Medium-Chain Acyl-CoA Dehydrogenase Deficiency (MCAD)

Fact: About one in every 15,000 babies in the United States is born with MCAD

What is MCAD? 
Medium chain acyl-CoA dehydrogenase (MCAD) deficiency is a rare genetic metabolic disorder characterized by a deficiency of the enzyme medium chain acyl-CoA dehydrogenase. This enzyme is found to be most active in the liver, certain white blood cells (leukocytes), and certain connective tissue cells (fibroblasts) and is necessary for the breakdown (oxidation) of certain fats (medium chain fatty acids). Failure to break down these fats can lead to the abnormal accumulation of fatty acids in the liver and the brain. Abnormally low levels of the MCAD enzyme may also hamper or interrupt other processes associated with the metabolism of fatty acids.

In infants with MCAD deficiency, symptoms may include recurrent episodes of unusually low levels of a certain sugar (glucose) in the blood (hypoglycemia), lack of energy (lethargy), vomiting, and/or liver malfunction. These symptoms are most frequently triggered when an affected infant does not eat for an extended period of time (fasting). In some cases, a viral illness (e.g., upper respiratory infection) that limits food intake may cause the symptoms to occur. MCAD deficiency is the most common disease in a group of disorders that involve abnormalities of fatty acid metabolism (fatty acid oxidation disorders [FODs]). MCAD deficiency is inherited as an autosomal recessive trait.

MCAD is also sometimes called:

  • MCADD deficiency
  • ACADM deficiency
  • MCADH deficiency

MCAD is inherited in an autosomal recessive manner. It affects both boys and girls equally. 

Everyone has a pair of genes that make the MCAD enzyme. In children with MCAD, neither of these genes works correctly. These children inherit one non-working gene for the condition from each parent.

Parents of children with MCAD rarely have the disorder. Instead, each parent has a single non-working gene for MCAD. They are called carriers. Carriers do not have MCAD because the other gene of this pair is working correctly.  When both parents are carriers, there is a 25% chance in each pregnancy for the child to have MCAD. There is a 50% chance for the child to be a carrier, just like the parents. And, there is a 25% chance for the child to have two working genes.

The first signs of medium-chain acyl-CoA dehydrogenase deficiency (MCAD) usually begin during infancy or early childhood (three months to three years of age). You might see signs such as: 

  • Sleeping longer or more often
  • Tiredness
  • Behavior changes
  • Irritable mood
  • Poor appetite
  • Fever
  • Diarrhea
  • Low blood sugar (referred to as hypoglycemia)
  • Vomiting

Many of these signs my occure when your baby eats foods that his or her body cannot break down. They can be triggered by long periods of time without eating, illnesses and infections.

If your baby shows any of these signs, be sure to contact your baby's doctor immedietely. 

Follow-Up Testing
Your baby’s doctor may ask you if your baby is showing any of the signs of MCAD. If your baby has certain signs, your baby’s doctor may suggest starting immediate treatment.

If your baby’s newborn screening result for medium-chain acyl-CoA dehydrogenase deficiency (MCAD) was out of the normal range, your baby’s doctor or the state screening program will contact you to arrange for your baby to have additional testing. It is important to remember that an out-of-range screening result does not necessarily mean that your child has the condition. An out-of-range result may occur because the initial blood sample was too small or the test was performed too early. However, as a few babies do have the condition, it is very important that you go to your follow-up appointment for a confirmatory test. Because the harmful effects of untreated MCAD can occur soon after birth, follow-up testing must be completed as soon as possible to determine whether or not your baby has the condition.

Follow-up testing will involve checking your baby’s urine and blood samples for harmful levels of acids and toxins. Certain acids and toxins build up in the body when a child has a fatty acid oxidation condition, so measuring the levels of these substances in your baby’s body can help doctors determine if your baby has a condition. A specific pattern of acylcarnitines in the blood and other organic acids in the urine might indicate that your baby has MCAD. Sometimes follow-up testing may also include testing a very small sample of skin 

Treatment for MCAD
Your baby’s primary doctor will work with a metabolic doctor to care for your child. Your doctor may also suggest that you meet with a dietician familiar with MCAD.  

Certain treatments may be advised for some children but not others. When necessary, treatment is usually needed throughout life. The following are treatments often recommended for children with MCAD: 

1.  Avoid going a long time without food.  
Infants and young children with MCAD need to eat frequently to prevent hypoglycemia or a metabolic crisis. Your metabolic doctor will tell you how often your child needs to be fed. In general, it is often suggested that infants be fed every four to six hours. Some babies need to eat even more frequently than this. It is important that infants be fed during the night. They may need to be woken up to eat if they do not wake up on their own. Your metabolic doctor and dietician will give you an appropriate feeding plan for your infant. Your doctor will also give you a ‘sick day’ plan, tailored to your child’s needs, for you to follow during illnesses or other times when your child will not eat.

Your metabolic doctor will continue to advise you on how often your child should eat as he or she gets older.  When they are well, many teens and adults with MCAD can go without food for up to 12 hours without problems. Most children do not have metabolic crises past the age of ten.  However, some may need to continue treatment throughout life. 

2.  Diet
Sometimes a low fat, high carbohydrate
 food plan is recommended. Carbohydrates give the body many types of sugar that can be used as energy. In fact, for children needing this treatment, most food in the diet should be carbohydrates (bread, pasta, fruit, vegetables, etc.) and protein (lean meat and low-fat dairy foods). Your dietician can create a food plan with the correct type and amount of fat your child needs.  Any diet changes should be made under the guidance of an experienced dietician. 
Ask your doctor whether or not your child needs to have any changes in his or her diet. 

3.  L-carnitine
Some children may be helped by taking L-camitine. This is a safe and natural substance that helps body cells make energy. It also helps the body get rid of harmful wastes. 

With prompt and careful treatment, children with MCAD usually live healthy lives with typical growth and development. The goal of treatment is to prevent long-term problems. However, children who have repeated metabolic crises may have life-long learning disabilities, spasticity, chronic muscle weakness or other effects.

Want to learn more?

The following are organizations and/or websites dedicated to providing information and education surrounding MCAD. These organizations are dedicated to research, education, awareness, and/or support. They are listed in Alphabetical order without any preference or prejudice. Listing these organizations is not a recommendation or referral in any regard for seeking treatment or consultation or support for treatment.

Baby's First Test
National Center for Biotechnology Information
National Organization for Rare Diseases
​STAR-G (Screening, Technology & Research in Genetics)
​U. S. Library of Medicine

Information on this page taken directly from​:
Baby's First Test
​National Organization for Rare Diseases
​STAR-G (Screening, Technology & Research in Genetics)