What Is MCAD Deficiency? A Plain-Language Guide to This Inherited Metabolic Disorder
Updated: 3 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 17, 2026
Quick Answer: Key Facts About MCAD Deficiency
MCAD deficiency is an inherited disorder that prevents the body from breaking down certain fats into energy, risking low blood sugar and metabolic crisis during illness or long gaps without eating. It is caused by an ACADM gene change and found through newborn screening. Frequent meals, limits on fasting and an emergency plan keep it well managed.
Feature | Details |
What it is | An inherited disorder that prevents the body from breaking down certain fats and turning them into energy. [1] |
Main risk | Low blood sugar and a sudden metabolic crisis during illness, high fever, stomach problems or long gaps without eating. [1] |
Cause | A change in the ACADM gene, inherited in an autosomal recessive pattern: both parents are carriers. [1] |
How it is found | |
Treatment | Diet and lifestyle: frequent feedings for infants, regular meals and snacks with complex carbohydrates for everyone else, limits on fasting, and an emergency plan for low blood sugar. [2] |
Outlook | If diagnosed and treated early, the disorder can be well managed through diet and lifestyle. [1] |
What the research says
Without newborn screening, serious harm was common. A systematic review of screened and unscreened cohorts found that, in the absence of newborn screening, premature death or serious disability occurred in an estimated 20% to 25% of children with the disorder. (Grosse et al., Genet Med, 2006)
Screening has improved outcomes. In a nationwide Czech study of 69 people with MCAD deficiency (15 diagnosed before and 54 after screening began in 2009), episodes of acute encephalopathy and the proportion with intellectual disability both fell significantly after screening was introduced. In a Utah cohort of 90 patients, people diagnosed clinically had more hypoglycemia-related events per patient than those diagnosed by screening (2.15 versus 0.62, a difference that was not statistically significant), and no irreversible complications were observed after diagnosis in any patient. (Rücklová et al., Nutrients, 2021; Anderson et al., Mol Genet Metab, 2020)
Screening does not remove every risk. Among the first 47 cases detected by the New England Newborn Screening Program, adverse events occurred in 5 children: two survived severe neonatal hypoglycemia, one survived a severe episode at 15 months, and two died, at 11 and 33 months. The authors concluded that most children do well but adverse outcomes have not been entirely avoided. (Hsu et al., Pediatrics, 2008)
What Is MCAD Deficiency?
Medium-chain acyl-coenzyme A dehydrogenase (MCAD) deficiency is an inherited disorder that prevents your body from breaking down certain fats and turning them into energy. MCAD is the enzyme, a protein that makes reactions happen, that your body needs to process these fats. Your metabolism involves the processes your body uses to produce energy, and MCAD deficiency can cause problems with it. [1]
When there is not enough of the MCAD enzyme, certain fats called medium-chain fatty acids cannot be broken down and turned into energy. This leads to low energy and low blood sugar, and fatty acids can build up in body tissues and cause damage. The level of sugar in the blood can drop dangerously low, a condition called hypoglycemia. If left untreated, MCAD deficiency may lead to severe lack of energy and tiredness, liver disease, coma and other serious health problems. [1]
MCAD deficiency is present from birth and is a lifelong condition. In the United States, all states test for it at birth as part of newborn screening, and many other countries also provide routine newborn screening. If it is diagnosed and treated early, the disorder can be well managed through diet and lifestyle. [1]
How Common Is MCAD Deficiency?
In the United States, the estimated incidence is 1 in 17,000 people, and the condition is more common in people of northern European ancestry than in other ethnic groups. [3] A systematic review of screening studies put the average US incidence at 1 in 15,000 to 1 in 20,000 births, with individual states reporting from 1 in 10,000 to 1 in 30,000. (Grosse et al., Genet Med, 2006) Rates can differ by region: a Utah cohort found MCAD deficiency in 1 in 9,266 newborns, compared with 1 in 17,759 for the US average. (Anderson et al., Mol Genet Metab, 2020)
What Are the Symptoms of MCAD Deficiency?
