Secretin and Gastrin: Your Digestive Juice Peptides

Gut Peptide Hormones

124 Years Studied

Secretin was the first hormone ever identified, discovered by Bayliss and Starling in 1902. It launched the entire field of endocrinology.

Chey & Chang, Journal of Gastroenterology, 2003

Chey & Chang, Journal of Gastroenterology, 2003

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Your stomach produces hydrochloric acid strong enough to dissolve metal. That acid is essential for breaking down protein and killing bacteria, but it would destroy your duodenum if left unchecked. Two peptide hormones manage this balance: gastrin drives acid production up, and secretin brings it back down. Together they form a feedback loop that has operated in every meal you have ever eaten.

Secretin holds a unique place in science. On January 16, 1902, physiologists William Bayliss and Ernest Starling extracted a substance from the duodenal mucosa of a dog, injected it intravenously, and observed pancreatic secretion. That substance was secretin, and its discovery introduced the concept of chemical messengers carried through the bloodstream, a class Starling later named "hormones."^[1] For context on the broader family of digestive peptides, see Every Peptide Hormone Your Gut Produces: A Complete Guide. For the peptide that drives the gut's cleaning waves between meals, see Motilin: The Peptide Behind Your Stomach's Cleaning Waves.

Gastrin: the acid accelerator

Gastrin is produced by G cells, specialized enteroendocrine cells concentrated in the pyloric antrum of the stomach, with smaller populations in the duodenum. The primary circulating form is gastrin-17 (G-17), a 17-amino-acid peptide, though a longer form, gastrin-34 (G-34, or "big gastrin"), is also released and has a longer half-life in circulation.

Three signals trigger gastrin release. First, amino acids and small peptides from partially digested protein directly stimulate G cells through calcium-sensing receptors and amino acid transporters on their apical surface. Anjom-Shoae et al. (2025) reviewed the calcium-sensing receptor's role in regulating GI hormone secretion, confirming that this receptor on G cells is a primary sensor for luminal amino acids.^[5] Second, mechanical distension of the stomach wall activates stretch receptors that relay signals to G cells via enteric neural pathways. Third, vagal nerve stimulation releases gastrin-releasing peptide (GRP, also called bombesin) from postganglionic neurons, which activates GRP receptors on G cells.^[3]

Once released, gastrin reaches parietal cells in the gastric fundus and body through the bloodstream. It binds cholecystokinin-B (CCK-B) receptors on parietal cells, stimulating the H+/K+-ATPase proton pump that exchanges potassium ions for hydrogen ions, creating hydrochloric acid. Gastrin also binds CCK-B receptors on enterochromaffin-like (ECL) cells, stimulating histamine release. That histamine then acts on H2 receptors on adjacent parietal cells, amplifying acid secretion. This gastrin-ECL-parietal cell axis is why H2 receptor blockers (famotidine, ranitidine) reduce acid output even though they don't directly target gastrin signaling.

The negative feedback loop is straightforward: as gastric pH falls below 3, somatostatin-producing D cells in the antrum are activated. Somatostatin directly inhibits G cells through paracrine signaling, shutting down gastrin release. This pH-somatostatin-gastrin circuit prevents runaway acid production during normal digestion. Dockray (2014) reviewed the dialogue between gut hormones and the brain, showing that gastrin and other GI peptides participate in integrated neural-hormonal circuits that extend beyond local feedback loops.^[3]

Beyond acid secretion, gastrin has trophic (growth-promoting) effects on the gastric mucosa. Khan et al. (2018) documented that gastrin family peptides are expressed in pancreatic islets and play roles in beta-cell function and survival, extending gastrin's biological significance beyond the stomach.^[4] This trophic activity becomes clinically relevant in conditions of chronic hypergastrinemia, where sustained elevated gastrin levels may promote mucosal hyperplasia.

Secretin: the acid neutralizer

Secretin is a 27-amino-acid peptide produced by S cells, enteroendocrine cells scattered through the duodenal and jejunal mucosa. Its primary release trigger is luminal acidity: when partially digested stomach contents (chyme) enter the duodenum and lower the local pH below approximately 4.5, S cells release secretin into the bloodstream.^[1] Fatty acids in the duodenal lumen also stimulate secretin release, though the acid trigger is dominant.

Secretin acts on three primary targets. On pancreatic ductal cells, it stimulates copious secretion of bicarbonate-rich fluid. This pancreatic juice neutralizes the acid arriving from the stomach, raising duodenal pH back toward 6-7 and protecting the intestinal mucosa from acid damage. On the stomach, secretin inhibits gastric acid secretion from parietal cells, reducing the flow of acid into the duodenum. On hepatocytes and biliary epithelium, secretin stimulates bile flow (choleresis), increasing the volume of bile delivered to the duodenum for fat digestion.

