Fatty Acid Metabolism

Definition

• Fatty acid metabolism is the coordinated set of pathways that store, mobilize, and oxidize long-chain fatty acids: lipolysis of adipose triglyceride, albumin-bound transport, carnitine-dependent mitochondrial import, the β-oxidation spiral yielding acetyl-CoA plus NADH/FADH₂, hepatic ketogenesis, and the opposing cytosolic pathway of de novo lipogenesis.
• Physiologically it is the fasting fuel system. Hepatic glycogen is substantially depleted after roughly 12–24 hours of fasting — sooner in infants and young children, whose higher metabolic rate per kilogram shortens the interval — and fat oxidation progressively predominates thereafter, supplying most ATP for muscle, heart, liver, and renal cortex. This spares glucose for the brain and red cells; the brain partially shifts to hepatically exported ketone bodies during prolonged fasting, while red cells remain obligate glucose consumers because they lack mitochondria.

Why it matters clinically

• Any block in the pathway converts an ordinary stressor — a viral illness, a skipped feed, a long run — into an energy-failure crisis: hypoketotic hypoglycemia, encephalopathy, cardiomyopathy, or rhabdomyolysis. Death or neurologic injury during a first decompensation is the reason these disorders are screened for rather than diagnosed clinically.
• The same pathway running unopposed produces diabetic ketoacidosis, the acute hyperglycemic emergency framed by the ADA Standards of Care, and chronic substrate overload underlies metabolic dysfunction-associated steatotic liver disease, addressed in AASLD guidance.

Epidemiology worth recalling

• Fatty-acid oxidation disorders are autosomal recessive and individually rare but collectively among the more common inborn errors detected at birth. MCAD deficiency is the most frequent, with highest carrier frequency in populations of Northern European ancestry; the classic presentation is a previously well infant or toddler.
• Newborn screening by tandem mass spectrometry acylcarnitine profiling detects MCAD, VLCAD, LCHAD/TFP, and carnitine uptake defects, all of which appear on the HRSA/ACHDNC Recommended Uniform Screening Panel; presentation in an older child or a patient born abroad should raise suspicion that screening was never performed.
• Acquired derangements are far more common than genetic ones: insulin deficiency, alcohol use, certain drugs, critical illness, and obesity all shift flux through these same steps.

Mobilization and activation

• Hormone-sensitive lipase / ATGL: glucagon and catecholamines raise cAMP → PKA phosphorylates and activates lipolysis in adipocytes; insulin dephosphorylates and shuts it off. Released free fatty acids travel bound to albumin (they do not cross the blood–brain barrier — the reason the CNS needs ketones).
• Acyl-CoA synthetase (outer mitochondrial membrane) activates the fatty acid at the cost of 2 ATP equivalents (ATP → AMP + PPi).

Carnitine shuttle — the regulated step

• CPT-I (CPT1A/CPT1B): outer membrane; converts acyl-CoA to acylcarnitine and is the rate-limiting step of β-oxidation, allosterically inhibited by malonyl-CoA. This is the reciprocal switch: in the fed state malonyl-CoA is high, so newly made fat is not immediately burned.
• Carnitine-acylcarnitine translocase moves acylcarnitine in; CPT-II on the matrix side regenerates acyl-CoA and frees carnitine.

• Odd-chain fatty acids end in propionyl-CoA → (biotin-dependent carboxylase) methylmalonyl-CoA → (B₁₂-dependent mutase) succinyl-CoA, the only genuinely glucogenic carbon from fat.
• Peroxisomes handle very-long-chain fatty acids (β-oxidation, generating H₂O₂ not ATP) and α-oxidation of branched phytanic acid.

Downstream and opposing flux

• Hepatic acetyl-CoA activates pyruvate carboxylase (sustaining gluconeogenesis) and feeds HMG-CoA synthase, the rate-limiting enzyme of ketogenesis. Liver lacks thiophorase (SCOT), so it exports ketones rather than using them.
• Synthesis runs in cytosol: citrate shuttle → ATP-citrate lyase → acetyl-CoA carboxylase (rate-limiting, biotin- and CO₂-dependent, activated by citrate/insulin, inhibited by AMPK and palmitoyl-CoA) → fatty acid synthase using NADPH from the HMP shunt and malic enzyme, yielding palmitate.

Carnitine shuttle defects

• Primary carnitine deficiency (SLC22A5/OCTN2): renal carnitine wasting → dilated cardiomyopathy, hypotonia, hypoketotic hypoglycemia; very low plasma free carnitine. Responds to L-carnitine.
• Secondary carnitine deficiency: valproate, chronic kidney disease/hemodialysis, pivalate-containing antibiotics.
• CPT-II deficiency: adult/adolescent form presents as exercise- or fever-induced rhabdomyolysis with myoglobinuria; the classic mimic of muscle glycogenoses, but here the trigger is prolonged low-intensity exertion or fasting rather than brief maximal effort.
• CPT-I deficiency: hypoketotic hypoglycemia with hepatomegaly and characteristically elevated free carnitine and low acylcarnitines (the mirror image of most oxidation defects).

Dehydrogenase (spiral) defects

• MCAD deficiency: the most common inherited fatty-acid oxidation disorder; a previously well infant/toddler decompensates after an intercurrent illness or a missed overnight feed with vomiting, lethargy, hypoketotic hypoglycemia, hyperammonemia and medium-chain dicarboxylic aciduria; ↑ C8 (octanoyl) acylcarnitine. It is on the HRSA/ACHDNC Recommended Uniform Screening Panel, and ACMG newborn-screening ACT sheets direct confirmatory acylcarnitine profile plus avoidance of fasting.
• VLCAD deficiency: earlier, more severe — cardiomyopathy, arrhythmia, hepatic failure, later rhabdomyolysis.