MCAD deficiency is usually first identified in babies and young children. In rare cases, it is not diagnosed until adulthood. Because all US states screen at birth, most people are found before any symptoms appear. Symptoms can vary and may include: [1]
Vomiting.
Low or no energy.
A weak feeling.
Low blood sugar (hypoglycemia).
What Triggers a Metabolic Crisis?
A sudden, severe episode, called a metabolic crisis, can be due to: [1]
Going too long without eating.
Missing meals.
Common infections.
High fever.
Ongoing stomach and digestive problems that may cause vomiting and diarrhea.
Intense exercise.
What Causes MCAD Deficiency?
MCAD deficiency is caused by a change in the ACADM gene. The condition is inherited from both parents in an autosomal recessive pattern. This means both parents are carriers: each has one changed gene and one unchanged gene, but they do not have symptoms of the condition. The child with MCAD deficiency inherits two copies of the changed gene, one from each parent. [1]
If you inherit only one changed gene, you will not develop MCAD deficiency. You are a carrier and can pass the changed gene to your children, but they would not develop the condition unless they also inherited a changed gene from their other parent. For two carriers, the chances with each pregnancy are: [1]
If both parents are carriers | Chance |
An unaffected child with two unchanged genes | 25% |
An unaffected child who is also a carrier | 50% |
A child with MCAD deficiency (two changed genes) | 25% |
A child is therefore at risk of MCAD deficiency only if both parents are carriers of a gene known to cause it. Children who inherit one copy typically do not develop the condition but will be carriers. [1]
What Complications Can MCAD Deficiency Cause?
If a metabolic crisis caused by MCAD deficiency is left untreated, it can lead to seizures, liver problems, brain damage, coma and sudden death. This is why preventing long fasting periods, especially during illness, matters so much. [1]
How Is MCAD Deficiency Diagnosed?
MCAD deficiency is diagnosed through newborn screening followed by genetic testing. [2]
Newborn screening. Many countries, including all US states, screen at birth. Using a heel prick, a few drops of your baby's blood are taken and analyzed. If screening levels are outside the standard range, additional testing can be done.
Genetic testing. This can reveal the changed gene that causes MCAD deficiency. A sample of blood or saliva, or cells from the inside of the cheek, the skin or other tissue, is sent to a lab. Your provider may also recommend testing family members, and genetic counseling can help you understand the testing process and what the results mean.
A metabolic specialist, a doctor who specializes in metabolism disorders, and often a registered dietitian are usually involved in diagnosis and treatment. Lab tests can help identify problems with metabolism, and the results can help treat or prevent complications. [2]
How Is MCAD Deficiency Treated?
Newborn screening allows MCAD deficiency to be diagnosed and treated early, before symptoms occur, preventing complications. Once identified, the disorder can be managed through diet and lifestyle. The main goal of treatment is to prevent the metabolic problems that can lead to low blood sugar. [2]
Infants need frequent feedings that include enough calories from complex carbohydrates.
Children and adults need enough complex carbohydrates on a regular schedule of meals and snacks.
Complex carbohydrates, such as brown bread, rice, pasta and cereal, release energy more slowly than simple carbohydrates like sugar. Work with your health care team, including a registered dietitian, to build a plan tailored to your energy needs, including extra complex carbohydrates during illness, stress or increased activity. [2]
Limit fasting. The length of time that is safe to go without eating varies with age and your care team's recommendations. [2]
Age | Maximum fasting time |
First 4 months of life (well infants) | No longer than 2 to 3 hours |
Up to 1 year of age | Gradually increased, up to 12 hours by age 1 |
Children and adults | Not longer than 12 hours |
Prevent low blood sugar. Recommendations generally include: [2]
Avoid fasting longer than your health care team recommends.
Eat regular meals and snacks with adequate calories from complex carbohydrates.
Choose foods that are high in complex carbohydrates and lower in fat.
Eat extra complex carbohydrates during illness, stress or increased activity, and before going to sleep.
Limit alcohol use.
