Digestion and absorption of fat and fat-soluble vitamins
The four steps of fat absorption (emulsification, lipolysis, micelle formation, chylomicron assembly), the bile acid enterohepatic circulation and why short and long ileal resections cause different diarrhoea, medium-chain triglycerides, fat-soluble vitamin deficiencies, and the tests that localise steatorrhoea, with a worked bile acid pool calculation.
Tier 2Medical sciences concept
Written with AI assistance and fact-checked against the sources below. For exam study, not clinical decisions. How notes are made
At a glance
- Fat absorption needs four things to work: lipase (with colipase), bile acids above their critical micellar concentration, a healthy mucosa, and lymphatic export of chylomicrons. Steatorrhoea localises to whichever step failed.
- The terminal ileum reclaims bile acids. Lose a short segment and bile acids spill into the colon and cause secretory diarrhoea; lose a long segment and the pool collapses and fat is malabsorbed.
- Vitamins A, D, E and K travel in micelles, so any cause of steatorrhoea or cholestasis can deplete them.
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Sources
- Sleisenger and Fordtran's Gastrointestinal and Liver Disease (11th ed.)Elsevier (Feldman, Friedman, Brandt, 2021), 2021
- Guyton and Hall Textbook of Medical Physiology (14th ed.)Elsevier (Hall, Hall, 2021), 2021
- Smith RC et al. Summary and recommendations from the Australasian guidelines for the management of pancreatic exocrine insufficiency. Pancreatology 2016;16(2):164-80AU/NZAustralasian Pancreatic Club, 2016
- Wedlake L et al. Systematic review: the prevalence of idiopathic bile acid malabsorption as diagnosed by SeHCAT scanning in patients with diarrhoea-predominant irritable bowel syndrome. Aliment Pharmacol Ther 2009;30(7):707-17Alimentary Pharmacology and Therapeutics, 2009
- Last updated
- 14 November 2026
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