Hydrogen sulfide small intestinal bacterial overgrowth — increasingly reclassified by leading gastroenterologists as Intestinal Sulfide Overproduction (ISO) — represents one of the most clinically challenging and underdiagnosed subtypes of gut dysbiosis. Unlike hydrogen-dominant SIBO (associated primarily with diarrhea) or methane-dominant SIBO/IMO (associated with constipation), ISO results from the unchecked proliferation of sulfate-reducing bacteria (SRBs) in the small intestine, producing hydrogen sulfide (H₂S) gas in pathological quantities. For decades, standard three-gas breath testing only measured hydrogen and methane, leaving ISO systematically invisible. The advent of the TrioSmart™ breath test (Gemelli Labs) — the first commercially available test to measure all three SIBO gases simultaneously — has finally given clinicians a diagnostic window into this condition. Even so, many patients with textbook ISO symptoms go years without a diagnosis, and formal treatment guidelines remain absent. This article synthesizes the current evidence base — including data presented at Digestive Disease Week (DDW) 2025 — to provide clinicians and informed patients with a practical, evidence-stratified approach to treating ISO/H₂S SIBO.

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Histamine is a biogenic amine with essential physiological roles — it mediates immune responses, modulates gastric acid secretion, functions as a neurotransmitter, and participates in the regulation of circadian rhythms. Under normal conditions, the enzyme diamine oxidase (DAO) — produced primarily in the intestinal mucosa — efficiently catabolizes dietary histamine before it can accumulate to symptomatic levels. A second enzyme, histamine N-methyltransferase (HNMT), handles intracellular histamine degradation. Histamine intolerance (HIT) occurs when this enzymatic capacity is overwhelmed, most commonly due to impaired DAO activity, excessive dietary histamine load, or gut dysbiosis that tips the balance toward histamine-producing bacteria. The result is a broad and frustrating symptom constellation that can mimic allergic disease, irritable bowel syndrome, migraine, and autonomic dysfunction — while standard allergy testing returns negative. Mast cell activation syndrome (MCAS) represents an overlapping but distinct condition in which mast cells pathologically release histamine and other mediators in excess. Though the mechanisms differ from classic HIT, the dietary and probiotic strategies are largely parallel, and clinically the two conditions frequently coexist. This article summarizes the evidence-based framework for a low histamine diet and explores strain-specific probiotic considerations — an area of growing importance in integrative gastroenterology, integrative oncology, and functional immune care. https://www.ifmsynergy.com/the-low-histamine-diet-and-low-histamine-probiotics-a-practical-clinical-guide/

https://www.ifmsynergy.com/functional-medicine-and-tcm-parallels/

https://youtu.be/N9u-1E7ZF8U