Treat low blood sugar. Talk with your provider about an emergency plan. Strategies may include taking simple carbohydrates, such as glucose tablets or sweetened drinks like juice that are not labeled diet, and seeking emergency medical care if you cannot eat or if you have diarrhea or vomiting. An intravenous (IV) line for extra glucose and additional treatment may be needed. [2]
For clinicians and parents, the American Academy of Pediatrics clinical report on acute illness in MCAD deficiency notes that appropriate treatment of children can lead to avoidance of morbidity and mortality, and that the primary care provider is crucial alongside the metabolic physician. (McGregor et al., Pediatrics, 2021)
What everyday steps help people with MCAD deficiency?
Parent and family understanding of the condition is critical to ongoing care and to preventing low blood sugar episodes and other complications. Prevention tips include: [2]
Avoid skipping meals.
Eat complex carbohydrates before extra activity or exercise.
Eat complex carbohydrates before going to bed.
Increase calories with extra complex carbohydrates during illness or stress.
Always carry complex carbohydrate snacks.
Always carry a source of simple carbohydrates to treat low blood sugar.
Know the warning signs of low blood sugar and other symptoms of a metabolic crisis.
If you or your child has MCAD deficiency, wear a medical alert bracelet or necklace and carry a medical card, so health care providers know about the condition even in an emergency. Ask your provider about information for health professionals that you can carry in case emergency care is needed. [2]
When Should You Contact Your Health Care Team?
Contact your health care team if you have been diagnosed with MCAD deficiency and have a high fever, no appetite, stomach or digestive symptoms, or a planned medical procedure that requires fasting. [1]
How can people cope with MCAD deficiency as a lifelong condition?
Caring for a child or family member with a lifelong disorder such as MCAD deficiency can be challenging. These strategies may help: [2]
Learn about the disorder. Then you can make the best choices and be an advocate for yourself or your child. Help family, caregivers, teachers and friends understand the care needed and the consequences of not following dietary instructions.
Find a team of trusted professionals. Medical centers with specialty teams can offer information, nutritional advice and support.
Seek out other families. Ask your provider about support groups, or whether they can put you in touch with a family who has dealt with the disorder.
Ask for help from family and friends. Take time for your own interests, and consider counseling with a mental health professional if it would help with coping.
Learn more. For more evidence-based health guides, visit Rinnit.com.
Frequently Asked Questions
Is MCAD deficiency something you are born with?
Yes. It is an inherited, lifelong condition present from birth, caused by changes in the ACADM gene inherited from both parents. [1]
What does MCAD deficiency do to the body?
Without enough of the MCAD enzyme, medium-chain fatty acids cannot be turned into energy. This causes low energy and low blood sugar, and fatty acids can build up in tissues and cause damage. [1]
How common is MCAD deficiency?
In the United States the estimated incidence is about 1 in 17,000 people, and it is more common in people of northern European ancestry. [3]
What are the symptoms?
Symptoms can include vomiting, low or no energy, a weak feeling and low blood sugar. Most people are found by newborn screening before symptoms appear. [1]
What triggers a metabolic crisis?
Going too long without eating, missing meals, common infections, high fever, ongoing vomiting and diarrhea, and intense exercise. [1]
How is MCAD deficiency found in babies?
Through newborn screening. A heel prick takes a few drops of blood for analysis, and all US states and many other countries screen for it. Genetic testing follows if levels are outside the standard range. [2]
Can a parent with no symptoms pass it on?
Yes. Both parents of an affected child are typically carriers with one changed gene and no symptoms. Two carriers have a 25% chance of having an affected child with each pregnancy. [1]
Can MCAD deficiency first appear in adulthood?
Rarely. It is usually identified in babies and young children, but in rare cases it is not diagnosed until adulthood. [1]
What happens if a metabolic crisis is not treated?
An untreated crisis can lead to seizures, liver problems, brain damage, coma and sudden death. [1]
How is MCAD deficiency treated?
Through diet and lifestyle: frequent feedings for infants, regular meals and snacks with complex carbohydrates for children and adults, limits on fasting, and an emergency plan for low blood sugar. [2]
How long can someone safely go without eating?
