Glucose-coupled sodium absorption remains intact even in secretory diarrhoea
Expansion
In secretory diarrhoea such as cholera, the toxin raises cyclic AMP and switches on chloride and water secretion. Crucially, it leaves the sodium-glucose cotransporter SGLT1 in the villus intact.
Providing glucose and sodium together therefore drives absorption:
- SGLT1 carries one glucose with two sodium ions into the enterocyte
- Water follows osmotically, by solvent drag
- Net absorption exceeds secretion, and the patient rehydrates despite continuing diarrhoea
Composition matters. Modern reduced osmolarity solution (about 245 mOsm/l, with 75 mmol/l each of sodium and glucose) is superior to the older hyperosmolar formula, which could itself worsen diarrhoea osmotically.
This is why plain water is inadequate (no sodium to absorb) and why sugary drinks such as cola are unsuitable (far too much sugar and too little sodium, so they are hyperosmolar and worsen losses).
Oral rehydration therapy is credited with saving tens of millions of lives, and is a striking example of a simple intervention derived directly from membrane physiology.