The Ultimate Study Guide for Gastrointestinal Hormones

A lot of people struggle with GI hormones during their Physiology course or during USMLE Step 1 Preparation. Hopefully this will help.

All together there are 8 major hormones in this table. We are going to group these into 3 groups:

1. Hormones that affect the stomach: the first 4 (Gastrin, CKK, Secretin, GIP)
2. The inhibitory hormone: Somatostatin
3. Hormones that affect intestinal motility: the bottom 3 (NO, VIP, and Motilin)

For the most part, the hormone's name will give away its function.
1. Gastrin: this is the only hormone that promotes GASTRIC functions (acid secretion and motility)
2. CKK: “cholecyst” means gallbladder, “kinin” means move. Therefore, this is the only hormone that causes gallbladder contraction. Remember that bile is important for lipid digestion (formation of micelles, etc.). What else is important for lipid digestion? Pancreatic lipase! Therefore, CKK also increases secretion of Pancreatic enzymes. The fact you need to memorize here is that CKK decreases gastric emptying.
3. Secretin: promotes pancreatic HCO3- secretions. Since this hormone promotes an alkaline pH in the duodenum, it also inhibits gastric acid secretion.
4. GIP: GIP stands for 2 things
     a. Gastric inhibitory polypeptide: referring to its function in reducing gastric acid secretion.
     b. Glucose dependent insulinotropic polypeptide.
5. Somatostatin: inhibits all secretions (acid, pepsinogen, HCO3-, pancreatic enzymes, insulin, glucagon). It does not affect motility.
6. NO: you know that nitric oxide is the ultimate smooth muscle relaxer, so it reduces motility and relaxes sphincters.
7. VIP: “active” indicates that it increases intestinal motility.
8. Motilin: you can tell from its name that this hormone also promotes intestinal motility.

Go over the actions one more time just to make sure you’ve got them. Then move on to Regulation:
1. Stomach distension increases motility of the stomach (via Gastrin) and intestine (via VIP). When the stomach’s full, you want the food to move along to the next part of the GIT.
2. Acidity: when the stomach is too acidic, you’ll want to bring the pH back to normal by decreasing acid secretion. This is done by decreasing Gastrin (which normally increases acid secretion), and increasing secretin and Somatostatin (which decrease acid secretion).
3. Glucose: when there’s glucose in your GIT, you’ll want to get ready for it by having insulin available. This is done via GIP. Note that GIP secretion is also stimulated by amino acids and fatty acids.
4. Fatty acids and amino acids each increase the secretion of 3 hormones.
     a. Both amino acids and fatty acids increase the secretion of CCK and GIP.
     b. Amino acids also increase secretion of Gastrin [think: proteins are digested by pepsin, which is derived from activation of pepsinogen by acid ].
     c. Fatty acids also increase secretion of Secretin [think fatty acids are digested by pancreatic lipases  which require an alkaline pH to function]
5. Vagal stimulation increases Gastrin and VIP. It also decreases Somatostatin.