Gut Peptide Dysregulation in IBS

Peptide Therapies for IBS

6+ peptides altered

At least six gut peptide hormones show abnormal levels in IBS patients, including reduced serotonin and PYY cell densities in the colon and elevated CCK and VIP in plasma.

El-Salhy et al., Digestive Diseases and Sciences, 2012

El-Salhy et al., Digestive Diseases and Sciences, 2012

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Irritable bowel syndrome affects 10 to 15% of the global population, yet no approved treatment addresses the underlying biology. IBS is classified as a "functional" disorder, meaning that standard diagnostic tests reveal no structural abnormality. But research over the past two decades has revealed that IBS patients have measurable, reproducible alterations in the peptide hormones that regulate gut motility, secretion, pain signaling, and the communication axis between gut and brain. These peptide abnormalities are not incidental findings; they map directly onto the cardinal symptoms of IBS: abdominal pain, altered bowel habits, visceral hypersensitivity, and disordered motility. For an overview of peptide-based treatments for this condition, see plecanatide: the uroguanylin analog for IBS-C.

Serotonin: The Gut's Master Regulator Gone Wrong

Approximately 95% of the body's serotonin (5-hydroxytryptamine, 5-HT) is produced in the gut by enterochromaffin (EC) cells embedded in the intestinal lining. Serotonin activates the submucosal sensory branch of the enteric nervous system and controls gastrointestinal motility and chloride secretion via interneurons and motor neurons. It is the single most important signaling molecule in gut function.

El-Salhy et al. (2012) quantified serotonin cell densities in colonic biopsies from 41 IBS patients and 20 controls using computerized image analysis.^[1] Serotonin cell densities were significantly reduced in the colon of IBS patients. This finding has been replicated across multiple studies and IBS subtypes, though the direction of the abnormality differs: circulating serotonin levels tend to rise after meals in IBS-D (diarrhea-predominant) patients and remain suppressed in IBS-C (constipation-predominant) patients. The common thread is dysregulation, not simply too much or too little.

The serotonin transporter (SERT), which removes serotonin from the synaptic space and terminates its signaling, is also altered in IBS. Reduced SERT expression means serotonin remains active longer, potentially explaining the exaggerated gut responses (cramping, urgency, hypersensitivity) characteristic of IBS-D. Cui et al. (2026) demonstrated that a novel hyaluronic acid-ethylenediamine-cinnamic acid conjugate attenuated IBS-D symptoms in a rat model by regulating 5-hydroxytryptamine signaling pathways, providing evidence that targeting serotonin dysregulation pharmacologically can reduce visceral hypersensitivity and restore normal motility patterns.^[2]

Peptide YY: The Brake That Fails

Peptide YY (PYY) is produced by L-cells in the ileum, colon, and rectum. It serves as the gut's primary brake mechanism, slowing motility (the "ileal brake"), stimulating water and electrolyte absorption, and inhibiting prostaglandin E2 and vasoactive intestinal peptide (VIP), both of which drive fluid secretion. When PYY signaling is deficient, the intestine moves content too quickly and absorbs too little fluid.

El-Salhy (2020) published a comprehensive review establishing that PYY concentration and PYY cell density are decreased in both the colon and rectum of IBS patients but remain unchanged in the ileum.^[3] The mechanism may involve decreased stem cell differentiation toward the endocrine cell lineage, reducing the total number of PYY-producing cells in the large intestine.

PYY also modulates visceral sensitivity through its interaction with serotonin. It regulates intestinal motility, secretion, and absorption as well as visceral sensitivity via modulating serotonin release. An abnormality in PYY therefore contributes to both the dysmotility and the visceral hypersensitivity seen in IBS.

A clinically relevant finding from El-Salhy's work is that dietary intervention with a low-FODMAP diet can restore PYY cell density in the large intestine and improve abdominal symptoms in IBS patients. This suggests the PYY abnormality is at least partially reversible through changes in the luminal environment, rather than being a fixed genetic defect.

CCK, VIP, and NPY: The Hormonal Triad

Zhang et al. (2008) measured five gut peptides in both plasma and sigmoid colon tissue from 40 IBS patients and 15 matched controls, producing one of the most comprehensive peptide profiles of the IBS gut.^[4]

Cholecystokinin (CCK) was elevated in both plasma and sigmoid tissue of IBS patients compared to controls. There was a significant positive correlation between mucosal CCK concentration and pain scores. CCK stimulates gallbladder contraction, pancreatic enzyme secretion, and gut motility. Elevated CCK in the sigmoid may directly contribute to the exaggerated colonic contractions and pain that characterize IBS. For more on CCK's normal function, see cholecystokinin: the peptide that tells your brain you're full.

