Biochem · Lipid Transport

Abetalipoproteinemia

The fat gets in. It just can't get out. MTP fails. Chylomicrons never form. Vitamins A, D, E, and K crash in sequence, and the clock starts ticking on the nervous system.

MTP Mutation No Chylomicrons TG ≈ 5 mg/dL Acanthocytes

Opening Vignette

An 8-month-old male is brought in for chronic diarrhea, foul-smelling stools, and failure to thrive since starting solid foods. Abdomen is distended. Serum triglycerides: 4 mg/dL. A peripheral blood smear shows red blood cells with multiple irregular spike-like projections.

What is the underlying molecular defect?

Deficiency of lipoprotein lipase

Mutation in microsomal triglyceride transfer protein (MTP)

Loss-of-function mutation in ABCA1

ApoB-100 truncation preventing VLDL secretion from hepatocytes

Three clues lock it in: infant with fat malabsorption plus near-zero serum TG plus acanthocytes (spiky RBCs). Lipoprotein lipase deficiency causes extremely high TG. ABCA1 is Tangier disease with low HDL and orange tonsils. ApoB-100 truncation hits VLDL from the liver, not chylomicrons from the gut. MTP is the enzyme that loads triglycerides onto ApoB-48 inside the enterocyte. Without it, chylomicrons never assemble, fat stays trapped, and TG in the blood is essentially zero.

1 / 6

The Mechanism

Follow the fat from gut lumen to bloodstream. Watch it get stuck.

The Setup

Normal Fat Absorption

Lipids are absorbed from the lumen into the enterocyte. MTP loads them onto ApoB-48 to form chylomicrons.

MTP Failure

Chylomicron Packaging Fails

Mutation in MTP prevents ApoB-48 loading. Triglycerides stay trapped as lipid vacuoles.

The Signature

Abetalipoproteinemia

Route Autosomal Recessive

Pattern MTP Mutation → Zero Chylomicrons

Pearl TG ≈ 5 mg/dL (Dead Giveaway)

Fat malabsorption + near-zero TG + acanthocytes = Abetalipoproteinemia.

Clinical Clues

Tap each card to flip it. Every finding has a mechanism behind it.

🚲

Steatorrhea

tap to reveal why

Why fat stays in the stool

Fat is absorbed into enterocytes but cannot be packaged into chylomicrons. It accumulates as lipid vacuoles and some passes through to the stool. Greasy, foul-smelling, high-volume diarrhea.

🧬

TG ≈ 5 mg/dL

the dead giveaway

Near-zero triglycerides

No chylomicrons enter the blood. Serum TG is essentially zero. Normal range is 50‑150 mg/dL. Seeing TG of 3‑8 mg/dL in an infant with fat malabsorption = abetalipoproteinemia until proven otherwise.

★

Acanthocytes

spiky smear finding

Why RBCs get spiky

Without proper lipoprotein delivery, RBC membrane lipid composition shifts. Abnormal cholesterol-to-phospholipid ratio causes irregular spike-like projections. This is a classic board finding paired with this diagnosis.

🔬

Lipid Vacuoles on Biopsy

enterocyte pathology

Enterocytes stuffed with fat

Small bowel biopsy shows enterocytes engorged with lipid vacuoles. Fat arrives normally from the gut, is esterified, but cannot leave because chylomicron assembly is blocked at the MTP step.

🛓

Failure to Thrive

presents in infancy

Caloric deficit from day one

Dietary fat is the densest energy source. Without fat absorption, infants cannot meet caloric demands. Symptom onset correlates with introduction of solid/fatty foods. Abdominal distension, chronic diarrhea, growth failure.

🧲

Inheritance

rare but testable

Autosomal Recessive

Both copies of MTP must be mutated for disease. Heterozygotes are carriers with mildly reduced lipoprotein levels but no clinical disease. Presents in infancy when fat intake begins.

