newborn babies.

During the screening test, the midwife takes some blood from your baby's heel. The amount of partially broken-down fat, called octanoylcarnitine (c8), is measured in the blood sample.

If the test result suggests that your baby may have MCADD, he or she will need further tests to confirm the diagnosis. You will be given an appointment to see a specialist or a dietitian.

Next appointment

The specialist or dietitian will:

• discuss the screening test result with you
• arrange for your baby to have a blood test and a urine test
• give you more advice about feeding your baby until the results of these further tests are known
• support you now and in the future if your baby has MCADD
• give you advice on what to do if your baby is not feeding well for any reason
• let your family doctor know about your baby's tests and MCADD
• give you written information about MCADD to share with your family and local hospital

Medium chain acyl dehydrogenase deficiency (MCADD) is treated by diet. When the child is well, there is no specific dietary management apart from avoiding long periods without food. During illness, they will need to drink glucose drinks very frequently to keep their blood sugar levels up. Eating regularly is more important than ever when they are unwell.

• Babies and young children with MCADD need to eat every three to six hours, including at night.
• Teenagers and adults may be able to go without food for longer periods of up to 12 hours.

Dietary treatment is usually lifelong.

Children should always receive prompt medical attention. If they are unable to eat or drink normally, take them to hospital.