The safe time without eating varies with age and your care team's advice. In the first 4 months, no longer than 2 to 3 hours for well infants; up to 12 hours by age 1; children and adults not longer than 12 hours. [2]
What should I do if blood sugar drops?
Follow the emergency plan from your provider, which may include glucose tablets or non-diet juice. Seek emergency care if you cannot eat or have vomiting or diarrhea, because an IV line for glucose may be needed. [2]
What should someone with MCAD deficiency always carry?
Complex carbohydrate snacks, a source of simple carbohydrates to treat low blood sugar, and a medical alert bracelet or card so providers know about the condition in an emergency. [2]
Can people with MCAD deficiency be managed well long term?
When diagnosed and treated early, the disorder can be well managed through diet and lifestyle. [1] In a Utah cohort of 90 patients, no irreversible complications were seen after diagnosis, although screening does not remove every risk. (Anderson et al., Mol Genet Metab, 2020)
When should I contact my health care team?
If you have been diagnosed and have a high fever, no appetite, stomach or digestive symptoms, or a planned medical procedure that requires fasting. [1]
Conclusion
MCAD deficiency is a lifelong, inherited disorder that stops the body from turning certain fats into energy. It is usually found before symptoms appear, thanks to routine heel-prick newborn screening. Once identified, it can be well managed with a steady routine: frequent meals and snacks built on complex carbohydrates, limits on how long you go without eating, and extra food during illness, stress or activity. Knowing the warning signs, carrying snacks and glucose, and wearing medical alert identification turn a potentially dangerous condition into a manageable part of daily life. If your baby's newborn screening result is outside the normal range, or you have been diagnosed and develop a high fever or a planned fasting procedure, contact your health care team. [1] [2]
Related Reading
References
Mayo Clinic Staff. MCAD deficiency - Symptoms and causes. Mayo Clinic. November 28, 2023. https://www.mayoclinic.org/diseases-conditions/mcad-deficiency/symptoms-causes/syc-20353745
Mayo Clinic Staff. MCAD deficiency - Diagnosis and treatment. Mayo Clinic. November 28, 2023. https://www.mayoclinic.org/diseases-conditions/mcad-deficiency/diagnosis-treatment/drc-20353747
National Library of Medicine. Medium-chain acyl-CoA dehydrogenase deficiency. MedlinePlus Genetics. Last updated February 1, 2015. https://medlineplus.gov/genetics/condition/medium-chain-acyl-coa-dehydrogenase-deficiency/
Grosse SD, et al. The epidemiology of medium chain acyl-CoA dehydrogenase deficiency: an update. Genet Med. 2006;8(4):205-212. PMID 16617240. https://doi.org/10.1097/01.gim.0000204472.25153.8d
Rücklová K, et al. Impact of Newborn Screening and Early Dietary Management on Clinical Outcome of Patients with Long Chain 3-Hydroxyacyl-CoA Dehydrogenase Deficiency and Medium Chain Acyl-CoA Dehydrogenase Deficiency-A Retrospective Nationwide Study. Nutrients. 2021;13(9):2925. PMID 34578803. https://doi.org/10.3390/nu13092925
Anderson DR, et al. Clinical and biochemical outcomes of patients with medium-chain acyl-CoA dehydrogenase deficiency. Mol Genet Metab. 2020;129(1):13-19. PMID 31836396. https://doi.org/10.1016/j.ymgme.2019.11.006
Hsu HW, et al. Spectrum of medium-chain acyl-CoA dehydrogenase deficiency detected by newborn screening. Pediatrics. 2008;121(5):e1108-e1114. PMID 18450854. https://doi.org/10.1542/peds.2007-1993
McGregor TL, et al. Management Principles for Acute Illness in Patients With Medium-Chain Acyl-Coenzyme A Dehydrogenase Deficiency. Pediatrics. 2021;147(1):e2020040303. PMID 33372121. https://doi.org/10.1542/peds.2020-040303
Medical disclaimer
This article is for general educational purposes only and is not a substitute for professional medical advice. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.
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