📷 Acanthocytes on peripheral smear · tap to expand

The Vitamin Cascade

No chylomicrons means no fat-soluble vitamin transport. All four go down. Each one takes a different system with it.

A

Retinol

Night blindness is the first sign. Without correction: retinitis pigmentosa, progressive visual field loss, eventually blindness. Retinal pigment epithelium depends on vitamin A for rhodopsin regeneration.

Eye damage

D

Cholecalciferol

Impaired calcium absorption leads to rickets in children (bowed legs, widened growth plates) or osteomalacia in adults. Bone demineralization from chronically low 1,25-OH vitamin D.

Bone damage

E

Tocopherol

The neurological time bomb. Vitamin E is the most tested. Its deficiency causes spinocerebellar ataxia (progressive, starts with gait), peripheral neuropathy, and acanthocytosis. This damage is largely irreversible if untreated.

Neuro time bomb

K

Phylloquinone

Clotting factors II, VII, IX, X require vitamin K for gamma-carboxylation. Without it: elevated PT/PTT, easy bruising, bleeding risk. Neonates are already at baseline risk from low vitamin K stores.

Bleeding risk

Board Priority Vitamin E deficiency is the most tested pearl. The question will give you ataxia, peripheral neuropathy, or acanthocytosis and ask which vitamin to supplement first or which deficiency explains the neuro findings. The answer is always Vitamin E. Start supplementation early because the spinocerebellar degeneration is irreversible once established.

Treatment

You can't fix MTP. You work around it. Tap each step to see the reasoning.

01

Restrict Long-Chain Fatty Acids

Cut out dietary LCFAs (from meat, dairy, oils). These require chylomicron packaging to be absorbed.

Why: Long-chain fatty acids (C12+) can only enter the body through chylomicrons via lymphatics. With no MTP, these fats pile up in enterocytes and worsen steatorrhea. Removing them reduces the substrate for the blocked pathway and eases GI symptoms.

02

MCT Oil as Calorie Source

Medium-chain triglycerides (C8-C12) bypass the defect entirely. They go straight portal vein, no chylomicron needed.

Why MCT works: Medium-chain fatty acids are absorbed directly into the portal circulation via albumin transport. They do NOT require chylomicron packaging or lymphatic transport. This is the board-tested mechanism. MCT oil delivers calories through the completely intact portal route, bypassing the broken chylomicron assembly line.

03

Massive Fat-Soluble Vitamin Supplementation

Supplement A, D, E, and K in high doses. Vitamin E is the priority to prevent spinocerebellar degeneration.

Why high doses: Normal fat-soluble vitamin absorption relies on chylomicrons. These patients have no chylomicrons, so absorption is severely impaired even with oral supplementation. You need to overwhelm the absorptive deficit with mass dosing, especially for vitamin E. Once spinocerebellar ataxia is established, it does NOT reverse. This is why early aggressive Vit E is the most board-tested pearl.

04

Goal: Protect the Nervous System

The ultimate outcome is preventing irreversible neurological damage from vitamin E deficiency. Ataxia and neuropathy are the feared long-term consequences.

The clock: From birth, the nervous system is at risk. Early diagnosis and aggressive Vit E supplementation can prevent the spinocerebellar degeneration entirely. Delayed treatment leads to progressive ataxia, proprioceptive loss, and peripheral neuropathy that may persist even after vitamins are replenished.

Comparators: Keep these straight Tangier Disease: ABCA1 mutation. Low HDL. Orange tonsils from cholesterol accumulation in macrophages. TG is normal.
Chylomicron Retention Disease (Anderson): ApoB-48 itself is defective, not MTP. Similar presentation. Rarer.
LP Lipase Deficiency: Cannot break DOWN chylomicrons. TG is extremely HIGH (opposite of abeta). Eruptive xanthomas, pancreatitis.

Quiz

Eight questions. Hit every major angle.

0/8

Keep pushing the concepts until they stick.

Board-Style Walkthrough

Original board-style vignettes. Shuffled, never-repeat, full explanations for every choice.