Fatty acid oxidation (FAO) disorders are inherited metabolic conditions that impair the body's ability to break down fats for energy, especially during fasting or illness. They commonly present with hypoketotic hypoglycemia, and early diagnosis is critical for preventing life-threatening crises.
Energy source: FAO provides ~80% of energy for skeletal and cardiac muscle, especially during fasting.
Ketone production: FAO generates ketone bodies, vital for brain and tissue energy when glucose is low.
Pathway complexity: Over 22 known enzyme defects affect FAO, ranging from severe neonatal forms to mild adult-onset variants.
Key regulators:
Malonyl-CoA inhibits CPTI during fed state.
Carnitine cycle is essential for long-chain fatty acid transport into mitochondria.
β-oxidation cycle: A 4-step mitochondrial process producing acetyl-CoA and electrons for ATP generation.
Long-chain FAO enzymes: VLCAD, LCHAD, LKAT
Medium/short-chain enzymes: MCAD, SCAD, MCHAD, SCHAD, MCKAT
Common defects:
MCAD (most prevalent)
CPTI/CPTII
LCHAD
VLCAD
SCAD
ACAD9
Trifunctional protein deficiency
Incidence: ~1 in 9,300 births; lower in Asian populations
Inheritance: Autosomal recessive
MCAD: Most common; one mutation accounts for 70–80% of cases
Trigger: Fasting, illness, vomiting
Hallmark: Hypoketotic hypoglycemia
Symptoms:
Vomiting, lethargy, seizures, apnea
Encephalopathy, respiratory arrest
Cardiac arrhythmias, rhabdomyolysis
Pregnancy risks: LCHAD-linked AFLP and HELLP syndromes
Findings:
Hepatomegaly (no splenomegaly)
Hypotonia, poor perfusion
Rare jaundice
Family history clues:
Sudden infant death, Reye syndrome, unexplained liver failure
Newborn screening: Tandem mass spectrometry (acylcarnitine profile)
Confirmatory tests:
Urine organic acids
Serum FFAs, β-hydroxybutyrate
Enzyme assays (liver, muscle, fibroblasts)
Genetic testing
Dextrose infusion to reverse hypoglycemia
Avoid FAO-inhibiting drugs (e.g., valproate, NSAIDs)
Avoid fat emulsions
Carnitine supplementation for transport defects
Avoid fasting; high-carb, low-fat diet
Overnight feeding support
Daily carnitine (100 mg/kg/day) for transport defects
Riboflavin (100 mg/day) for ACAD9 or early β-oxidation defects
Neonatal onset: Poor prognosis; up to 60% mortality by age 4–5
Early diagnosis: Prevents crises and improves outcomes
| Enzyme | Function | Associated Disorder | Key Clinical Features |
|---|---|---|---|
| MCAD | Medium-chain acyl-CoA dehydrogenase | MCAD deficiency | Hypoketotic hypoglycemia, vomiting, lethargy, seizures |
| VLCAD | Very-long-chain acyl-CoA dehydrogenase | VLCAD deficiency | Cardiomyopathy, rhabdomyolysis, hypoglycemia |
| LCHAD | Long-chain 3-hydroxyacyl-CoA dehydrogenase | LCHAD deficiency | Liver failure, retinopathy, neuropathy, pancreatitis |
| SCAD | Short-chain acyl-CoA dehydrogenase | SCAD deficiency | GERD, failure to thrive, cyclic vomiting |
| CPTI | Carnitine palmitoyltransferase I | CPTI deficiency | Hepatic dysfunction, hypoglycemia, no ketones |
| CPTII | Carnitine palmitoyltransferase II | CPTII deficiency | Muscle pain, rhabdomyolysis, myoglobinuria |
| ACAD9 | Acyl-CoA dehydrogenase 9 | ACAD9 deficiency | Cardiomyopathy, lactic acidosis, liver failure |
| CAT | Carnitine-acylcarnitine translocase | CAT deficiency | Neonatal hypoglycemia, liver failure, cardiomyopathy |
