Protein digestion

Gastric phase: pepsins form oligopeptides and free amino acids, which stimulate secretin and cholecystokinin secretion.

Intestinal phase:

Luminal phase: pancreatic proteases and peptidases (three endopeptidases and two exopeptidases).

Brush border phase: amino acid absorption. Aminopeptidases in the brush border membrane hydrolyse oligopeptides further. Amino acid transport systems transport free amino acids. A peptide transport system transports dipeptides and tripeptides.

Fat-soluble vitamin absorption

Vitamin A: passive diffusion Vitamin D: passive diffusion Vitamin E: passive diffusion Vitamin K:

K1 or phytomenadione: carrier mediated uptake K2 or multiprenyl menaquinones: passive diffusion

Tests for intestinal absorption l

Fat absorption e

72 hour stool fat: >6 grams per day on a 60-100 gram fat diet is abnormal t

C14-triolein breath test e

Prothrombin time and international normalised ratio Serum vitamin D

Carbohydrate absorption

Lactose/hydrogen breath test Flat oral glucose tolerance curve

Vitamin B12

Schilling test

Bile acid

SeHCAT test

There are five major sodium-dependent amino acid transport systems in the brush border of the enterocyte: Neutral amino acids Anionic amino acids Imino acids Neutral amino acids Cationic amino acids and cystine Basolateral membrane amino acid transport systems export amino acids into the portal circulation.

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