Hydrogen Sulfide: Your Gut Bacteria Are Poisoning Your Mitochondria
Cyanide kills by binding cytochrome c oxidase — Complex IV of the mitochondrial electron transport chain, the final step of ATP production. Without Complex IV, electrons pile up, the proton gradient collapses, ATP synthase stops, and the cell dies. Cyanide is fast and unambiguous. Nobody debates whether it causes fatigue.
Hydrogen sulfide (H₂S) binds the same site. The same enzyme. The same mechanism. The difference is dose: cyanide arrives acutely and kills. H₂S, produced chronically by gut bacteria at sub-lethal concentrations, arrives continuously and impairs. Not enough to kill cells. Enough to reduce ATP output by 10, 20, maybe 30 percent — silently, without a measurable biomarker in standard clinical panels, for months and years.
In a healthy person with adequate mitochondrial reserve, this impairment might be compensated and invisible. In an ME/CFS patient operating at the edge of their energy capacity, it can be the difference between functional and bedbound.
1 Where the H₂S comes from
The human gut harbours sulfate-reducing bacteria — organisms that use sulfate as a terminal electron acceptor instead of oxygen, producing H₂S as their metabolic waste product. The dominant genera are Desulfovibrio, Bilophila, and Fusobacterium. They are normal residents of the colon in small numbers.
When the gut microbiome shifts — when inflammatory conditions, antibiotic use, dietary changes, or motility dysfunction alter the balance of organisms — sulfate-reducing bacteria can overgrow. They are fed by dietary sulfur: sulfate from preservatives and processed foods, sulfur amino acids from meat and eggs, sulfur-containing supplements (N-acetyl cysteine, alpha-lipoic acid, methylsulfonylmethane), and sulfur-rich vegetables (garlic, onion, broccoli, cauliflower, Brussels sprouts).
The irony is bitter. Several of these foods and supplements are recommended for ME/CFS patients on “anti-inflammatory” or “mitochondrial support” protocols. A patient taking NAC to boost glutathione (a sulfur-containing antioxidant) while eating garlic and broccoli (widely recommended for their anti-inflammatory properties) may be feeding the bacteria that are poisoning the mitochondria they’re trying to support.
2 The mechanism: Complex IV inhibition
Cytochrome c oxidase (Complex IV) is the final complex of the electron transport chain. It accepts electrons from cytochrome c and transfers them to molecular oxygen, producing water. This reaction is what makes aerobic respiration “aerobic” — without it, the chain backs up and the cell reverts to anaerobic glycolysis.
H₂S inhibits Complex IV by binding to the copper centre (CuB) in the binuclear active site — the same site where oxygen normally binds (Nicholls 1982; Cooper and Brown 2008). The inhibition is reversible: when H₂S is removed, Complex IV function recovers. But in a situation where H₂S is produced continuously — as it is with persistent gut dysbiosis — the inhibition is chronic.
The degree of inhibition depends on concentration. Low nanomolar H₂S may actually be beneficial — endogenous H₂S at physiological concentrations acts as a gaseous signalling molecule, regulating vasodilation, inflammation, and neuronal function. It is micromolar concentrations — the range produced by bacterial overgrowth — that become inhibitory to Complex IV (Szabo et al. 2014).
This is a dose-response relationship with a narrow therapeutic window: too little H₂S and you lose its signalling functions, too much and your mitochondria are poisoned. Gut dysbiosis pushes the concentration past the therapeutic range into the inhibitory range — not acutely toxic, but chronically impairing.
3 The portal circulation: from gut to everywhere
H₂S produced in the gut enters the portal circulation — the venous blood that flows directly from the intestines to the liver. The liver is the first line of defence: it oxidises H₂S via the sulfide oxidation pathway, converting it to thiosulfate and sulfate. Under normal conditions, the liver clears most gut-derived H₂S before it reaches the systemic circulation.
When H₂S production exceeds hepatic clearance capacity — which can occur with significant sulfate-reducing bacterial overgrowth — the excess enters systemic circulation. It reaches skeletal muscle, cardiac muscle, the brain, and every other mitochondria-containing tissue. The Complex IV inhibition becomes systemic, not just local.
Hepatic clearance capacity is not unlimited, and it may already be reduced in ME/CFS patients. The liver’s sulfide oxidation pathway requires oxygen and mitochondrial function — if hepatic mitochondria are already compromised by other ME/CFS mechanisms, the liver’s ability to clear H₂S is reduced, creating a vicious cycle: gut H₂S → hepatic mitochondrial impairment → reduced H₂S clearance → more systemic H₂S → more widespread mitochondrial impairment.
