By Gilang Cahya 
A groundbreaking study spearheaded by scientists at the University of Cambridge has identified high levels of a specific gut hormone, Insulin-Like Peptide 5 (INSL5), as a potential underlying cause for a significant proportion of chronic diarrhea cases. The research, published in the esteemed journal Gut, suggests that INSL5 could be implicated in up to 40% of patients diagnosed with Irritable Bowel Syndrome with Diarrhea (IBS-D). This pivotal discovery not only sheds light on a previously elusive mechanism driving these debilitating conditions but also paves the way for the development of novel diagnostic tools and therapeutic interventions.
Unraveling the Mystery of Bile Acid Diarrhea
The digestive process involves a complex interplay of hormones and biochemicals, with bile acids playing a crucial role in fat digestion. Following consumption, the liver releases bile acids into the upper small intestine to emulsify fats, facilitating their absorption into the bloodstream. Subsequently, these bile acids are largely reabsorbed in the lower sections of the small intestine, creating a cyclical process essential for nutrient uptake.
However, for a subset of the population, approximately one in 100 individuals, this reabsorption process falters, leading to a condition known as bile acid diarrhea (BAD), also referred to as bile acid malabsorption. In these cases, excess bile acids bypass their intended reabsorption pathway and spill into the large intestine (colon). This influx of bile acids into the colon acts as a potent irritant, triggering frequent, urgent, and watery bowel movements. The severity of these symptoms can significantly impact quality of life, with some patients experiencing episodes of incontinence, leading to profound social and psychological distress.
The diagnostic challenge surrounding BAD has historically been a significant hurdle. The absence of readily available and routine clinical blood tests has meant that many individuals suffering from its symptoms often receive a diagnosis of Irritable Bowel Syndrome (IBS), a broad category encompassing various gastrointestinal disorders. It is estimated that as many as one in 20 individuals worldwide live with IBS. Within this population, a substantial proportion – an estimated one in three patients whose primary symptom is diarrhea – may actually be experiencing undiagnosed BAD. This diagnostic ambiguity has led to delayed or incorrect treatment, prolonging patient suffering and perpetuating a cycle of misunderstanding regarding the true cause of their ailments.
The Emerging Role of INSL5
Previous research conducted in animal models, specifically mice, had hinted at the involvement of INSL5, a gut hormone produced by specialized cells located in the distal colon and rectum. These studies suggested that INSL5 might play a role in the development of chronic diarrhea, with its release being triggered by the presence of bile acids. When these cells in the colon encounter the irritant effect of unabsorbed bile acids, they respond by secreting INSL5.
Building upon these animal studies, researchers at the Institute of Metabolic Science at the University of Cambridge embarked on a mission to investigate whether this same hormonal mechanism was at play in human chronic diarrhea. The breakthrough in their investigation was facilitated by a sophisticated new antibody test developed by the pharmaceutical company Eli Lilly, with whom the Cambridge team is collaborating. This innovative test possesses the remarkable sensitivity to quantify even minute concentrations of INSL5 in biological samples, thereby enabling researchers to precisely measure its levels in human subjects.
Chronology of Discovery and Key Findings
The journey to understanding INSL5’s role in diarrhea has been a methodical process, involving the analysis of data from multiple studies and patient cohorts.
Early Animal Models (Pre-2010s): Initial observations in mice suggested a link between gut hormones and diarrhea, with INSL5 being identified as a potential mediator stimulated by bile acids.
University of Adelaide Study (Circa 2010s): A study at the University of Adelaide explored methods to stimulate the release of the gut hormone GLP-1, a key player in appetite regulation and the target of modern weight-loss medications. In healthy volunteers, administering a bile acid enema to trigger GLP-1 release inadvertently induced diarrhea. Crucially, the Cambridge team’s subsequent analysis of samples from this study revealed a significant temporary surge in INSL5 levels following the bile acid enema. Furthermore, a direct correlation was observed: the higher the INSL5 levels, the more immediate and urgent the need for bowel movements among the volunteers. This finding provided compelling early evidence that INSL5 is indeed a key player in the physiological response to bile acid exposure, particularly in the context of diarrhea.
Imperial College London Cohort Analysis (Recent Years): The Cambridge team then turned their attention to a cohort of patients studied by Professor Julian Walters at Imperial College London. This cohort included individuals diagnosed with bile acid diarrhea. When researchers analyzed blood samples from this group, a striking difference emerged. While INSL5 levels were found to be almost undetectable in healthy control volunteers, they were significantly elevated in patients with confirmed bile acid diarrhea. Moreover, the severity of their diarrhea, as indicated by the water content of their stool samples, was directly proportional to their INSL5 levels. This provided robust human data confirming the strong association between elevated INSL5 and bile acid diarrhea.
