Beyond Hydrogen and Methane

For years, small intestinal bacterial overgrowth (SIBO) was understood as existing in two forms: hydrogen-dominant (associated with diarrhoea) and methane-dominant (now called intestinal methanogen overgrowth or IMO, associated with constipation). But a growing body of research, led largely by Dr. Mark Pimentel at Cedars-Sinai, has identified a third gas — hydrogen sulfide — produced by sulfate-reducing bacteria in the small intestine that represents a distinct and previously undetected form of SIBO.

This discovery has significant implications because many patients with persistent SIBO-like symptoms who test negative on standard hydrogen and methane breath tests may actually have hydrogen sulfide SIBO that was simply not being measured.

What Causes Hydrogen Sulfide SIBO

Hydrogen sulfide SIBO is caused by an overgrowth of sulfate-reducing bacteria (SRB) in the small intestine. The primary species involved are Desulfovibrio, Bilophila wadsworthensis, and Fusobacterium. These bacteria consume the hydrogen gas produced by other gut bacteria and reduce dietary sulfate to hydrogen sulfide (H2S) as their primary energy-producing pathway.

Sulfate-reducing bacteria thrive on sulfur-containing compounds. Diets high in animal protein (which contains the sulfur amino acids methionine and cysteine), cruciferous vegetables (which contain sulfur compounds), garlic, onions, eggs, and wine can provide abundant substrate for these organisms. However, it is important to note that these are healthy foods in most contexts — it is the bacterial overgrowth that creates the problem, not the foods themselves.

The Hydrogen Sulfide Pathway

In a healthy gut, small amounts of hydrogen sulfide are normal and even beneficial. H2S at physiological concentrations supports intestinal motility, reduces inflammation, and protects the mucosal barrier. The problem arises when sulfate-reducing bacteria overgrow in the small intestine and produce H2S in excess, leading to toxic concentrations that damage the intestinal lining and produce systemic symptoms.

Hydrogen sulfide SIBO creates a diagnostic blind spot because standard breath tests only measure hydrogen and methane. A patient with hydrogen sulfide SIBO may show a flat-line on the standard breath test — low hydrogen (because the SRB are consuming it) and low methane — and be told they do not have SIBO, when in fact they have an unmeasured form of it.

Symptoms of Hydrogen Sulfide SIBO

The symptom profile of hydrogen sulfide SIBO has some distinctive features:

  • Diarrhoea — often the predominant bowel pattern, similar to hydrogen-dominant SIBO
  • Sulfurous gas — gas that smells like rotten eggs is the hallmark symptom. While not everyone with occasional sulfur-smelling gas has SIBO, persistent foul-smelling gas combined with other symptoms is a strong clinical indicator
  • Brain fog and fatigue — hydrogen sulfide has direct neurotoxic effects at elevated concentrations and can impair mitochondrial function by inhibiting cytochrome c oxidase
  • Bladder pain and urgency — H2S can sensitise bladder nerves, and there is clinical overlap with interstitial cystitis
  • Halitosis (bad breath) — sulfur compounds contribute to persistent bad breath that does not respond to oral hygiene measures
  • Intolerance to sulfur-rich foods — symptoms worsen after consuming eggs, cruciferous vegetables, garlic, onions, and high-protein meals
  • Weight gain or difficulty losing weight — unlike hydrogen SIBO which tends to cause weight loss, hydrogen sulfide SIBO has been associated with weight gain in some clinical observations

Testing for Hydrogen Sulfide SIBO

The diagnostic landscape has improved significantly with the introduction of the trio-smart breath test, which measures hydrogen, methane, and hydrogen sulfide simultaneously. This is currently the only commercially available breath test that includes H2S measurement. Prior to this test, hydrogen sulfide SIBO could only be inferred indirectly (a flat-line breath test in a symptomatic patient) or through clinical suspicion based on symptom patterns.

Interpreting the Trio-Smart Results

A hydrogen sulfide level above 3 parts per million at any point during the breath test is considered positive. However, the test is still relatively new, and reference ranges may be refined as more data accumulates. Some clinicians use a lower threshold of 2 ppm in the presence of compelling symptoms.

Treatment Approaches

Treatment of hydrogen sulfide SIBO requires a different approach than standard SIBO treatment:

  • Bismuth compounds — bismuth subsalicylate (Pepto-Bismol) directly inhibits sulfate-reducing bacteria and is often used as part of the treatment protocol
  • Antimicrobials — rifaximin is commonly used, sometimes in combination with other agents targeting SRB specifically
  • Low-sulfur diet — temporary reduction of sulfur-containing foods to starve the SRB while antimicrobial treatment takes effect. This includes reducing red meat, eggs, cruciferous vegetables, alliums, and dairy during the treatment phase
  • Molybdenum supplementation — this trace mineral is a cofactor for the enzyme sulfite oxidase, which helps the body process sulfur compounds. Some practitioners find it helpful during treatment
  • Prokinetics — as with all forms of SIBO, restoring proper motility is essential for preventing recurrence

Monitoring Your Response

Tracking symptoms during and after treatment for hydrogen sulfide SIBO is particularly important because the condition has a high recurrence rate and treatment responses can be gradual. Monitoring the frequency and character of your gas, bowel patterns, energy levels, and cognitive function over weeks and months provides essential data for you and your practitioner. GutIQ supports this ongoing monitoring by providing structured symptom tracking that captures the specific symptoms relevant to hydrogen sulfide SIBO, helping you and your healthcare team evaluate treatment effectiveness and catch early signs of recurrence.