Expansion
Emulsification, lipolysis, micelle formation, absorption and chylomicron export
Expansion
- Emulsification by bile salts and mechanical churning, increasing surface area
- Lipolysis by pancreatic lipase with its cofactor colipase, producing free fatty acids and 2-monoglycerides
- Micelle formation with bile salts, carrying products across the unstirred water layer to the brush border
- Absorption by passive diffusion into the enterocyte, with bile salts left behind and reabsorbed later in the terminal ileum
- Re-esterification to triglyceride in the smooth endoplasmic reticulum
- Chylomicron assembly with apoproteins, and export into the lacteal and thence the thoracic duct
Failure at any step causes steatorrhoea:
- No bile salts: obstructive jaundice, ileal resection
- No lipase: chronic pancreatitis, cystic fibrosis
- Damaged mucosa: coeliac disease
- Blocked lymphatics: intestinal lymphangiectasia, Whipple disease
Medium-chain triglycerides bypass steps 3 and 6 entirely and enter the portal blood, which is why they are useful therapeutically in most of these conditions.