Targeting the Gut–Brain Axis in IBS-Associated Anxiety: Mechanistic Role of Short-Chain Fatty Acids and Oxygen-Based Microenvironment Modulation in SIBO

Irritable Bowel Syndrome (IBS) represents a prototypical disorder of gut–brain axis dysfunction, frequently accompanied by anxiety and autonomic instability. Emerging evidence implicates small intestinal bacterial overgrowth (SIBO) and microbial dysbiosis as central drivers of this bidirectional pathology. Dysbiosis disrupts epithelial homeostasis, promotes aberrant fermentation with excessive gas production, and induces a state of chronic low-grade inflammation, collectively resulting in increased intestinal permeability, visceral hypersensitivity, and neuroendocrine dysregulation.

A critical downstream consequence of dysbiosis is depletion of short-chain fatty acids (SCFAs), particularly butyrate, a key regulator of intestinal and neuroimmune function. Butyrate serves as the primary energy substrate for colonocytes and reinforces epithelial barrier integrity through upregulation of tight junction proteins (occludin, claudins, ZO-1) and stimulation of mucin production. At the molecular level, butyrate exerts potent anti-inflammatory effects via histone deacetylase (HDAC) inhibition, leading to suppression of NF-κB signaling and downstream cytokines such as IL-6 and TNF-α, while promoting regulatory T-cell (Treg) differentiation. Beyond the gut, butyrate modulates the gut–brain axis through vagal afferent signaling and regulation of the hypothalamic–pituitary–adrenal (HPA) axis, contributing to normalization of cortisol dynamics, attenuation of neuroinflammation, and stabilization of serotonergic pathways. These effects correlate clinically with reduction in anxiety, autonomic overactivity (including hyperhidrosis), and fatigue.

In parallel, luminal hypoxia within the small intestine represents a critical but under-recognized factor sustaining SIBO and dysbiosis. Oxygel, a structured oxygen delivery system, is proposed to modulate this microenvironment by increasing local oxygen tension, thereby suppressing obligate anaerobic overgrowth and reducing pathological fermentation and gas production. Enhanced epithelial oxygenation supports enterocyte metabolism and promotes the expression of antimicrobial peptides (AMPs), including defensins, which are essential for mucosal immunity and microbial regulation. Restoration of AMP activity contributes to re-establishment of microbial homeostasis and repair of epithelial barrier dysfunction.

The integration of SCFA restoration and oxygen-based microenvironment modulation represents a synergistic therapeutic paradigm. While butyrate primarily acts on epithelial integrity, immune regulation, and neuroendocrine balance, oxygen delivery targets the upstream drivers of dysbiosis and fermentation. Together, these interventions address both luminal and systemic components of IBS-associated gut–brain axis dysfunction. This framework provides a mechanistically coherent model for the treatment of IBS with comorbid anxiety, emphasizing the need to simultaneously restore microbial ecology, epithelial function, and neuroimmune signaling.