Hydrogen sulfide is the gas behind rotten eggs, swamp stink, and industrial accidents. It is also, at the right dose, one of the molecules keeping the lining of your gut alive. New work reframes a gas we spent decades fearing as a quiet, load-bearing part of a healthy intestine, and as a window into how the microbiome manages oxidative stress.
Lit Review Friday · Hydrogen Sulfide and Oxidative Stress in the Gut · Published 2026 · Reading time: ~16 minutes
- Is hydrogen sulfide bad for your gut? Not at the concentrations your gut actually maintains. The free hydrogen sulfide your colon cells see is roughly 10 to 190 micromolar, and at that level it acts as a fuel and a signaling molecule, not a poison (Kumar and Banerjee, 2026).
- Why does a "toxic" gas show up in almost everyone? Because a low baseline of hydrogen sulfide is a normal feature of a working gut. In a 2026 nationwide breath-testing study, sulfide was spread across nearly all patients and only tracked with symptoms at the very highest end (Pimentel et al., 2026).
- Can hydrogen sulfide help the gut heal? In animal and early human studies, hydrogen-sulfide-releasing drugs protected the stomach and accelerated ulcer healing. In one Phase 2 trial, 3% of people on the hydrogen-sulfide-releasing drug developed an ulcer, versus 42% on standard naproxen (Wallace et al., 2020).
- What flips hydrogen sulfide from helpful to harmful? Capacity. It helps when production stays within what your cells can oxidize, and it causes damage only when the sulfur load overwhelms that system (Kumar and Banerjee, 2026).
🎧 Listen to the full episode: Your Gut Runs on Rotten Eggs
Also on Spotify.
- Hydrogen sulfide, the rotten-egg gas, was added to gut breath tests as a third "bad" gas tied to diarrhea and SIBO (Pimentel et al., 2026).
- But in that study's own data, sulfide sits at similar levels in almost everyone and only tracks with symptoms at the extreme. It behaves like a baseline, not a poison.
- A 2026 biochemistry review shows the old "toxic" gut-sulfide numbers were largely a lab artifact. At the real, much lower dose, your colon cells burn the gas for fuel and use it to build the low-oxygen home good bacteria need (Kumar and Banerjee, 2026).
- The problem is not the gas. It is sulfide production outrunning your gut's capacity to use it. The target is balance, not eradication.
The Gas We Got Backwards
Your nose is built to hate hydrogen sulfide. The rotten-egg smell triggers a fast, almost involuntary disgust, and for good reason: at high concentrations the gas is a genuine respiratory poison. It binds the same spot in your mitochondria that oxygen needs, and at industrial doses it can kill quickly.
So when researchers and clinicians started measuring hydrogen sulfide in the gut, the story almost wrote itself. A toxic gas, made by bacteria, sitting in your colon. It had to be a problem. That assumption shaped decades of textbooks, breath tests, and treatment logic.
A 2026 review by Roshan Kumar and Ruma Banerjee at the University of Michigan, published in Gut Microbes, makes the case that we got the dose, and therefore the story, badly wrong. Read alongside a body of work on sulfur, redox balance, and tissue repair, it points to a different picture: hydrogen sulfide as a tightly managed signal that helps build a healthy gut, defend against oxidative stress, and even accelerate the healing of damaged tissue. This is a reference page on that science. It is the long version behind the episode.
For decades, textbooks put colonic hydrogen sulfide at roughly 1.0 to 2.4 millimolar, a frankly catastrophic concentration for a toxic gas. Kumar and Banerjee argue most of that is an artifact: only 1 to 8% of measured sulfide is actually free. The biologically real free concentration is closer to 10 to 190 micromolar, ten to a hundred times lower, and squarely inside the range where sulfide is useful rather than toxic.
Wait, Isn't Hydrogen Sulfide a Poison?
It is, at the wrong dose. This is the oldest idea in toxicology: the dose makes the poison. Oxygen is essential and also damages your DNA. Water is essential and can kill you. Hydrogen sulfide is no different. The question was never "is it toxic," it was "at what concentration, and is that the concentration your gut actually maintains."
