The reasoning: luminal → mucosal → post‑mucosal
| Nutrient | Luminal phase | Mucosal (brush border and enterocyte) | Exit |
|---|---|---|---|
| Carbohydrate | salivary and pancreatic α‑amylase → maltose, maltotriose, α‑limit dextrins | lactase, sucrase–isomaltase, maltase–glucoamylase; SGLT1 (glucose, galactose), GLUT5 (fructose) | GLUT2 → portal blood |
| Protein | pepsin, trypsin, chymotrypsin, elastase, carboxypeptidases | aminopeptidases; amino‑acid transporters (Na⁺‑dependent and independent); PepT1 (H⁺‑coupled di/tripeptides) | amino acids → portal blood |
| Fat | emulsification (bile salts), lipase + colipase → 2‑monoglyceride + FFA; micelles | diffusion into enterocyte; re‑esterification in smooth ER; apoB‑48 + MTP → chylomicron | lacteals → thoracic duct (MCTs go directly to portal blood) |