Malabsorption and diarrhoea
- Topic
Diarrhoea can come from water being pulled in, pumped out, food not being absorbed, or an inflamed bowel; each has its own diseases.
What it is
Malabsorption is defective absorption of fats, vitamins, proteins, carbohydrates, minerals and water; its hallmark is steatorrhoea. It arises when one of four phases fails: digestion in the lumen, terminal digestion at the brush border, transport across the epithelium, or lymphatic transport of fat. Diarrhoea, an increase in stool mass, frequency or fluidity, is grouped into four types that a few bedside questions separate. The pathologist's role is to find the lesion behind the pattern.
Function
- Fat-soluble vitamin loss: vitamin K (bleeding), vitamin D and calcium (osteopenia, tetany), vitamin A (night blindness).
- Iron, folate and B12 loss: anaemia and mucositis.
- Protein loss: oedema and muscle wasting.
The four types of diarrhoea
| Type | Mechanism | With fasting | Examples |
|---|---|---|---|
| Secretory | Active chloride and water secretion; isotonic stool | Continues | Cholera, ETEC, neuroendocrine tumours |
| Osmotic | Unabsorbed solute pulls water; stool osmotic gap | Stops | Lactase deficiency, osmotic laxatives |
| Malabsorptive | General failure of nutrient uptake; steatorrhoea | Stops | Coeliac disease, pancreatic insufficiency |
| Exudative (inflammatory) | Damaged mucosa leaks pus and blood | Continues | Shigella, amoebiasis, inflammatory bowel disease |
Clinical notes
- In cystic fibrosis, the defective CFTR channel thickens secretions, blocks pancreatic ducts and causes exocrine pancreatic insufficiency in most patients.
- Irritable bowel syndrome is diagnosed only after the structural causes on this page have been excluded; its biopsies are normal.
Test yourself
Think of the answer, then tap the question to see it.
What are the four phases of nutrient absorption that can fail?
Intraluminal digestion, terminal (brush-border) digestion, transepithelial transport and lymphatic transport of lipids.
Which types of diarrhoea continue during fasting?
Secretory and exudative diarrhoea.
Why does cystic fibrosis cause malabsorption?
Thick secretions block pancreatic ducts, leading to pancreatic insufficiency and failed intraluminal digestion.
Elsewhere in DurgiPedia
- Patterns of malabsorption
- Water and electrolyte absorption and how diarrhea happens
- Fat digestion and absorption
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Celiac Disease (StatPearls (NCBI Bookshelf), 2025); Lactose intolerance: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2025).
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Representative model, for educational purposes only.