Expansion
Amylase to oligosaccharides, brush border enzymes to monosaccharides, then transport
Expansion
- Salivary and pancreatic amylase break starch into maltose, maltotriose and alpha-limit dextrins. Amylase cannot digest cellulose, which passes on as fibre
- Brush border enzymes complete the job: maltase, sucrase-isomaltase and lactase
- Absorption: only monosaccharides are absorbed
- Glucose and galactose by SGLT1, a sodium-dependent cotransporter
- Fructose by GLUT5, facilitated diffusion
- All leave the cell basolaterally by GLUT2
Lactase is the most vulnerable enzyme: it sits at the villus tip, is the last to recover after mucosal injury, and declines with age in most of the world’s population. Undigested lactose is osmotically active and fermented by colonic bacteria, giving bloating, cramps and acidic diarrhoea.
The SGLT1 mechanism underlies oral rehydration therapy: glucose and sodium are absorbed together, dragging water with them, which is why oral rehydration solution contains both, and why it works even in cholera when secretory mechanisms are switched on.