Irritable Bowel Syndrome (IBS) is currently the most common diagnosis amongst gastroenterologists. It is estimated that 10-15% of the US population has IBS, yet only 7% may have a correct diagnosis. Once the hurdle of diagnosis has been conquered, treatment is next. A novel approach may be addressing gastrointestinal motility and pain via serotonin receptors in the gut as the majority of this neurotransmitter is made in the gut. Colonic serotonin synthesis is dependent upon specific bacteria, in conjunction with tryptophan containing foods.
IBS diagnosis is largely made via Rome III criteria, with the exclusion of Celiac Disease as an additional consideration. IBS is categorized into four main subtypes based on the predominant bowel habit: IBS with constipation (IBC-C); IBS with diarrhea (IBS-D); IBS with mixed symptomology (IBS M); and unclassified IBS. Treatment strategies include dietary modifications, a low FODMAP diet, fiber, increased water intake, bulking agents and antispasmodics. Associated stress induced via an IBS diagnosis and symptoms, may be addressed via relaxation techniques. Exercise has shown proven benefits as well.
It is estimated that 95% of serotonin is made in the gut, corresponding to at least seven gut serotonin receptors. Serotonin exerts profound influence over colonic peristaltic and secretory reflexes via the enteric nervous system independent of the central nervous system. Within the human colon, significant serotonin receptors 5HT3 and 5HT4 are involved in motility, secretion, and pain sensation. Colonic serotonin reuptake transporter (SERT) regulates re-uptake of serotonin. Research has shown that patients with IBS may have a significant decrease in SERT resulting in increased serotonin associated with loose stools, cramping and pain. Those with IBS that present with elevated colonic levels of serotonin may fall into the IBS-D subtype. Conversely, individuals with IBS-C may have lower levels of colonic serotonin. Potentially, the colonic muscles are less reactive to serotonin and this may result in hard stools.
Increasing serotonin synthesis may be an appropriate approach to IBS-C. An abundant and diverse gut microbiome is associated with proper serotonin synthesis. In this respect, broad spectrum probiotics may prove fruitful in the short-term, yet have limited long-term viability. Uniquely, commensal Clostridium species may hold the key to colonic serotonin synthesis. In a germ-free model, mice secreted absolutely zero serotonin. It was noted the addition of commensal Clostridium species promoted serotonin synthesis. Specifically, proper colonization of Clostridium species may be beneficial to IBS-C. Commensal Clostridium spp. significantly contribute to butyrate production and overall gut homeostasis. Daily dietary soluble fiber, including chick peas, inulin-like fructans, and partially hydrolyzed guar gum (galactomannan), may be recommended to support commensal Clostridium clusters, and butyrate production.
Colonic enterochromaffin cells (EC) are responsible for serotonin synthesis. Tryptophan hydroxylase is the rate limiting enzyme in serotonin synthesis. EC release serotonin in the presence of dietary tryptophan. Adding tryptophan rich foods may support serotonin synthesis. Foods rich in tryptophan include oats, chicken, turkey, fish, prunes, apples and bananas. Improved serotonin synthesis may in turn influence colonic motility kinetics associated with IBS-C.
As many individuals are dealing with IBS, novel treatment approaches may be welcomed. Serotonin influence over the gut is significant and provides a potential avenue to address IBS pathophysiology. Application of off label use of SSRIs, at low dose for specific populations while considering side effects, may prove promising. As diet and microbiota influence serotonin release in the gut, consider tryptophan rich foods, specifically for IBS-C. A diverse microbiome, containing an abundance of commensal Clostridium species is essential to serotonin synthesis.