For years the answer looked terrifying, because the measured numbers were enormous. But those numbers came with a hidden flaw baked into the lab method. To measure sulfide in a stool or tissue sample, researchers traditionally acidified it. Acid does something specific and disruptive: it tears sulfur off of proteins, particularly off the iron-sulfur clusters where sulfur sits safely bound inside enzymes, and releases it as free gas. The instrument then reads all of that liberated sulfur as if it had been floating free in the living gut. It never was.
Think of weighing a soaking-wet sponge to estimate how much water you could actually drink from it. The total weight is real, but most of that water is locked into the structure and will never reach your mouth. The old sulfide numbers were the weight of the whole sponge. When you correct for the bound fraction, the free hydrogen sulfide your colon cells are actually exposed to drops into the 10 to 190 micromolar range. That correction is the hinge the entire reframe turns on.
How Does Hydrogen Sulfide Protect the Gut?
Once you accept the lower dose, the biology gets elegant. At these concentrations, hydrogen sulfide is not poisoning your colon cells. Below roughly 20 micromolar, it actually stimulates them. The cells lining your colon, called colonocytes, carry a mitochondrial enzyme named sulfide quinone oxidoreductase, usually shortened to SQOR. Its job is to grab incoming hydrogen sulfide, strip the electrons off it, and feed those electrons into the cell's energy-producing chain. In other words, your gut cells burn the gas for fuel.
The Hypoxic Moat
Burning hydrogen sulfide consumes oxygen. That is the part that matters most. As colonocytes oxidize the gas, they pull oxygen out of the gut interior, keeping the lumen severely oxygen-poor. That low-oxygen zone is not a malfunction. It is the exact environment your most valuable bacteria need.
The bacteria that produce butyrate, the short-chain fatty acid that feeds your colon lining and tamps down inflammation, are obligate anaerobes. Oxygen is lethal to them. So the system runs as a loop: bacteria make hydrogen sulfide, colonocytes burn it and strip out oxygen, the resulting low-oxygen moat protects the anaerobes, the anaerobes make butyrate, and butyrate feeds the colonocytes so they can keep burning sulfide. The gas that smells like death is one of the things holding that peaceful arrangement together.
- Gut bacteria ferment dietary sulfur and release hydrogen sulfide.
- Colonocytes use SQOR to oxidize that sulfide, consuming oxygen as they do.
- Oxygen is stripped from the gut interior, creating a protective low-oxygen "moat."
- Oxygen-intolerant anaerobes thrive in that moat and produce butyrate.
- Butyrate feeds the colonocytes, which keep burning sulfide. The loop sustains itself.
The Mucus Sponge
There is a second layer of protection. The colon is coated in a thick mucus barrier built mostly from a netlike protein called MUC2, which is heavily cross-linked by sulfur-to-sulfur bonds. That structure lets the mucus act as a chemical sponge: when excess hydrogen sulfide rises up from the bacterial layer, the mucus traps it before it can reach and overwhelm the cell surface. The barrier is literally made of the element it is managing. When sulfide production stays moderate, the sponge holds. When it surges past capacity, those bonds start to break down, the mucus frays, and the protection fails. Capacity, again, is the whole game.
Is Hydrogen Sulfide an Antioxidant?
This is where the gut story connects to the much larger story of oxidative stress. Every cell in your body runs on reactions that, as a byproduct, throw off reactive oxygen species: the molecular sparks that, left unchecked, damage DNA, proteins, and membranes. Your body counters them with antioxidant systems, and it turns out a meaningful slice of that defense is run by your microbiome through sulfur chemistry.
A 2022 study by Jun Uchiyama and colleagues at Keio University, published in Cell Reports, demonstrated this directly. Gut bacteria, especially members of the Lachnospiraceae and Ruminococcaceae families that we associate with a healthy gut, take sulfur-containing amino acids and convert them into reactive sulfur species such as cysteine persulfide. These molecules are powerful antioxidants. The researchers found that gut bacteria maintain a large share of the body's steady-state reactive sulfur species: germ-free and antibiotic-treated mice had markedly lower levels.
The functional test was striking. When the team gave mice cystine, a sulfur amino acid, and then triggered oxidative liver injury, the sulfur supply reduced the damage, lower liver enzymes and lower markers of oxidative stress, but only when the gut bacteria were intact. Wipe out the microbiome with antibiotics and the protection disappeared. The antioxidant effect was, in a precise and testable way, microbiome-dependent.
