By Gilang Cahya 
A groundbreaking study spearheaded by researchers at the University of Cambridge has unveiled a compelling connection between elevated levels of a specific hormone found in gut cells and a significant portion of chronic diarrhea cases, potentially explaining up to 40% of presentations in patients diagnosed with Irritable Bowel Syndrome with Diarrhea (IBS-D). This discovery, published in the esteemed journal Gut, holds substantial promise for the development of novel diagnostic tools, including a blood test, and opens avenues for targeted therapeutic interventions.
Unraveling the Mystery of Bile Acid Diarrhea
The digestive system is a complex and finely tuned machinery, with the liver playing a crucial role in fat metabolism. Upon ingestion of food, the liver diligently releases bile acids into the upper section of the small intestine. These bile acids are essential for breaking down dietary fats, facilitating their absorption into the bloodstream. The body then efficiently reabsorbs most of these bile acids in the lower reaches of the small intestine, completing a vital digestive cycle.
However, for a notable segment of the population, approximately one in every 100 individuals, this intricate process falters. They are afflicted by a condition known as bile acid diarrhea (BAD), also referred to as bile acid malabsorption. In these individuals, the bile acids are not adequately reabsorbed in the small intestine and consequently travel into the large intestine, or colon. This unchecked presence of bile acids in the colon acts as a potent irritant, triggering a cascade of debilitating symptoms. The most prominent among these is urgent and watery diarrhea, often accompanied by a distressing risk of incontinence episodes, profoundly impacting patients’ quality of life.
The diagnostic challenge posed by BAD cannot be overstated. Currently, there are no routine clinical blood tests available to accurately identify this condition. This diagnostic void often leads to individuals experiencing persistent and severe diarrhea being misdiagnosed or broadly categorized under the umbrella term of Irritable Bowel Syndrome (IBS). IBS itself is a multifaceted gastrointestinal disorder affecting an estimated one in 20 people globally, characterized by a range of symptoms including abdominal pain, bloating, and altered bowel habits. Within the IBS spectrum, it is estimated that as many as one in three patients whose primary symptom is diarrhea may be unknowingly suffering from undiagnosed bile acid diarrhea. This diagnostic ambiguity not only delays appropriate treatment but also contributes to patient frustration and the perpetuation of debilitating symptoms.
The Role of Insulin-Like Peptide 5 (INSL5)
Previous research, primarily conducted in animal models, had begun to hint at the involvement of a gut hormone named Insulin-Like Peptide 5 (INSL5) in the pathogenesis of chronic diarrhea. INSL5 is naturally produced and stored in specialized cells located at the distal end of the colon and rectum. These cells are known to release INSL5 in response to irritation, and early studies suggested that bile acid accumulation in the colon could act as such an irritant, triggering INSL5 release.
The recent investigation by scientists at the Institute of Metabolic Science, University of Cambridge, sought to definitively establish whether this observed correlation in animal studies extended to human physiology and played a significant role in chronic diarrhea in humans. A critical enabler of this research was the development of a novel antibody test by the pharmaceutical company Eli Lilly, a collaborator in this study. This sophisticated assay possesses the sensitivity to detect and quantify minute quantities of INSL5, thereby bridging a crucial gap in the ability to study this hormone in human biological samples.
Chronology of Discovery and Key Findings
The research journey began with an analysis of samples from a previously conducted study at the University of Adelaide. This earlier research focused on understanding the mechanisms that trigger the release of Glucagon-Like Peptide-1 (GLP-1), a hormone that forms the basis of many modern weight-loss medications. In that study, healthy volunteers received a bile acid enema, a procedure designed to introduce bile acid directly into the lower bowel. While the enema successfully stimulated GLP-1 release, it also unexpectedly induced diarrhea.
Upon re-examining the samples from this Adelaide study, the Cambridge team made a pivotal discovery. They observed that the administration of the bile acid enema caused a temporary but significant surge in INSL5 levels. Crucially, the intensity of this INSL5 elevation directly correlated with the speed at which the volunteers experienced the urge to defecate. This finding provided strong initial evidence supporting the hypothesis that INSL5 plays a causal role in the acute diarrheal response triggered by bile acids.
