The gut microbiome consists of approximately 100 trillion microorganisms — bacteria, archaea, fungi, and viruses — that colonize the gastrointestinal tract, with the highest density in the colon. This community influences immune system development, intestinal barrier integrity, neurotransmitter production (90% of the body's serotonin is produced in the gut), short-chain fatty acid synthesis, and metabolism of xenobiotics including medications. Dysbiosis — disruption of the microbial community — is associated with inflammatory bowel disease, obesity, metabolic syndrome, depression, and autoimmune conditions.

Diet is the primary modifiable determinant of microbiome composition. High dietary fiber intake — from diverse plant sources — is the most evidence-supported microbiome intervention. Fiber is fermented by colonic bacteria to produce short-chain fatty acids (SCFAs), particularly butyrate, which serve as the primary fuel for colonocytes and have anti-inflammatory, anti-proliferative, and intestinal barrier-strengthening effects. A diverse plant intake (30+ different plant foods per week) is associated with higher microbiome diversity, which is generally correlated with better health outcomes.

Probiotic supplements have evidence in specific, narrow indications: prevention of antibiotic-associated diarrhea (Lactobacillus rhamnosus GG and Saccharomyces boulardii have the strongest evidence), management of IBS subtypes (mixed evidence, strain-specific), and H. pylori eradication adjunct therapy. The evidence for probiotics in general wellness, weight management, or immune enhancement in healthy adults is weak. Most commercial probiotic products do not survive gastrointestinal transit in meaningful quantities, and the strains marketed rarely match the strains studied.

IBS (Irritable Bowel Syndrome) affects 10–15% of adults and is a functional disorder driven by gut-brain axis dysregulation, visceral hypersensitivity, and microbiome alterations rather than structural pathology. It is significantly undertreated. Evidence-based interventions include a low-FODMAP elimination diet (effective in 70% of IBS patients when properly implemented), gut-directed hypnotherapy, specific probiotic strains, and antispasmodics. The Rome IV criteria provide a clinical diagnostic framework — IBS should not be diagnosed without ruling out inflammatory, infectious, or structural causes first.