Kumar and Banerjee add the cellular mechanism behind this protective tilt. When colonocytes process hydrogen sulfide, they shift the balance of their internal chemistry toward the reduced, protective side of the ledger, a "reductive shift" in the cofactor pools that repair oxidative damage. The same gas that powers the oxygen-scrubbing moat also helps load the antioxidant defenses. Two independent lines of evidence, the same conclusion: this sulfur chemistry is part of how the body holds back oxidative stress, not a source of it.
Uchiyama et al. found gut bacteria maintain roughly 30 to 50% of plasma reactive sulfur species, and Kumar and Banerjee note that an estimated 70% of systemic sulfide metabolism traces back to the gut. The sulfur chemistry happening in your colon does not stay in your colon. It circulates, and it shows up in the antioxidant capacity of distant organs like the liver.
What Does This Mean for SIBO and Breath Testing?
If you follow gut health, you have probably heard hydrogen sulfide discussed as the third gas in breath testing, the one tied to diarrhea. A large 2026 study by Mark Pimentel and colleagues in the Journal of Clinical Gastroenterology tested more than three thousand people with an at-home, three-gas breath test measuring hydrogen, methane, and hydrogen sulfide. They defined a category they called ISO, intestinal sulfide overproduction, at a threshold of hydrogen sulfide at or above two parts per million, and they found that higher sulfide tracked with more diarrhea, urgency, and abdominal pain.
That data is real and useful. But the framing deserves a closer look, and the most interesting evidence is inside Pimentel's own paper. When the team mapped their patients with a machine-learning visualization, hydrogen and methane formed clean clusters, distinct regions where the gas tracked the disease pattern. Hydrogen sulfide did not. It was, in the paper's own description, dispersed almost at random across the entire map, present at similar levels in sicker and healthier patients alike, and significantly linked to symptoms only at the very highest quantile.
Hold that pattern against the biochemistry. A marker that shows up in nearly everyone, at similar levels, and only tracks with disease at the extreme, is not behaving like a poison. It is behaving like a baseline. Through Kumar and Banerjee's lens, that is exactly what you would predict: a low background of free sulfide is a normal sign of a functioning, oxygen-scrubbing gut. The disease is not the presence of the gas. It is sulfide production that has outrun the host's capacity to oxidize it. The breath test may be partly reading the natural exhaust of a healthy system.
None of this means breath testing is useless or that a high result should be ignored. At the genuine high end, a flooded sulfide system reflects a real dysbiosis worth taking seriously, and Pimentel's data captures that. The caution is about the leap from "elevated sulfide" to "eradicate the sulfide producers." Kumar and Banerjee review evidence that collapsing this system can backfire: in animal studies, removing the gut's sulfide-handling capacity let oxygen back into the lumen and increased susceptibility to Salmonella roughly threefold.
If you are working through a breath-test result, that is a conversation for you and a clinician who knows your full picture. This page is here to add biochemical context, not to direct your care.
Can Hydrogen Sulfide Help the Gut Heal?
Some of the strongest evidence that hydrogen sulfide is protective comes from an unexpected place: the long effort to make safer painkillers. Nonsteroidal anti-inflammatory drugs like naproxen, ibuprofen, and aspirin relieve pain by blocking prostaglandins, but those same prostaglandins help defend the stomach lining. That is why chronic NSAID use causes ulcers and bleeding. Decades of work, much of it from John Wallace and Stefano Fiorucci, showed that two gaseous molecules, nitric oxide and hydrogen sulfide, carry out many of the same protective jobs in the stomach that prostaglandins do.
That insight led to a clever class of drugs: hydrogen-sulfide-releasing NSAIDs, which deliver the painkiller and a slow trickle of hydrogen sulfide at the same time. The first was a diclofenac derivative (ATB-337) reported in Gastroenterology in 2007; it suppressed prostaglandins just as hard as plain diclofenac but barely touched the stomach. A naproxen version (ATB-346), reported in 2010, did something even more telling: it did not just spare the stomach, it accelerated the healing of pre-existing ulcers, while plain naproxen and a COX-2 drug did not.
