New Study Identifies Gut Hormone INSL5 as Key Factor in Chronic Diarrhea and Irritable Bowel Syndrome with Diarrhea

High levels of a hormone found in cells in the gut could underlie many cases of chronic diarrhea and help explain up to 40% of cases of patients with irritable bowel syndrome with diarrhea, according to a new study led by scientists at the University of Cambridge. This groundbreaking research, published in the esteemed journal Gut, not only sheds new light on the complex mechanisms of digestive disorders but also holds significant promise for the development of a much-needed diagnostic blood test and points towards a novel therapeutic avenue for millions of sufferers worldwide.

The Enigma of Bile Acid Diarrhea

The digestive process, a marvel of biological engineering, begins with the consumption of food. The liver, a vital organ, plays a crucial role by releasing bile acids into the upper part of the small intestine. These potent compounds are essential for breaking down dietary fats, facilitating their absorption into the bloodstream. Once their primary function is fulfilled in the small intestine, bile acids are efficiently reabsorbed into the body at its lower reaches, completing a vital cyclical process.

However, for a significant portion of the population, this intricate system falters. Approximately one in every 100 individuals is affected by a condition known as bile acid diarrhea (BAD), also referred to as bile acid malabsorption (BAM). In these cases, the bile acid fails to be properly reabsorbed in the small intestine. Instead, an excess amount of bile acid travels into the large intestine, or colon. This presence in an area where it is not meant to be triggers a cascade of uncomfortable and often debilitating symptoms, including urgent, watery diarrhea, and in severe instances, can lead to episodes of incontinence, profoundly impacting a patient’s quality of life.

The diagnostic challenge posed by BAD has long been a significant hurdle. Currently, there are no routine clinical blood tests that can definitively identify this condition. This diagnostic void has led to a considerable number of individuals with BAD being misdiagnosed with irritable bowel syndrome (IBS), a broad umbrella term encompassing a variety of functional gastrointestinal disorders. It is estimated that as many as one in 20 people globally may have IBS. Within this large patient group, a staggering one in three individuals whose primary symptom is diarrhea is believed to be suffering from undiagnosed bile acid diarrhea. This means a substantial number of people are living with a treatable condition without knowing its true cause, often receiving management strategies that do not address the root of their discomfort.

Unveiling the Role of INSL5: A Decade of Research and Collaboration

The Cambridge-led study builds upon earlier research, primarily conducted in animal models, that hinted at the involvement of a specific gut hormone in the pathology of chronic diarrhea. This hormone, known as Insulin-Like Peptide 5 (INSL5), is naturally present in specialized cells located at the distal end of the colon and in the rectum. Previous observations in mice suggested that these INSL5-producing cells are stimulated to release the hormone when irritated by the presence of bile acid. This initial discovery laid the groundwork for the Cambridge team to investigate whether INSL5 plays a similar, crucial role in chronic diarrhea experienced by humans.

The feasibility of this investigation was significantly enhanced by the development of a sophisticated new antibody test by the pharmaceutical company Eli Lilly. The University of Cambridge researchers, in collaboration with Eli Lilly, leveraged this innovative test, which possesses the remarkable sensitivity required to detect and quantify even minute quantities of INSL5 in biological samples. This technological advancement was pivotal in enabling the detailed analysis that underpinned the current findings.

Chronology of Discovery: From Mouse Models to Human Validation

The journey to understanding INSL5’s role in human digestion has been a progressive one, marked by key studies and collaborations.

• Early Pre-clinical Observations: Studies in mice first identified INSL5 in gut cells and suggested its release in response to irritation, potentially linking it to diarrheal symptoms.
• The GLP-1 Enema Study (circa early 2010s): A study conducted at the University of Adelaide explored the release of the gut hormone GLP-1, a hormone known for its role in weight regulation and the basis for modern weight-loss medications. In this study, healthy volunteers were administered a bile acid enema. While the enema successfully triggered GLP-1 release, it also had the unintended consequence of inducing diarrhea.
• Cambridge Team’s Analysis of Adelaide Data (circa early 2020s): When the University of Cambridge researchers analyzed samples from this earlier Adelaide study, they made a crucial discovery. They found that the bile acid enema led to a temporary but significant surge in INSL5 levels. Crucially, the intensity of the diarrheal response in the volunteers correlated directly with the elevation in INSL5. This observation provided strong correlational evidence suggesting that INSL5 is indeed implicated in the physiological mechanisms underlying chronic diarrhea.
• Analysis of Patient Samples from Imperial College London (circa early 2020s): Further strengthening the link between INSL5 and bile acid diarrhea, the Cambridge team examined samples from patients with diagnosed BAD, provided by Professor Julian Walters at Imperial College London. The results were striking: while INSL5 levels were almost undetectable in healthy individuals, they were markedly elevated in patients suffering from bile acid diarrhea. Moreover, the degree of stool water content, a key characteristic of diarrhea, was directly proportional to the elevated INSL5 levels. This provided compelling evidence that INSL5 is not merely correlated with BAD but is likely a direct contributor to its watery nature.
• Investigating Potential Therapeutic Targets with Nottingham Samples (circa early 2020s): The research then pivoted towards exploring INSL5 as a therapeutic target. Samples from patients with IBS who had been treated with ondansetron, an anti-sickness medication known to block INSL5 activity in mice, were obtained from Professor Robin Spiller at the University of Nottingham. The Cambridge team’s analysis revealed that approximately 40% of these IBS patients exhibited elevated INSL5 levels, even though their BAD had been ruled out through standard diagnostic procedures. Significantly, these patients with raised INSL5 responded best to ondansetron, indicating a potential role for INSL5 in a subset of IBS patients and suggesting ondansetron as a viable treatment option.

