VLCAD stands for very long-chain acyl-CoA dehydrogenase deficiency, a rare inherited condition where the body cannot properly break down certain fats for energy. It affects an estimated 1 in 30,000 to 1 in 100,000 births and is the most common subtype of long-chain fatty acid oxidation disorders, accounting for roughly 59% of all cases. People with VLCAD lack a working version of an enzyme needed to process fats with carbon chains 14 to 20 units long, which means their cells struggle to produce energy during times when the body normally relies on stored fat.
How the Body Normally Uses Fat for Energy
Your cells break down fat through a process called beta-oxidation, which happens inside mitochondria, the energy-producing structures in every cell. One of the first steps in this process requires the VLCAD enzyme to begin chopping long-chain fatty acids into smaller pieces. Those smaller pieces then feed into the energy cycle that powers your heart, muscles, and liver.
When this enzyme is missing or not working well, the body hits a wall. It can’t efficiently convert stored fat into usable energy, and it can’t produce enough ketones, which are the backup fuel your brain and muscles rely on during fasting or intense exercise. At the same time, partially broken-down fats accumulate in the blood and tissues, where they can damage organs, particularly the heart and skeletal muscles.
What Causes It
VLCAD is caused by mutations in the ACADVL gene, located on chromosome 17. The condition follows an autosomal recessive inheritance pattern, meaning a child must inherit one defective copy of the gene from each parent to develop the disorder. Parents who each carry one mutated copy typically have no symptoms themselves.
More than one type of mutation can cause VLCAD. Missense mutations, where a single building block in the gene is swapped, are the most common and can lead to either severe or milder forms depending on how much working enzyme the body still produces. Nonsense mutations, which completely shut down enzyme production, tend to cause the most severe, earliest-onset form of the disease. Splicing and frameshift mutations also occur. This genetic variability is one reason VLCAD looks so different from person to person.
Three Forms Based on Age and Severity
Doctors recognize three broad clinical forms of VLCAD, largely defined by when symptoms first appear and which organs are most affected.
Severe Infantile Form
This is the most dangerous presentation, appearing in the first months of life. Infants may develop an enlarged, weakened heart (hypertrophic cardiomyopathy) and liver failure. Without prompt treatment, this form can be life-threatening. It is associated with mutations that eliminate nearly all enzyme activity.
Childhood-Onset Form
Children between ages 1 and 13 may present with episodes of dangerously low blood sugar that the body cannot compensate for by producing ketones. These episodes, called hypoketotic hypoglycemia, often occur during illness or after skipping meals. The liver is the primary organ affected.
Adolescent and Adult-Onset Form
The mildest form primarily affects skeletal muscles. People experience exercise intolerance, muscle pain, and recurring episodes of rhabdomyolysis, a condition where muscle fibers break down and release their contents into the bloodstream. These episodes are triggered by prolonged physical activity, fasting, cold exposure, or illness. In one published case, a 13-year-old girl developed severe muscle pain and dark-colored urine after extended exercise, with muscle enzyme levels rising to more than 1,000 times the normal range. Her 19-year-old brother, later diagnosed with the same condition, had experienced repeated similar episodes after moderate exercise.
How VLCAD Is Detected
Most cases today are identified through newborn screening, which is routine in many countries. The test measures levels of a specific fat fragment in a drop of blood taken from the baby’s heel. The primary marker is a compound called C14:1 acylcarnitine, a long-chain fat-carnitine molecule that accumulates when the VLCAD enzyme isn’t working. Ratios of C14:1 to other acylcarnitines help distinguish true cases from false positives. Newer research from Japan has shown that comparing C14:1 to medium-chain acylcarnitines is particularly effective at reducing false alarms.
Newborn screening catches many cases before symptoms develop, but some people with milder mutations aren’t diagnosed until childhood or adulthood, often after a first episode of rhabdomyolysis or unexplained muscle pain. Genetic testing of the ACADVL gene confirms the diagnosis.
Daily Management and Diet
The cornerstone of VLCAD management is dietary. Because the body can’t process long-chain fats efficiently, those fats need to be limited while still meeting the body’s energy and nutritional needs.
Current guidelines recommend restricting long-chain fat intake to about 10% of total daily calories. Going below that threshold risks essential fatty acid deficiency, since the body still needs small amounts of specific long-chain fats (linoleic acid and linolenic acid) that it cannot make on its own. To replace the missing energy from restricted fats, medium-chain triglycerides (MCT oil) are supplemented at 10 to 25% of total calories. MCTs bypass the broken VLCAD enzyme entirely because they’re short enough to be processed by different enzymes. The combined fat intake from both sources should fall in the 20 to 35% range of total energy, similar to a typical diet’s fat percentage but with a very different composition.
In 2020, the FDA approved triheptanoin (sold as Dojolvi), a specially designed medium-chain triglyceride, for people with confirmed long-chain fatty acid oxidation disorders including VLCAD. It provides both calories and a type of fat the body can use through alternative metabolic pathways.
Avoiding Metabolic Crises
Fasting is one of the biggest risks for people with VLCAD. When you go without food, your body switches from burning glucose to burning stored fat. For someone with VLCAD, that switch leads to an energy crisis because the fat can’t be properly broken down. Research using tissue monitoring found that fat breakdown begins as early as 4.5 hours into a fast in children with VLCAD, earlier than in some other fatty acid oxidation disorders.
Published guidelines generally allow older children and adults a nightly fast of 10 to 12 hours during stable health, but younger children and infants need to eat more frequently. During illness, even mild infections that reduce appetite, the safe fasting window shrinks considerably. Many families keep cornstarch-based drinks or other slow-release carbohydrate sources on hand for overnight use or sick days.
Prolonged exercise is the other major trigger, especially for the adolescent and adult-onset form. People with VLCAD learn to eat carbohydrate-rich snacks before and during physical activity, stay well hydrated, and avoid pushing through muscle pain. Cold temperatures also increase energy demand and can provoke episodes.
What Happens During a Crisis
When a metabolic crisis does occur, whether from illness, fasting, or overexertion, the body needs an immediate alternative energy source. In a hospital setting, this means intravenous glucose, typically a 10% dextrose solution given at a rate of at least 1.5 times normal fluid maintenance. The goal is to flood the body with carbohydrate energy so it stops trying to burn fat. In most documented cases involving infants and children, this approach successfully resolves rhabdomyolysis and stabilizes heart function. More severe cases, such as one involving a comatose 32-year-old woman with both heart involvement and rhabdomyolysis, have required much higher concentrations of intravenous glucose.
Long-Term Outlook
The prognosis for VLCAD has improved dramatically since newborn screening became widespread. Children identified before their first crisis and started on dietary management early often do well, particularly those with milder genetic variants that leave some residual enzyme activity. The severe infantile form still carries significant risk, especially for heart complications, but early detection and aggressive nutritional support have changed outcomes even for these patients.
People with the adult-onset muscular form can generally lead active lives with careful management, though they remain vulnerable to rhabdomyolysis episodes throughout life. Learning individual triggers, maintaining consistent eating patterns, and having a clear emergency plan for illness are the practical tools that make the biggest difference in day-to-day life with VLCAD.