Digestion and absorption of nutrients primarily occurs in the small intestine.
Carbohydrate
Fat
Lipase - secreted from the exocrine pancreas
Protein
ingested food => polypeptides
Hydrolysis of these peptides by proteolytic enzymes - pepsin and trypsin which are secreted as inactive proenzymes pepsinogen and trypsinogen from the stomach and pancreas, respectively.
Activation of trpysinogen to trypsin is achieved by enteropeptidase (or enterokinase), an enzyme produced in the duodenum.
Enteropeptidase deficiency results in defective conversion of the proenzyme trypsinogen to the active enzyme trypsin.
Trypsin, in turn, activates other proteolytic enzymes including chymotrypsin, carboxypeptidase and elastase.
Activation of pepsinogen to pepsin requires an acidic pH (in stomach) and a small amount of preexisting pepsin.
Pepsin initiates the digestive process of proteins in the stomach that is completed by trypsin and other enzymes in the proximal small intestine.
Below is a brief summary of the major hormones involved in food digestion:
| Source | Stimulus | Actions | |
|---|---|---|---|
| Gastrin | G cells in antrum of the stomach | Distension of stomach, vagus nerves (mediated by gastrin-releasing peptide), luminal peptides/amino acids |
Inhibited by: low antral pH, somatostatin | Increases acid secretion by gastric parietal cells, pepsinogen and IF secretion, increases gastric motility, stimulates parietal cell maturation | | CCK | I cells in upper small intestine | Partially digested proteins and triglycerides | Increases secretion of enzyme-rich fluid from pancreas, contraction of gallbladder and relaxation of sphincter of Oddi, decreases gastric emptying, trophic effect on pancreatic acinar cells, induces satiety | | Secretin | S cells in upper small intestine | Acidic chyme, fatty acids | Increases secretion of bicarbonate-rich fluid from pancreas and hepatic duct cells, decreases gastric acid secretion, trophic effect on pancreatic acinar cells | | VIP | Small intestine, pancreas | Neural | Stimulates secretion by pancreas and intestines, inhibits acid secretion | | Somatostatin | D cells in the pancreas & stomach | Fat, bile salts and glucose in the intestinal lumen | Decreases acid and pepsin secretion, decreases gastrin secretion, decreases pancreatic enzyme secretion, decreases insulin and glucagon secretion inhibits trophic effects of gastrin, stimulates gastric mucous production |
Role of the main gastrointestinal hormones