Full mechanism, in text

How fat is digested and absorbed

1 · Bile emulsifies → micrometer droplets

Oil and water do not mix, so bile from the gallbladder acts like a detergent, emulsifying large oil droplets into micrometer-scale droplets that enzymes can reach.When fat enters the duodenum, oil (lipid) and water (intestinal fluid) do not mix — large oil droplets have small surface area, and enzymes cannot reach them.

The gallbladder receives the fatty-meal signal → releases bile (containing bile acids + phospholipids + cholesterol) → bile acids are natural amphiphiles (one end hydrophilic, the other lipophilic) → they act like detergents and emulsify the large oil droplets into micrometer-scale droplets.

Surface area increases 10,000× → enzymes can act efficiently.

The health of bile-acid synthesis + secretion depends on hepatobiliary function — cirrhosis / biliary obstruction / cholecystectomy significantly reduce emulsification efficiency, dampening fat digestion and dropping absorption of fat-soluble vitamins (A/D/E/K) along with it.

2 · Pancreatic lipase + colipase hydrolyze

Pancreatic lipase, anchored to the droplet surface by colipase, cuts each into one monoacylglycerol and two free fatty acids.Pancreatic lipase is the enzyme that actually cleaves fat — it cuts (TG) into:

TG → monoacylglycerol (MG) + 2 free fatty acids (FFA)

But lipase alone is unstable at the water-oil interface — so it has a partner: colipase — a small accessory protein that anchors lipase at the lipid-phase interface of the emulsified droplet.

The pancreas secretes it in zymogen form, alongside the colipase precursor; both are activated in the small intestine by trypsin.

Orlistat (weight-loss drug) directly inhibits pancreatic lipase → 30% of fat is not digested → simply excreted (this is the chemical basis of weight loss, and also why the side effect is steatorrhea).

3 · Mixed micelles deliver to gut wall

Bile acids wrap FA, MG and fat-soluble vitamins into mixed micelles, which carry them across the watery gut lumen to the enterocyte brush border.The cleaved FA + MG cannot swim across the aqueous phase (they are lipid-soluble), yet they must cross the watery gut lumen to reach the enterocyte brush border — what now?

Bile acids wrap them into mixed micelles —

· Diameter 4-6 nm
· Inner layer: FA + MG + cholesterol + fat-soluble vitamins (A/D/E/K) + carotenoids
· Outer layer: bile acids (hydrophilic heads outward, hydrophobic tails inward)

This micelle is a "water-soluble lipid package" — it can freely diffuse through water to the enterocyte brush border, where it releases its contents into the enterocyte.

Key: fat-soluble vitamins (A/D/E/K) absorb best when taken with fat — they need to ride the micelle.

Practical: meal fat can raise vitamin D absorption. Dawson-Hughes 2015 gave 50 healthy older adults a single 50,000 with a fat-free meal vs 30% fat meals: 12-hour plasma D3 peak was 32% higher after the fat meals (95% 11-52%). That is not less than half absorbed when fasting, and not a doubling.

4 · Reassemble → ride lymph (not portal)

Enterocytes reassemble FA and MG into and pack it into chylomicrons, which are too large for normal capillaries and so travel by lymph, not the portal vein.Inside the enterocyte, after FA + MG are absorbed, the smooth endoplasmic reticulum reassembles them into + coats them with apolipoprotein B-48 (-48) + phospholipid + cholesterol → forming large chylomicrons (CM) — diameter 80-1200 nm, hundreds of times larger than a micelle.

CM do not enter the portal vein; they enter the lacteal (central lymphatic capillary in the intestinal villus) → travel via the thoracic duct → drain into the veins where the left subclavian and internal jugular veins meet → enter systemic circulation.

Why the lymph route: because CM particles are too large to pass the endothelial gaps of normal capillaries, but the endothelial clefts of lymphatic capillaries are wide enough to admit them.

Downstream:
· TG on CMs is hydrolyzed by lipoprotein lipase () at the capillary walls of peripheral tissues (adipose / muscle) → FFA released for tissue uptake
· CM remnants are taken up by the liver
· Bile acids are efficiently reabsorbed back to the liver at the terminal ileum — the enterohepatic cycle repeats 6-8 times per day

Practical implications:
· Post-meal blood lipid peaks 3-4 hours late — this is why lipid panels are drawn fasting
· Fat-soluble vitamins need fat and a healthy hepatobiliary system
· People post-cholecystectomy tolerate large-fat meals poorly (continuous bile drip vs pulsatile release) — they can manage by small frequent meals + MCT oil (medium-chain triglycerides, which don't need bile and go directly through the portal vein)

Full mechanism, in text

1 · Bile emulsifies → micrometer droplets

2 · Pancreatic lipase + colipase hydrolyze

3 · Mixed micelles deliver to gut wall

4 · Reassemble → ride lymph (not portal)

Fat digestion → micelles → lymph

Full mechanism, in text

1 · Bile emulsifies → micrometer droplets

2 · Pancreatic lipase + colipase hydrolyze

3 · Mixed micelles deliver to gut wall

4 · Reassemble → ride lymph (not portal)

Fat digestion → micelles → lymph