4 The clinical picture
H₂S-dominant gut dysbiosis presents differently from hydrogen- or methane-dominant SIBO. The classic features:
- Rotten egg gas/belching/flatulence. H₂S smells like rotten eggs. Patients and their families notice it, though many are too embarrassed to report it to their doctor.
- Diarrhoea. H₂S is directly toxic to colonocytes and disrupts mucosal integrity, producing loose stools.
- Intolerance to sulfur-rich foods. Garlic, onion, eggs, cruciferous vegetables, wine — foods that increase sulfate availability to sulfate-reducing bacteria — produce disproportionate symptoms.
- Intolerance to sulfur-containing supplements. NAC, alpha-lipoic acid, MSM, taurine (to a lesser degree) — supplements commonly recommended for ME/CFS — may worsen symptoms in this subset.
- Fatigue that worsens with “healthy eating.” The cruelest presentation. A patient following an anti-inflammatory diet rich in garlic, broccoli, and cauliflower gets worse, not better. The diet is feeding the problem.
The overlap with ME/CFS symptoms is substantial, and the H₂S contribution can easily be attributed to “just the disease” rather than to a specific, treatable driver.
5 Testing
Standard breath tests for SIBO measure hydrogen and methane. Until recently, H₂S was not measured. The trio-smart breath test (Gemelli Biotech) now measures all three gases, allowing identification of H₂S-dominant SIBO specifically (Pimentel, Chow, and Lin 2003).
Urinary thiosulfate — a downstream metabolite of H₂S — can serve as a rough proxy for systemic H₂S exposure, though it is not specific and is not widely available.
Clinical suspicion — rotten-egg gas, sulfur food intolerance, diarrhoea-predominant bowel symptoms, fatigue that worsens with sulfur-containing supplements — is often sufficient to justify an empirical trial of sulfur reduction, even without formal testing.
6 Treatment
Dietary sulfur restriction. Reduce intake of high-sulfur foods (garlic, onion, cruciferous vegetables, eggs, high-sulfur processed foods) and sulfur-containing supplements (NAC, ALA, MSM). This is counterintuitive and conflicts with standard anti-inflammatory dietary advice — which is precisely why it’s missed. The patient who improves on a “less healthy” diet may have been poisoning their mitochondria with the “healthy” one.
Bismuth subsalicylate. Bismuth binds H₂S in the gut lumen, forming insoluble bismuth sulfide (the same compound that turns stool black). This reduces luminal H₂S concentration and symptomatically reduces gas, diarrhoea, and — in some patients — fatigue. It is cheap, available over the counter, and has been used safely for decades.
Targeted probiotics. Increasing populations of non-sulfate-reducing bacteria — particularly butyrate-producing species (Faecalibacterium prausnitzii, Roseburia) — can competitively displace sulfate reducers. This is slower than antibiotic approaches but addresses the ecological imbalance rather than just the symptoms.
Rifaximin. The standard SIBO treatment has some efficacy against sulfate-reducing bacteria, though data specific to H₂S-dominant SIBO are limited.
Addressing the underlying motility dysfunction. As with all forms of SIBO, treating the overgrowth without addressing the motility failure that caused it leads to recurrence. Prokinetics, vagal tone rehabilitation, and meal spacing remain important.
7 The connection to the energy failure landscape
H₂S from gut dysbiosis connects three pillars of ME/CFS pathophysiology through a single molecule:
- Mitochondrial dysfunction — direct Complex IV inhibition, reducing ATP ceiling
- Gut dysbiosis — the microbiome composition shift that produces the H₂S
- Immune activation — H₂S-driven mucosal damage increases intestinal permeability, allowing bacterial endotoxin (LPS) translocation into the bloodstream, triggering systemic immune activation
This is not “the gut-brain axis” as a vague metaphor. It is a specific molecule, produced by specific bacteria, inhibiting a specific enzyme complex, through a specific binding mechanism, with specific downstream consequences for energy production, intestinal integrity, and immune activation. Every step is biochemically characterised. Every step is, in principle, targetable.
For the ME/CFS patient who has tried “everything” — CoQ10, carnitine, B vitamins, anti-inflammatory diets — and hasn’t improved: has anyone checked whether their gut bacteria are producing the very toxin that makes all those interventions irrelevant? If Complex IV is chronically inhibited by H₂S, no amount of upstream cofactor support will restore the ATP output the patient needs. You have to stop the poison before you can fix the engine (Loth 2026).