Peroxisomal disease

• X-linked adrenoleukodystrophy (ABCD1): VLCFA accumulation → adrenal insufficiency plus progressive demyelination in a boy.
• Zellweger spectrum (PEX genes): absent functional peroxisomes → profound hypotonia, seizures, hepatomegaly, craniofacial dysmorphism.
• Refsum disease: phytanoyl-CoA hydroxylase defect → retinitis pigmentosa, anosmia, neuropathy, ataxia; treat by restricting dairy/ruminant fat.

Downstream metabolic disease

• DKA: unrestrained lipolysis and ketogenesis from insulin deficiency; the ADA Standards of Care frame therapy as fluids, IV insulin (which suppresses lipolysis and ketogenesis), and potassium repletion.
• Steatotic liver disease (MASLD): substrate overload plus de novo lipogenesis; AASLD guidance emphasizes weight loss and cardiometabolic risk reduction.

• Hypoketotic hypoglycemia is the signature phrase for any fatty-acid oxidation defect: the patient cannot make ketones, so the brain has no backup fuel and glucose is consumed unopposed. Ketones that are low or absent when they should be high is the whole diagnosis.
• Single best next step in an acute crisis: IV dextrose — it both corrects hypoglycemia and suppresses lipolysis, removing the toxic substrate. Long-term management is avoidance of fasting (frequent feeds, sick-day carbohydrate plan), not carnitine for every patient.
• MCAD deficiency: ↑ C8 acylcarnitine and dicarboxylic aciduria with absent ketones; detected on the HRSA/ACHDNC-recommended newborn screening panel, which is why examiners pair it with an infant who "passed screening" or was born abroad.
• The one regulatory fact tested most: malonyl-CoA inhibits CPT-I. High insulin → active acetyl-CoA carboxylase → malonyl-CoA → β-oxidation off. AMPK does the reverse during energy stress.
• Common distractor — glycogen storage disease type I, which also causes fasting hypoglycemia but with ketosis, lactic acidosis, hyperuricemia and hyperlipidemia. Ketones present ⇒ think glycogen/gluconeogenesis, not β-oxidation.
• Second distractor: even-chain fatty acids cannot be converted to glucose — acetyl-CoA cannot cross pyruvate dehydrogenase backwards. Only propionyl-CoA from odd-chain fats (and glycerol) is glucogenic.
• Liver lacks thiophorase (SCOT), so it exports ketones and cannot burn them; conversely the brain cannot oxidize albumin-bound fatty acids and adapts by using β-hydroxybutyrate in prolonged fasting.
• Adult with recurrent exertional myoglobinuria and normal lactate rise on exercise → CPT-II deficiency; McArdle disease is the distractor (flat lactate, second-wind phenomenon).
• Drug associations: valproate → secondary carnitine deficiency; ackee fruit (hypoglycin A) → inhibited acyl-CoA dehydrogenase; biotin is the cofactor for acetyl-CoA carboxylase and propionyl-CoA carboxylase.

• β-oxidation occurs in mitochondria (long-chain) and peroxisomes (very long-chain); produces acetyl-CoA and NADH/FADH₂
• Carnitine is essential for transporting long-chain fatty acids across the inner mitochondrial membrane
• Ketone bodies (acetoacetate, β-hydroxybutyrate, acetone) produced during prolonged fasting/starvation and diabetic ketoacidosis (DKA)
• Fatty acid synthesis occurs in cytoplasm, requires malonyl-CoA and acetyl-CoA carboxylase (ACC), is inhibited by AMP-activated protein kinase (AMPK)
• Lipoprotein lipase on capillary endothelium hydrolyzes triglycerides in chylomicrons and VLDL

• Carnitine deficiency: weakness, myopathy, cardiomyopathy, hypoketotic hypoglycemia (impaired β-oxidation)
• Diabetic ketoacidosis: fruity breath, Kussmaul respiration, elevated ketones and anion gap metabolic acidosis
• Refsum disease: elevated phytanic acid (branched-chain fatty acid accumulation in peroxisomes)
• Jamaican vomiting sickness: hypoglycemia from ackee fruit toxin inhibiting β-oxidation via acyl-CoA dehydrogenase

• Confusing β-oxidation location: Long-chain in mitochondria (primary), very long-chain in peroxisomes (then to mitochondria); peroxisomes do NOT produce ATP directly
• Assuming all ketosis is harmful: Nutritional ketosis (fasting, ketogenic diet) is physiologic; only DKA with acidemia is dangerous; ketones are efficient brain fuel
• Missing carnitine deficiency: Think of this in myopathy + hypoglycemia + low ketones; primary (genetic) vs. secondary (renal disease, medication)

• Carnitine deficiency: L-carnitine supplementation (oral or IV)
• DKA: IV fluids (rehydration), insulin (suppresses ketogenesis), electrolyte replacement (especially K⁺)
• Fatty liver/metabolic syndrome: Weight loss, exercise (increases AMPK activity), consider GLP-1 agonists or pioglitazone
• Lipoprotein lipase deficiency: Very low-fat diet (<20g fat/day), fish oil (omega-3 triglycerides), avoid alcohol