The secretin receptor (SCTR) is a class B G protein-coupled receptor that activates adenylyl cyclase, raising intracellular cAMP. This places it in the same receptor superfamily as receptors for VIP, glucagon, GLP-1, and GLP-2, all peptides discussed elsewhere on this site. For VIP specifically, see Vasoactive Intestinal Peptide (VIP): The Gut's Master Regulator. For the intestinal repair peptide GLP-2, see GLP-2: The Intestinal Growth Factor That Repairs Your Gut Lining.

The 2003 comprehensive review by Chey and Chang catalogued a century of secretin research milestones: isolation and purification, amino acid sequencing, chemical synthesis, radioimmunoassay development, receptor cloning, and identification of a secretin-releasing peptide. They also documented secretin's role in regulating gastric motility and its presence in the central nervous system, where its function as a neuropeptide is still being defined.^[1]

How secretin and gastrin oppose each other

The gastrin-secretin interaction is a classic example of peptide hormone counterbalancing. After a meal, the sequence unfolds in a predictable order.

Phase 1 (gastric phase): Protein in the stomach triggers gastrin release from G cells. Gastrin stimulates acid secretion. Stomach pH drops to 1.5-2.0, creating the acidic environment needed for pepsin activation and protein denaturation.

Phase 2 (intestinal phase): Acid-rich chyme enters the duodenum. The low pH triggers secretin release from S cells. Secretin simultaneously stimulates pancreatic bicarbonate secretion (to neutralize the arriving acid) and inhibits gastric acid output (to slow the source).

Phase 3 (feedback resolution): As duodenal pH rises from bicarbonate neutralization and gastric acid secretion slows, secretin release diminishes. The system resets for the next bolus of chyme.

This reciprocal arrangement means that gastrin dominates the gastric phase of digestion and secretin dominates the intestinal phase. They do not operate in isolation. Somatostatin from D cells acts as a master brake on both peptides, activated when luminal acidity exceeds what the current digestive phase requires. Cholecystokinin (CCK), released by I cells in response to fats and proteins in the duodenum, works alongside secretin in the intestinal phase, stimulating gallbladder contraction and pancreatic enzyme secretion while secretin handles the bicarbonate and volume component. For more on CCK's satiety signaling, see Cholecystokinin (CCK): The Peptide That Tells Your Brain You're Full.

Secretin and gastric emptying

Beyond its acid-neutralizing role, secretin affects how fast the stomach empties its contents. Brandler et al. (2020) conducted a double-blind, randomized, saline-controlled crossover trial in 10 healthy volunteers and 10 patients with functional dyspepsia, measuring gastric accommodation (by SPECT imaging) and gastric emptying (by scintigraphy) after secretin or placebo administration.^[2]

Compared with placebo, secretin delayed gastric emptying at 30 minutes by 11% (p = 0.004) in healthy subjects and by 8% (p = 0.03) in functional dyspepsia patients. Gastric accommodation, satiation volume, and postprandial symptom scores did not differ between secretin and placebo. Plasma GLP-1, GIP, and hepatic polypeptide levels were also unaffected.

The researchers concluded that secretin receptor activation may represent a novel therapeutic target for patients with functional dyspepsia and rapid gastric emptying, estimated at roughly 20% of functional dyspepsia patients. They noted that sacubitril (a neprilysin inhibitor that prevents secretin degradation, already used in heart failure) could theoretically raise endogenous secretin levels and slow gastric emptying as a secondary benefit.

This finding connects secretin to an active area of GI therapeutics. For comparison, GLP-1 receptor agonists also delay gastric emptying, though through a different receptor system and with more pronounced effects on appetite and body weight.

The secretin stimulation test

One of secretin's most valuable clinical applications is diagnostic. Under normal physiology, secretin suppresses gastrin release from G cells, partly through stimulation of somatostatin secretion. In Zollinger-Ellison syndrome (ZES), a condition caused by gastrin-secreting tumors (gastrinomas), this relationship is inverted: intravenous secretin causes a paradoxical rise in serum gastrin.

The secretin stimulation test works by administering synthetic secretin (2 U/kg intravenously) and measuring serum gastrin at baseline and at 2, 5, 10, 15, and 30 minutes. In patients with gastrinomas, gastrin rises by 120 pg/mL or more above baseline, typically within 2-10 minutes. In healthy individuals, gastrin levels remain flat or decrease. This paradoxical response occurs because gastrinoma cells express secretin receptors that, unlike normal G cells, trigger gastrin release rather than suppression.

The test is particularly useful in patients with intermediate fasting gastrin levels (150-1000 pg/mL) where the diagnosis is uncertain. Very high gastrin levels (above 1000 pg/mL) combined with elevated gastric acid output are usually sufficient for diagnosis without provocation testing. False-positive results can occur in patients with Helicobacter pylori infection, gastric atrophy, or chronic proton pump inhibitor use, all conditions that elevate baseline gastrin through non-tumor mechanisms.