| Enzyme | Clinical Function | Key Features | Diagnostic Labs/Findings |
|---|---|---|---|
| 2,4-Dienoyl-CoA Reductase | Metabolizes unsaturated fatty acids during β-oxidation | Rare; hypoglycemia, lethargy, metabolic acidosis | Elevated unsaturated acylcarnitines; abnormal urine organic acids |
| Carnitine-Acylcarnitine Translocase (CAT) | Transports acylcarnitines into mitochondria | Neonatal hypoglycemia, cardiomyopathy, liver failure | ↑ C16–C18 acylcarnitines; low free carnitine |
| CPTII | Converts acylcarnitines to acyl-CoA inside mitochondria | Muscle pain, rhabdomyolysis, myoglobinuria | ↑ C16–C18 acylcarnitines; normal carnitine |
| CPTI | Initiates carnitine shuttle for long-chain fatty acids | Hepatic dysfunction, hypoketotic hypoglycemia | Low ketones; ↑ free fatty acids; low acylcarnitines |
| Carnitine Uptake Defect | Transports carnitine into cells | Fasting intolerance, hypotonia, cardiomyopathy | Low plasma carnitine; high urinary carnitine |
| LCHAD | Oxidizes long-chain 3-hydroxyacyl-CoA | Liver failure, retinopathy, neuropathy, pancreatitis | ↑ C16–OH and C18–OH acylcarnitines; HADHA mutation |
| MCKAT | Cleaves medium-chain 3-ketoacyl-CoA | Hypoglycemia, liver dysfunction, metabolic crisis | ↑ medium-chain acylcarnitines; abnormal organic acids |
| MCAD | Oxidizes medium-chain acyl-CoA | Hypoketotic hypoglycemia, vomiting, seizures | ↑ C6–C10 acylcarnitines; common mutation c.985A>G |
| MCHAD | Oxidizes medium-chain 3-hydroxyacyl-CoA | Similar to MCAD; less common | ↑ medium-chain hydroxyacylcarnitines |
| SCHAD | Oxidizes short-chain 3-hydroxyacyl-CoA | Persistent vomiting, failure to thrive, hyperinsulinism | ↑ short-chain hydroxyacylcarnitines; abnormal insulin levels |
| Multiple Acyl-CoA Dehydrogenase | Oxidizes multiple chain-length acyl-CoAs | Severe neonatal onset, metabolic acidosis, muscle weakness | ↑ C4–C18 acylcarnitines; ↑ urine organic acids |
| SCAD | Oxidizes short-chain acyl-CoA | GERD, cyclic vomiting, developmental delay | ↑ C4 acylcarnitine; ethylmalonic acid in urine |
| Trifunctional Protein | Performs final steps of long-chain FAO | Neuropathy, cardiomyopathy, liver dysfunction | ↑ long-chain hydroxyacylcarnitines; HADHA/HADHB mutations |
| VLCAD | Oxidizes very-long-chain acyl-CoA | Cardiomyopathy, rhabdomyolysis, hypoglycemia | ↑ C14:1 acylcarnitine; VLCAD gene mutation |
| HMG-CoA Synthase | Initiates ketogenesis from acetyl-CoA | Hypoglycemia, no ketones, vomiting | Low ketones; ↑ FFAs; normal acylcarnitines |
| HMG-CoA Lyase | Cleaves HMG-CoA to acetoacetate | Metabolic acidosis, hypoglycemia, vomiting | ↑ 3-hydroxy-3-methylglutarate in urine; low ketones |
| ACAD9 | Oxidizes long-chain acyl-CoA; supports ETC complex I | Cardiomyopathy, lactic acidosis, liver failure | ↑ long-chain acylcarnitines; low complex I activity |
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Merritt, J. L., Chang, I. J., & Sweeney, M. L. (2018). Fatty acid oxidation disorders. In M. P. Adam et al. (Eds.), GeneReviews® [Internet]. University of Washington, Seattle.
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National Organization for Rare Disorders (NORD). (2023). Fatty acid oxidation disorders. Retrieved from
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