VLCAD deficiency (very long-chain acyl-CoA dehydrogenase deficiency) is a rare genetic condition that prevents the body from converting certain long-chain fats into energy. It affects roughly 1 in 31,500 births and is now routinely detected through newborn screening in the United States. The condition ranges from severe forms that appear in infancy to milder forms that may not cause problems until childhood or adulthood.
How VLCAD Deficiency Works
Your body breaks down fat for energy through a process that happens inside mitochondria, the power plants of your cells. This process works like an assembly line: enzymes chop long fatty acid chains into smaller pieces, two carbon atoms at a time, releasing energy with each pass. VLCAD is the enzyme responsible for the very first step when those fatty acids are long, between 14 and 20 carbon atoms in length.
When the VLCAD enzyme is missing or not working well, those long-chain fatty acids pile up instead of being converted to fuel. This is especially dangerous during fasting, illness, or heavy exercise, because those are the times your body leans hardest on fat for energy. Without that fuel source, blood sugar can drop dangerously low. Meanwhile, the unprocessed fats can accumulate in organs like the heart, liver, and muscles, causing direct damage.
Genetics and Inheritance
VLCAD deficiency is caused by mutations in the ACADVL gene, located on chromosome 17. It follows an autosomal recessive pattern, meaning a child must inherit a faulty copy of the gene from each parent to develop the condition. Parents who each carry one mutated copy typically have no symptoms themselves. If both parents are carriers, each pregnancy carries a 25% chance of producing an affected child.
Three Forms of the Disease
VLCAD deficiency appears in three distinct forms, depending on how much working enzyme a person has.
The severe form shows up in the first months of life. Infants may develop an enlarged, weakened heart, an enlarged liver, and dangerously low blood sugar. Without treatment, this form can be life-threatening. The heart problems in particular are a hallmark of severe VLCAD deficiency, though they can often improve with dietary management.
The moderate form typically appears in early childhood. It tends to cause episodes of low blood sugar and extreme tiredness, usually triggered by illness or going too long without eating. Heart involvement is less common in this form.
The mildest form may not surface until later childhood, adolescence, or even adulthood. Its primary feature is exercise-induced muscle pain and breakdown (a condition called rhabdomyolysis), where muscle tissue releases its contents into the bloodstream. This can cause dark-colored urine and severe muscle soreness after intense physical activity.
What Triggers a Crisis
People with VLCAD deficiency can feel perfectly fine most of the time. Problems arise when the body’s demand for fat-based energy spikes. The most common triggers are:
- Fasting or missed meals: After the body burns through its sugar reserves, it turns to fat. Without functioning VLCAD, that transition fails.
- Illness with fever: Infections increase metabolic demand and often reduce appetite, creating a double problem.
- Prolonged or intense exercise: Muscles rely heavily on fatty acids during sustained activity.
During a metabolic crisis, symptoms include extreme lethargy, vomiting, low blood sugar, and in severe cases, seizures or loss of consciousness. People with lower enzyme activity tend to have more frequent hospital admissions related to these episodes.
How It Is Detected
Most cases are now caught through newborn screening before any symptoms develop. The screening uses a blood sample from the baby’s heel to measure levels of specific fat-related molecules called acylcarnitines. An elevated level of a marker called C14:1 is the key red flag for VLCAD deficiency.
The C14:1 level at screening also turns out to be the most sensitive predictor of how much enzyme activity a child has, which helps doctors estimate disease severity and guide early treatment decisions. After an abnormal screening result, follow-up typically includes blood and urine tests, heart imaging, and genetic testing to confirm the diagnosis and identify the specific gene mutations involved. A small skin sample may also be taken to directly measure enzyme activity in the cells.
The expansion of newborn screening has shifted the landscape of VLCAD deficiency considerably. More children are now identified with milder forms that might have gone undiagnosed in the past.
Dietary Management
The cornerstone of VLCAD treatment is dietary modification. The goal is straightforward: limit the long-chain fats the body cannot process and replace them with medium-chain fats (called MCT, or medium-chain triglycerides) that bypass the broken enzyme entirely. Medium-chain fats are short enough to be handled by different enzymes in the same energy pathway.
In practice, this means a carefully controlled diet where long-chain fats are restricted to only essential fatty acids the body needs but cannot make on its own. MCT oil becomes a primary fat source. In one common approach, about 60% of dietary fat comes from MCT oil and 40% from long-chain sources, with total fat making up around 40% of daily calories. During times of physical stress or exercise, MCT intake may need to increase to prevent the body from pulling long-chain fats out of its own fat stores.
Frequent meals and snacks are essential. Avoiding prolonged fasting is one of the simplest and most important strategies. For infants, this means regular feeding schedules, sometimes around the clock. Older children and adults learn to recognize their limits and plan meals around activity.
Long-Term Outlook
Early detection through newborn screening has dramatically improved outcomes. In a retrospective study of screened patients, none presented with metabolic crisis at diagnosis, and their growth remained on track over time. Dietary treatment can reverse even serious complications: in one documented case, an infant with severe heart enlargement saw the condition resolve after starting an MCT-based diet.
The relationship between specific gene mutations and long-term outcomes is still not perfectly predictable. Some people with mutations that suggest severe disease do well with treatment, while others with seemingly milder genetic profiles still need close management. Long-term follow-up into adulthood is generally recommended, since the full picture of how the condition evolves over a lifetime is still emerging. People with higher residual enzyme activity tend to have fewer hospitalizations and a more stable course overall.