Selective arterial secretin injection (SASI) takes this principle further by injecting secretin into specific arteries supplying the pancreas, liver, or duodenum, then measuring gastrin in hepatic vein samples. A localized rise in gastrin identifies which vascular territory harbors the gastrinoma, providing anatomical localization that standard imaging may miss in tumors smaller than 1 cm.

Proton pump inhibitors and gastrin elevation

Proton pump inhibitors (PPIs) like omeprazole, esomeprazole, and lansoprazole work by irreversibly blocking the H+/K+-ATPase proton pump on parietal cells. By suppressing acid secretion, they also remove the negative feedback signal that keeps gastrin in check. With less acid in the stomach, D cells produce less somatostatin, and G cells receive less inhibitory input. The result is elevated fasting gastrin levels, a condition called secondary hypergastrinemia.

The magnitude varies by individual and PPI dose, but fasting gastrin levels typically rise 2- to 6-fold during chronic PPI therapy. Waldum et al. (2018) documented this phenomenon and reviewed its implications. In most patients, gastrin levels stabilize at a new elevated baseline and return to normal within 1-2 weeks of PPI discontinuation. In some patients, abrupt PPI withdrawal triggers rebound acid hypersecretion, where acid production temporarily exceeds pre-treatment levels, driven by the trophic effect of hypergastrinemia on ECL and parietal cell mass.^[5]

The trophic concern is more nuanced. Chronic hypergastrinemia stimulates ECL cell proliferation. In animal models (particularly rats), prolonged extreme hypergastrinemia can induce ECL cell carcinoid tumors. Whether this translates to clinically meaningful risk in humans taking standard-dose PPIs remains debated, with large epidemiological studies showing inconsistent results. The current evidence does not support a causal link between PPI use and gastric cancer in most patients, but long-term PPI use without a clear indication remains a topic of pharmacovigilance.

Secretin beyond digestion

Secretin receptors are expressed outside the gastrointestinal tract, including in the brain, heart, and kidneys. In the central nervous system, secretin is present in the cerebellum, hippocampus, and hypothalamus. Research has investigated secretin's potential roles in water homeostasis (through interactions with vasopressin signaling) and in social behavior.

The autism-secretin episode remains one of the more cautionary tales in peptide therapeutics. In 1998, a case report described behavioral improvements in an autistic child who received secretin during a diagnostic GI procedure. Media coverage triggered enormous public demand for secretin as an autism treatment. Between 1999 and 2006, at least 13 placebo-controlled trials tested single or multiple doses of secretin in children with autism spectrum disorders. Williams et al. (2011) conducted a systematic review of these trials and found no evidence of benefit for language, cognition, behavior, communication, or socialization. The strength of evidence for lack of effectiveness was rated high, and the authors concluded that secretin for autism warranted no further study.^[6]

This episode illustrates a recurring pattern in peptide biology: a legitimate physiological role in one system does not predict therapeutic value in another. Secretin's functions in pancreatic and gastric physiology are well-established. Its role as a CNS peptide remains poorly understood and has not translated into any approved neurological application.

The enteroendocrine cell connection

Both G cells and S cells are members of the enteroendocrine cell system, the largest endocrine organ in the body by total cell count. These cells are scattered individually throughout the GI epithelium rather than grouped into a discrete gland, which delayed their recognition as a coordinated hormonal system. Crooks et al. (2021) reviewed the enteroendocrine system's role in appetite regulation, gastrointestinal disease, and obesity, showing that this diffuse network coordinates dozens of peptide hormones that regulate motility, secretion, and metabolism.^[6] Miedzybrodzka et al. (2022) further detailed how the enteroendocrine system adapts in obesity, with altered hormone profiles affecting both gut function and systemic metabolism.^[7]

Nwako et al. (2025) demonstrated that enteroendocrine cells also regulate intestinal barrier permeability, a function beyond their classical hormonal role. This finding suggests that the cells producing gastrin and secretin participate in mucosal defense mechanisms independent of their hormone secretion.^[8]

The enteroendocrine system does not operate as isolated cell types producing single hormones. Individual enteroendocrine cells can co-express multiple peptide hormones, and their hormone expression profiles can change in response to local signals. This plasticity means the clean textbook separation of "G cells make gastrin" and "S cells make secretin" is an oversimplification, useful for understanding the dominant output of each cell type but incomplete as a description of how the system actually operates.

For the incretin-producing members of this same cell family, see L-Cells and K-Cells: Where Your Gut Makes Incretin Hormones. For how peptide hormones differ mechanistically from steroid hormones, see Peptide vs Steroid Hormones: Two Fundamentally Different Systems.

What happens when the system fails

When the gastrin-secretin balance breaks down, the clinical consequences are predictable from the biology.