Building upon this promising lead, the Cambridge researchers proceeded to analyze samples obtained from patients with diagnosed bile acid diarrhea, courtesy of Professor Julian Walters at Imperial College London. The results were striking. In stark contrast to healthy volunteers, in whom INSL5 levels were virtually undetectable, patients with bile acid diarrhea exhibited markedly elevated INSL5 concentrations. Furthermore, the degree of INSL5 elevation in these patients directly correlated with the water content of their stool samples, indicating a more severe diarrheal phenotype.
Expert Reactions and Diagnostic Implications
Dr. Chris Bannon, the first author of the study and a clinical fellow at the University of Cambridge’s Institute of Metabolic Science, articulated the significance of these 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. "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."
The implications for clinical practice are profound. Currently, when a patient presents with chronic diarrhea, physicians typically embark on a diagnostic pathway that includes testing for food intolerances, ruling out infectious causes, and screening for inflammatory bowel diseases. While significant research has been dedicated to understanding the gut microbiome, the role of gut hormones in digestive health has historically received less attention. 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 identification of INSL5 as a key player offers a much-needed new diagnostic avenue, potentially redirecting diagnostic efforts and improving patient outcomes.
Therapeutic Potential and Future Directions
Beyond its diagnostic utility, the INSL5 hormone also presents a promising target for novel therapeutic interventions. The Cambridge team further investigated this potential by analyzing samples from patients enrolled in a study led by Professor Robin Spiller at the University of Nottingham. In that study, patients with IBS had been treated with ondansetron, an anti-sickness medication known to inhibit the action of INSL5 in mice.
The analysis of these samples revealed that approximately 40% of the IBS patients in Professor Spiller’s study had elevated INSL5 levels, even though their bile acid malabsorption had been ruled out through standard tests. More importantly, these patients with elevated INSL5 responded most favorably to ondansetron treatment. This suggests that a subset of IBS-D patients, potentially those with subclinical or atypical bile acid issues, might be experiencing diarrhea driven by INSL5, and that ondansetron could offer them relief.
The precise mechanism by which ondansetron exerts its therapeutic effect in this context is still under investigation. However, a known side effect of ondansetron is constipation, which aligns with the hypothesis that blocking INSL5 activity could reduce bowel motility and fluid secretion. The Cambridge team is now actively pursuing further research to elucidate these mechanisms, with the ultimate goal of either repurposing ondansetron for this specific patient population or developing entirely new drugs that more effectively target INSL5 pathways.
Currently, the first-line treatment for bile acid diarrhea typically involves the use of bile acid sequestrants. While effective for a majority of patients, these medications only achieve success in about two-thirds of individuals, leaving a significant proportion without adequate symptom relief. The prospect of a targeted therapy for INSL5-driven diarrhea offers hope for those who do not respond to existing treatments.
The "Poison Sensor" Hypothesis
Dr. Bannon offered an insightful perspective on the biological rationale behind a hormone that appears to induce diarrhea. "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 evolutionary perspective suggests that INSL5’s diarrheal effect is a protective mechanism gone awry, becoming a source of chronic illness when the trigger—excess bile acid—is persistent.
Broader Impact and Future Research
The research, supported by prestigious bodies including the Medical Research Council, Wellcome, and the National Institute for Health and Care Research Cambridge Biomedical Research Centre, represents a significant leap forward in understanding a common yet often poorly understood group of gastrointestinal disorders. The potential to develop a simple blood test for INSL5 could revolutionize the diagnosis of bile acid diarrhea, allowing for earlier and more accurate identification of affected individuals. This, in turn, would facilitate timely access to appropriate treatment, whether it be existing therapies or novel drugs targeting INSL5.
The implications extend beyond the immediate patient population. By shedding light on the role of gut hormones in digestive health, this study could catalyze further research into other neglected hormonal pathways that may influence conditions ranging from functional bowel disorders to metabolic diseases. The collaborative effort between the University of Cambridge, Eli Lilly, Imperial College London, and the University of Nottingham underscores the power of interdisciplinary research in tackling complex medical challenges. The ongoing work by Dr. Bannon and his colleagues, under the guidance of Professors Fiona Gribble and Frank Reimann at the Institute of Metabolic Science, promises to further unravel the intricacies of gut endocrinology and pave the way for improved patient care. The journey from identifying a key hormone to developing a diagnostic test and targeted treatment is a testament to the perseverance and ingenuity of scientific inquiry.