Vasoactive intestinal peptide (VIP) was elevated in both plasma and sigmoid tissue of IBS patients. VIP relaxes smooth muscle and stimulates intestinal fluid secretion. Elevated VIP in IBS could contribute to the secretory component of diarrhea and the bloating that patients frequently report.

Neuropeptide Y (NPY) was decreased in both plasma and sigmoid tissue. NPY normally counteracts VIP's secretory effects and promotes fluid absorption. The combination of elevated VIP and decreased NPY creates a pro-secretory imbalance that favors diarrhea. Plasma NPY was lower in IBS-D patients than in IBS-C patients, consistent with its role in fluid balance.

Substance P and somatostatin showed trends in IBS patients but did not reach statistical significance in this study. Other research has documented elevated substance P in IBS-D, where it may contribute to mast cell activation and neurogenic inflammation.

GLP-1 and the L-Cell Hypothesis

O'Malley (2019) proposed a unifying hypothesis for GLP-1's role in IBS pathophysiology, published in Experimental Physiology.^[5] The central argument is that L-cells, the enteroendocrine cells that secrete GLP-1 (and co-secrete PYY), function as biosensors that detect IBS-related changes in the luminal environment.

In the small intestine, L-cells respond primarily to nutrients. In the distal gut, they are activated by luminal factors including short-chain fatty acids, bile acids, and microbial metabolic products. All three of these are altered in IBS patients: the microbiome composition shifts, bile acid metabolism changes, and short-chain fatty acid production is modified. GLP-1 released from these activated L-cells can act as a hormone, a paracrine factor, or a neuromodulatory factor, influencing gut motility, visceral sensitivity, and signaling along the gut-brain axis.

GLP-1 also interacts with the hypothalamic-pituitary-adrenal (HPA) stress axis and the immune system, both of which are activated in IBS. A GLP-1 mimetic (exenatide) has been found to alleviate acute pain symptoms in IBS patients in a small clinical study, providing direct evidence that manipulating GLP-1 signaling can affect IBS symptoms. This positions GLP-1 not just as a metabolic hormone but as a potential integrator of the multiple pathophysiological pathways that converge in IBS. For more on the gut's complete peptide signaling network, see every peptide hormone your gut produces.

Insulin-Like Peptide 5: A Newly Identified Player

Bannon et al. (2026) published a study in Gut describing insulin-like peptide 5 (INSL5) as a newly measurable enteroendocrine hormone relevant to diarrheal conditions.^[6] INSL5 is produced by the same colonic L-cells that secrete GLP-1 and PYY. Using a novel immunoassay, the researchers measured INSL5 in human blood for the first time and found that rectal bile acid administration stimulated INSL5 release.

In patients with bile acid diarrhea (a condition that overlaps with IBS-D), INSL5 levels were elevated and correlated with diarrhea severity. Animal studies have shown that INSL5 promotes colonic motility. If INSL5 is chronically elevated in patients with bile acid-driven diarrhea, it may represent an additional pro-motility signal that accelerates colonic transit and worsens diarrhea symptoms.

This study opens a new dimension in the gut peptide picture. The traditional focus on serotonin, PYY, and CCK may be missing important signals from less-studied peptides like INSL5 that could explain why targeting any single peptide pathway has produced only modest clinical results. The gut's signaling network is more redundant and interconnected than any single-peptide model suggests.

The Gut-Brain Axis: Where Peptides Meet Psychology

Lorsch et al. (2026) published a comprehensive review in the Journal of Clinical Investigation examining the mechanisms and clinical implications of gut-brain interactions.^[7] The gut-brain axis is bidirectional: psychological stress alters gut peptide secretion, and altered gut peptide signaling feeds back to the central nervous system, affecting mood, pain perception, and stress responses.

This bidirectionality explains a persistent clinical observation in IBS: stress worsens symptoms, and symptoms worsen stress. The peptide hormones discussed throughout this article are not merely local regulators of gut function. Serotonin, PYY, CCK, GLP-1, and others all have receptors in the central nervous system and influence brain circuits involved in pain processing, anxiety, and autonomic regulation. When these peptide signals are dysregulated in the gut, the information transmitted to the brain is also distorted, contributing to the visceral hypersensitivity, amplified pain perception, and psychological comorbidity that characterize IBS.