Then it was tested in people. A 2019 Phase 2 trial in the British Journal of Pharmacology gave healthy volunteers either the hydrogen-sulfide-releasing naproxen or standard naproxen for two weeks, at doses that suppressed the pain target equally. The difference in damage was dramatic.
After two weeks at equal pain-target suppression, 42% of people taking standard naproxen developed at least one stomach ulcer. On the hydrogen-sulfide-releasing version, only 3% did. Same painkilling effect, a fraction of the gut damage, with the only meaningful difference being the slow release of hydrogen sulfide.
The thread that ties this back to oxidative stress was tightened in 2023. A study in Antioxidants tested a hydrogen-sulfide-releasing indomethacin derivative (ATB-344) in a model of ischemia-reperfusion injury, the kind of oxidative damage that happens when blood flow to tissue is cut off and then restored. The hydrogen-sulfide version protected the stomach lining where plain indomethacin did not, and it did so by raising the cell's own antioxidant machinery, enzymes like heme oxygenase-1 and superoxide dismutase, and preventing oxidative damage to RNA. That is the same antioxidant axis Uchiyama's bacteria feed into. The painkiller story and the oxidative-stress story are the same story.
A practical caution worth stating plainly: these are drug-development findings in animals and early human trials, not a reason to take hydrogen sulfide supplements or to change how you use any medication. The value here is conceptual. The body uses hydrogen sulfide as a tissue-protective, pro-healing signal, and that is hard to square with the idea that the gas is simply toxic waste.
Even Your Food Dyes Run Through It
One more line of work shows how far the reach of gut sulfur chemistry extends. In 2022, Sarah Wolfson and colleagues at Albert Einstein College of Medicine published a paper in Nature Metabolism showing that the hydrogen sulfide your bacteria make can chemically reduce azo compounds, a class that includes common Western food dyes like Red 40, without any enzyme involved. The sulfide reacts directly with the dye.
There is a feedback wrinkle that matters for the whole picture. Because the dye consumes sulfide when it reacts, feeding mice more Red 40 transiently lowered their gut sulfide levels. In other words, what you eat does not just set how much hydrogen sulfide your bacteria produce. It can also pull sulfide out of the system through side reactions you would never think to track. The chemical landscape of the gut is a web of interacting molecules, and hydrogen sulfide sits near the center of it.
When the System Tips: Capacity, Not Eradication
Everything above describes a balanced system. So what actually goes wrong? The damage does not come from the presence of hydrogen sulfide. It comes from production overrunning the host's ability to handle it. When the sulfur load gets too high, too fast, the SQOR machinery and the mucus sponge are both overwhelmed, the protective loop tips into injury, and the redox balance shatters. This is the state where sulfide genuinely tracks with disease.
Diet is the main dial. A pattern very high in animal protein and low in fiber and vegetables, sometimes called a sulfur microbial diet, delivers a heavy, fast load of sulfur amino acids and taurine-rich bile acids to the sulfide-producing bacteria. That is the kind of input that can outrun capacity. Plant-forward eating delivers sulfur in slower, more managed forms and feeds the bacteria that keep production smooth. This is part of the long-running picture, built by researchers like Sebastian Winter and Andreas Bäumler, that dysbiosis is better understood as a shift in the gut's metabolic and oxygen climate than as a simple roster of good and bad species.
The reframe has a practical edge. If hydrogen sulfide were just a villain, the goal would be to wipe it out. But the moment you appreciate that a baseline of it is protective, the target changes. The aim is not zero sulfide. It is a system with the capacity to use the sulfide it makes: a diverse community that produces it smoothly, a colon lining with the enzymes to oxidize it, and a mucus barrier intact enough to buffer the surges. You are tuning a system, not eliminating a molecule.
The Honest Limitations
This is an emerging field, and intellectual honesty requires naming what we do not yet know.
First, the corrected free-sulfide range of 10 to 190 micromolar is an estimate derived from bioenergetic studies on colon-derived cells, not a direct, real-time measurement inside a living human gut. Measuring free sulfide in place, moment to moment, is still a technical frontier. The exact concentration any given person's colon cells see is genuinely unknown.
Second, much of the mechanistic detail comes from cell cultures and mouse models. Mice are an imperfect stand-in here in a specific way: they do not metabolize some plant sulfur compounds the way humans do, which means dietary sulfur studies in standard lab mice should be read with caution. Humanized models are needed.