Zollinger-Ellison syndrome (gastrinoma): Unregulated gastrin secretion from a neuroendocrine tumor produces massive acid hypersecretion. Patients develop severe peptic ulcers, often multiple and in atypical locations (distal duodenum, jejunum), along with diarrhea from acid-mediated inactivation of pancreatic enzymes. Fasting gastrin levels exceed 1000 pg/mL in many cases. Prior to the development of PPIs, ZES carried high surgical morbidity. Current management combines PPIs for acid suppression with surgical resection of localized gastrinomas.

Autoimmune gastritis: Antibodies against parietal cells destroy acid-producing capacity. Without acid, the negative feedback on G cells is lost, resulting in chronic hypergastrinemia. The combination of achlorhydria and hypergastrinemia leads to ECL cell hyperplasia and, in some cases, type 1 gastric carcinoid tumors. These are typically small, multiple, and indolent, a direct consequence of sustained gastrin-driven ECL cell proliferation in the absence of acid.

Chronic Helicobacter pylori infection: H. pylori infection of the antrum can suppress somatostatin production from D cells, removing the brake on gastrin release and leading to relative hypergastrinemia and increased acid output. This mechanism contributes to duodenal ulcer formation in H. pylori-positive patients. Eradication of H. pylori typically restores normal somatostatin-gastrin dynamics.

Functional dyspepsia: As the Brandler et al. (2020) trial demonstrated, a subset of functional dyspepsia patients have rapid gastric emptying. For these patients, secretin's ability to slow gastric emptying represents a potential therapeutic mechanism, though secretin receptor agonists have not yet entered clinical development for this indication.^[2]

The Bottom Line

Secretin and gastrin form a two-peptide feedback system that balances acid production against acid neutralization in every digestive cycle. Gastrin drives acid up; secretin brings it down and stimulates the pancreatic bicarbonate that protects the duodenum. Their interaction with somatostatin creates a three-peptide axis that has been the foundation of GI endocrinology since secretin's discovery in 1902. Clinical applications range from the secretin stimulation test for gastrinoma diagnosis to understanding PPI-induced hypergastrinemia, but no secretin-based therapeutic has reached the market for GI indications despite over a century of research.

Frequently Asked Questions

What is the difference between secretin and gastrin?

Gastrin is a peptide hormone released by G cells in the stomach that stimulates hydrochloric acid production from parietal cells. Secretin is released by S cells in the duodenum when acid arrives from the stomach, and it stimulates pancreatic bicarbonate secretion while inhibiting gastric acid output. They have opposing effects: gastrin increases acidity, secretin neutralizes it. Together they maintain the pH balance needed for digestion.

What triggers the release of secretin?

Secretin is released when the pH in the duodenal lumen drops below approximately 4.5, signaling that acidic stomach contents have arrived. Fatty acids in the duodenum also stimulate secretin release, though to a lesser degree. Chey and Chang (2003) documented that secretin release involves both direct chemosensory detection by S cells and neural pathways, with vagal afferents playing a role in amplifying the response.

What is the secretin stimulation test used for?

The secretin stimulation test diagnoses Zollinger-Ellison syndrome, a condition caused by gastrin-secreting tumors (gastrinomas). Intravenous secretin (2 U/kg) normally suppresses gastrin, but gastrinoma cells respond paradoxically by releasing gastrin. A rise of 120 pg/mL or more above baseline within 10 minutes is considered positive. The test is most useful when fasting gastrin is elevated but not high enough for definitive diagnosis (150-1000 pg/mL range).

Do proton pump inhibitors raise gastrin levels?

Yes. PPIs suppress stomach acid, removing the negative feedback that normally keeps gastrin production in check. Fasting gastrin levels typically rise 2- to 6-fold during chronic PPI therapy. Levels return to normal within 1-2 weeks of stopping the medication, though abrupt discontinuation can trigger rebound acid hypersecretion in some patients. Whether long-term PPI-induced hypergastrinemia carries meaningful clinical risk in humans remains debated.

Was secretin really the first hormone discovered?

Secretin was the first substance identified as a hormone. William Bayliss and Ernest Starling demonstrated on January 16, 1902, that an extract from duodenal mucosa could stimulate pancreatic secretion when injected intravenously, proving that a chemical messenger traveled through the blood independently of nerve signals. Starling coined the term "hormone" (from the Greek for "to arouse") three years later. This single experiment launched the field of endocrinology.

Does secretin have any effect on the brain?

Secretin and its receptor are present in several brain regions including the cerebellum, hippocampus, and hypothalamus, but its CNS functions remain poorly defined. Following a 1998 case report suggesting behavioral improvements in an autistic child, 13 placebo-controlled trials tested secretin for autism between 1999 and 2006. A 2011 systematic review found no evidence of benefit for language, cognition, behavior, or social function, with high confidence in this negative conclusion.