Supporting Data and Analytical Insights

The findings of the Cambridge study are bolstered by robust data analysis, revealing critical correlations that underscore the significance of INSL5.

• Correlation between INSL5 levels and Diarrheal Urgency: The initial analysis of samples from the Adelaide study demonstrated a clear link: the higher the INSL5 levels detected after a bile acid enema, the more rapidly volunteers felt the urge to defecate. This suggests a direct role for INSL5 in modulating gut motility and signaling the need for bowel evacuation.
• Elevated INSL5 in Bile Acid Diarrhea Patients: Comparing healthy volunteers to individuals with confirmed bile acid diarrhea, the study found INSL5 levels to be "almost undetectable" in the former and "much higher" in the latter. This stark contrast highlights INSL5 as a potential biomarker for BAD.
• INSL5 Levels and Stool Water Content: The study further quantified the impact of INSL5 on stool consistency. It was observed that "the higher the INSL5 level, the more watery their stool samples." This provides a direct physiological link between elevated INSL5 and the hallmark symptom of watery diarrhea.
• INSL5 Prevalence in IBS Patients Responding to Ondansetron: In a crucial step towards therapeutic application, the analysis of samples from Professor Spiller’s cohort revealed that approximately 40% of IBS patients treated with ondansetron had elevated INSL5 levels. This suggests that a significant portion of individuals presenting with IBS-like symptoms may, in fact, have an underlying issue related to INSL5 dysregulation, potentially stemming from undiagnosed BAD or a related pathway. The finding that these specific patients responded best to ondansetron further solidifies INSL5’s relevance as a therapeutic target.

Expert Commentary and Potential Implications

Dr. Chris Bannon, the study’s first author and a clinical fellow at the University of Cambridge’s Institute of Metabolic Science, expressed considerable enthusiasm about the findings. "This was a very exciting finding because it showed us that this hormone could be playing a big part in symptoms of this misunderstood condition," Dr. Bannon stated. He elaborated on the potential for a diagnostic breakthrough: "It also meant it might allow us to develop a blood test to help diagnose bile acid diarrhea if INSL5 levels are only high in these individuals."

Dr. Bannon highlighted the current diagnostic landscape, noting that physicians typically investigate food intolerances, infections, or inflammation when a patient presents with chronic diarrhea. While research into the gut microbiome has garnered significant attention, he pointed out that "gut hormones have been neglected." However, he emphasized the growing recognition of their critical role in "gut health and weight management," suggesting a paradigm shift in understanding digestive disorders.

The discovery of INSL5’s role opens up a promising new avenue for treatment. The study’s exploration of ondansetron, an existing anti-sickness medication, suggests its potential repurposing for patients with elevated INSL5. While the precise mechanism by which ondansetron alleviates diarrhea in these individuals is still under investigation, its known side effect of constipation offers a clue. The Cambridge team plans to delve deeper into this, aiming to either repurpose ondansetron effectively or develop more targeted therapies.

Currently, the primary treatment for bile acid diarrhea involves bile acid sequestrants. While effective for about two-thirds of patients, this leaves a substantial proportion without adequate relief. The identification of INSL5 as a potential target offers hope for improved treatment efficacy for these non-responders.

Dr. Bannon offered an insightful perspective on the evolutionary purpose of INSL5: "I often get asked why we would have a hormone that gives you diarrhea. I think of it as a kind of poison sensor. Bile acids aren’t meant to be in the colon – they’re an irritant to the colon and they’re toxic to the microbiome. It makes sense that you would have something that detects toxins and helps the body rid itself of them. But a problem develops if it’s always being triggered by bile acid, causing very dramatic symptoms." This analogy elegantly frames INSL5’s function as a protective mechanism gone awry in the context of bile acid malabsorption.

Broader Impact and Future Directions

The implications of this research extend far beyond the immediate diagnosis and treatment of bile acid diarrhea.

• Improved Diagnostic Accuracy: The potential for a simple blood test to diagnose BAD could revolutionize how this condition is identified, reducing misdiagnosis and ensuring patients receive appropriate care sooner. This could significantly alleviate the burden on healthcare systems and improve patient outcomes.
• Personalized Treatment Strategies: By identifying a specific hormonal marker, the study paves the way for more personalized treatment approaches. Patients could be stratified based on their INSL5 levels, allowing clinicians to tailor interventions more effectively, potentially using ondansetron or developing novel drugs that specifically target INSL5 pathways.
• Understanding IBS Heterogeneity: The finding that a significant percentage of IBS patients may have elevated INSL5 levels underscores the heterogeneity of IBS. It suggests that what is currently grouped under the broad IBS umbrella may, in fact, encompass distinct underlying pathophysiological mechanisms, including hormonal dysregulation. This could lead to a reclassification or subtyping of IBS, enabling more precise management.
• Therapeutic Innovation: The identification of INSL5 as a key player in diarrheal signaling presents a novel target for drug development. Beyond repurposing existing medications, this research could spur the creation of entirely new classes of drugs designed to modulate INSL5 activity, offering hope to those who do not respond to current therapies.
• Public Health Significance: Chronic diarrhea and IBS are highly prevalent conditions that significantly impact an individual’s quality of life, leading to lost productivity and increased healthcare costs. By addressing a significant underlying cause, this research has the potential to improve the well-being of millions globally.

The research was generously supported by the Medical Research Council and Wellcome, with additional crucial support from the National Institute for Health and Care Research (NIHR) Cambridge Biomedical Research Centre. This collaborative funding environment underscores the importance and potential impact of this scientific endeavor. The ongoing work by Dr. Bannon and his colleagues, under the leadership of Professors Fiona Gribble and Frank Reimann at the Institute of Metabolic Science, University of Cambridge, promises to further unravel the complexities of gut hormone signaling and its profound influence on digestive health.