Corticotropin-releasing factor (CRF), a peptide that mediates the body's stress response, has direct effects on colonic motility and is elevated in IBS patients under stress. The interplay between CRF from the brain and local peptide signals from the gut creates a feedback loop where psychological state and gut function become mutually reinforcing.

What This Means for Treatment

The peptide data in IBS reveals a multi-system signaling disorder, not a single broken pathway. At least six peptide hormones (serotonin, PYY, CCK, VIP, NPY, GLP-1) are altered in IBS patients, with emerging evidence for additional players like INSL5. Current approved treatments target only fragments of this picture: linaclotide and plecanatide act on guanylate cyclase C to increase chloride secretion; alosetron (5-HT3 antagonist) targets serotonin signaling in IBS-D. For details on how linaclotide treats IBS with constipation, see our dedicated article.

The finding that dietary intervention (low-FODMAP) can restore PYY cell density suggests that some peptide abnormalities are consequences of the altered luminal environment rather than primary causes of IBS. This has implications for how aggressively pharmaceutical versus dietary interventions should be pursued.

No treatment currently addresses the full spectrum of peptide dysregulation in IBS. Whether a multi-target peptide approach or a single upstream intervention (perhaps targeting the L-cell as a master regulator) would be more effective remains an open question. For the full landscape of peptide therapies for IBS, see our overview.

The Bottom Line

IBS patients show reproducible abnormalities in at least six gut peptide hormones, including reduced serotonin and PYY cell densities, elevated CCK and VIP, and decreased NPY. GLP-1 and the newly characterized INSL5 add additional layers of complexity. These peptide changes map onto the core symptoms of IBS and operate within the bidirectional gut-brain axis. Current treatments target individual peptide pathways, but the evidence increasingly suggests IBS is a multi-peptide signaling disorder that may require broader therapeutic approaches.

Frequently Asked Questions

What peptides are altered in IBS?

At least six gut peptide hormones show abnormal levels in IBS patients. Serotonin and peptide YY (PYY) cell densities are reduced in the colon (El-Salhy et al., 2012). Cholecystokinin (CCK) and vasoactive intestinal peptide (VIP) are elevated, while neuropeptide Y (NPY) is decreased in both plasma and sigmoid tissue (Zhang et al., 2008). GLP-1 signaling through colonic L-cells is also altered (O'Malley, 2019).

Does serotonin cause IBS symptoms?

Serotonin dysregulation contributes to IBS symptoms but does not cause the disorder alone. About 95% of the body's serotonin is produced in the gut, where it controls motility and secretion. IBS-D patients tend to have exaggerated postprandial serotonin release, while IBS-C patients show suppressed release (El-Salhy et al., 2012). Reduced serotonin transporter expression prolongs serotonin signaling, potentially explaining urgency and cramping.

What is PYY and why is it low in IBS?

Peptide YY (PYY) is a hormone produced by L-cells in the colon and rectum that slows gut motility, promotes fluid absorption, and acts as the intestine's "brake." PYY cell density is decreased in the colon and rectum of IBS patients, possibly due to reduced stem cell differentiation toward endocrine cells (El-Salhy, 2020). A low-FODMAP diet has been shown to restore PYY cell density and improve symptoms.

Can diet fix gut peptide problems in IBS?

Dietary intervention with a low-FODMAP diet has been shown to restore PYY cell density in the large intestine of IBS patients and improve abdominal symptoms (El-Salhy, 2020). This suggests that at least some peptide abnormalities in IBS are reversible consequences of the altered luminal environment rather than fixed defects. The luminal environment directly influences enteroendocrine cell function and peptide secretion.

How does the gut-brain axis relate to IBS?

The gut-brain axis is a bidirectional communication system where gut peptide hormones signal the brain and brain-derived stress hormones alter gut function. In IBS, dysregulated gut peptides send distorted signals to brain circuits involved in pain processing and stress responses, while stress hormones like CRF alter colonic motility and peptide secretion (Lorsch et al., 2026). This creates a self-reinforcing cycle.

What is INSL5 and how does it relate to diarrhea?

Insulin-like peptide 5 (INSL5) is a newly characterized hormone produced by colonic L-cells that promotes colonic motility. A 2026 study found INSL5 levels were elevated in patients with bile acid diarrhea and correlated with diarrhea severity (Bannon et al., 2026). INSL5 is released in response to bile acids in the rectum, adding a new dimension to the understanding of diarrheal conditions that overlap with IBS-D.