Irritable Bowel Syndrome Patient Data Analysis (Recent Years): Seeking to understand the broader implications for IBS-D, the Cambridge researchers examined samples from patients enrolled in a study led by Professor Robin Spiller at the University of Nottingham. In this study, patients with IBS had been treated with ondansetron, an anti-sickness medication known to block the action of INSL5 in mice. The analysis of these samples yielded a significant revelation: approximately 40% of these IBS patients exhibited elevated INSL5 levels, even though their bile acid malabsorption had been ruled out by conventional testing. Crucially, these patients with high INSL5 levels demonstrated the most pronounced positive response to ondansetron treatment. This suggests that a subset of IBS-D patients, previously undiagnosed, may be suffering from a form of BAD or a condition with a similar INSL5-driven mechanism.
Implications for Diagnosis and Treatment
The identification of INSL5 as a potential biomarker for bile acid diarrhea holds profound implications for clinical practice.
Development of a Blood Test:
Dr. Chris Bannon, the study’s first author and a clinical fellow at the Institute of Metabolic Science, University of Cambridge, 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," he stated. "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."
Currently, the diagnostic pathway for chronic diarrhea often involves a battery of tests to rule out other causes such as food intolerances, infections, or inflammatory bowel diseases. While research into the gut microbiome has garnered significant attention, the role of gut hormones has, until now, been relatively underexplored. Dr. Bannon emphasized this point: "When you go to the doctor with chronic diarrhea, it’s likely they’ll test for food intolerances, rule out an infection or look for signs of inflammation. There has been significant research interest in the microbiome, but gut hormones have been neglected. But it’s becoming increasingly clear that gut hormones play an important role in things like gut health and weight management."
The prospect of a simple, non-invasive blood test to detect elevated INSL5 levels could revolutionize the diagnosis of BAD. This would enable clinicians to accurately identify patients suffering from this specific condition, leading to more targeted and effective treatment strategies. Early and accurate diagnosis could alleviate years of misdiagnosis and the associated emotional and physical burden on patients.
A Novel Therapeutic Target:
Beyond its diagnostic potential, INSL5 also presents a promising target for novel therapeutic interventions. The observation that ondansetron, a drug known to inhibit INSL5 activity in mice, provided relief to a significant proportion of IBS-D patients with elevated INSL5 levels is particularly noteworthy. While the exact mechanism by which ondansetron exerts its beneficial effect in humans remains under investigation, a known side effect of the drug is constipation. This suggests that by blocking INSL5, the drug may be reducing the excessive stimulation of the colon that leads to diarrhea.
The Cambridge team plans to further investigate this therapeutic avenue, with the ultimate goal of either repurposing ondansetron for this specific patient subgroup or developing entirely new drugs that more effectively and specifically target INSL5. Current treatments for bile acid diarrhea primarily involve bile acid sequestrants. While these medications are effective for approximately two-thirds of patients, a significant minority do not respond adequately. The development of INSL5-targeting therapies could offer a much-needed alternative for these non-responders, potentially improving outcomes for a wider range of individuals.
Understanding the "Poison Sensor" Analogy
Dr. Bannon offered an insightful perspective on the evolutionary purpose of a hormone that, in excess, causes diarrhea. He posited that INSL5 might function as a "poison sensor" within the gut. "I often get asked why we would have a hormone that gives you diarrhea," he explained. "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 highlights the delicate balance of the gut ecosystem. While INSL5’s role in expelling harmful substances is likely protective under normal circumstances, its persistent activation by unabsorbed bile acids can lead to a pathological state characterized by chronic diarrhea. Understanding this nuanced function is crucial for developing treatments that modulate INSL5 activity without disrupting its essential protective roles.
Broader Impact and Future Directions
The implications of this research extend beyond the immediate scope of bile acid diarrhea and IBS-D. It underscores the critical, and often overlooked, role of gut hormones in maintaining gastrointestinal health and regulating various bodily functions. As research continues to unravel the intricate signaling pathways within the gut, it is likely that other gut hormones will be identified as key players in a range of digestive disorders and metabolic conditions.
The collaborative nature of this research, involving institutions like the University of Cambridge, University of Adelaide, Imperial College London, and the University of Nottingham, alongside pharmaceutical partners like Eli Lilly, exemplifies the power of interdisciplinary and cross-institutional cooperation in advancing scientific understanding. Support from major funding bodies such as the Medical Research Council, Wellcome, and the National Institute for Health and Care Research Cambridge Biomedical Research Centre has been instrumental in enabling this vital work.
Future research will likely focus on further elucidating the precise molecular mechanisms by which INSL5 influences gut motility and secretion, as well as refining diagnostic assays and therapeutic strategies. Understanding the genetic and environmental factors that predispose individuals to bile acid malabsorption and elevated INSL5 levels will also be crucial. Ultimately, this line of inquiry holds the promise of transforming the lives of millions suffering from chronic diarrhea, offering them hope for accurate diagnosis, effective treatment, and a return to a better quality of life. The identification of INSL5 as a central player in this narrative marks a significant leap forward in our understanding of the complex and often mysterious workings of the human gut.