Third, the hydrogen-sulfide-releasing drug data, however striking, is about engineered pharmaceuticals delivering controlled doses to specific tissues. It demonstrates that hydrogen sulfide can be protective and pro-healing. It does not translate into a recommendation to supplement the gas, and it does not mean more sulfide is always better.
Finally, the Pimentel reframe is an interpretation. His data is solid; the argument here is that the "overproduction" framing and a single fixed threshold do not fully fit the dispersed, baseline-like pattern the gas actually shows. That is a hypothesis the literature is consistent with, not a settled conclusion a single trial has proven.
Frequently Asked Questions
Is the hydrogen sulfide in my gut the same gas that smells like rotten eggs?
Yes, it is the identical molecule. The difference is concentration. At industrial levels hydrogen sulfide is a lethal respiratory poison, but the free concentration in your colon is estimated at roughly 10 to 190 micromolar, a range where it acts as a fuel and signaling molecule rather than a toxin (Kumar and Banerjee, 2026).
Does a positive hydrogen sulfide breath test mean I have too much sulfide?
Not necessarily. In a 2026 nationwide study, sulfide was present at similar levels across nearly all patients and only tracked clearly with symptoms at the very highest levels (Pimentel et al., 2026). A baseline of the gas appears to be normal, so a result above a threshold is a starting point for a conversation with a clinician, not a verdict on its own.
Should I cut out sulfur or high-protein foods to lower gut hydrogen sulfide?
The science points toward balance rather than elimination. A very high animal-protein, low-fiber pattern can push sulfide production past what the gut can manage, while plant-forward eating tends to deliver sulfur in slower, better-managed forms (Kumar and Banerjee, 2026). Major dietary changes for a diagnosed condition should be made with a clinician, not based on a single marker.
Is hydrogen sulfide actually an antioxidant?
In effect, yes, through related molecules. Gut bacteria convert sulfur amino acids into reactive sulfur species that raise the body's antioxidant capacity, and in mice this microbial sulfur chemistry reduced oxidative liver injury in a microbiome-dependent way (Uchiyama et al., 2022). Processing hydrogen sulfide also shifts colon cells toward a protective, reduced internal chemistry (Kumar and Banerjee, 2026).
Can Pepto-Bismol affect my gut bacteria?
Possibly, at the level of the sulfide system. The bismuth in Pepto-Bismol binds free hydrogen sulfide, and in animal studies removing the gut's sulfide supply collapsed the protective low-oxygen environment and increased susceptibility to Salmonella roughly threefold (Kumar and Banerjee, 2026). This is mechanistic and animal-model evidence, not a clinical directive about occasional use.
What is the "hypoxic moat" in the gut?
It is the low-oxygen zone your colon cells actively maintain. By burning hydrogen sulfide and butyrate for energy, colonocytes consume oxygen and keep the gut interior oxygen-poor, which protects the oxygen-intolerant bacteria that produce butyrate (Kumar and Banerjee, 2026). It is a deliberate, self-reinforcing arrangement, not a sign of suffocating tissue.
The Bottom Line
Hydrogen sulfide is a near-perfect example of how biology refuses to fit our clean categories of poison and cure. The same gas that can kill at high concentration is, at the microdose your gut maintains, a fuel, a signal, an architect of the low-oxygen environment your best bacteria depend on, and part of the antioxidant system that protects tissue far beyond the colon.
That reframe matters because it changes the question. For a generation we asked how to get rid of the bad gas. The better question is whether the gut has the capacity to use the gas it makes, and what diet, mucus integrity, and microbial diversity it would take to keep that capacity intact.
- The "toxic" colonic sulfide numbers (1 to 2.4 millimolar) were largely a lab artifact; real free sulfide is closer to 10 to 190 micromolar
- At that low dose, hydrogen sulfide is a fuel: colon cells burn it, consume oxygen, and build a protective low-oxygen "moat" for butyrate-producing anaerobes
- Gut bacteria turn sulfur into reactive sulfur species that raise the body's antioxidant capacity, reducing oxidative damage in distant organs like the liver
- Hydrogen-sulfide-releasing painkillers protected the stomach and accelerated ulcer healing, with one human trial showing 3% ulcers versus 42% on standard naproxen
- In breath testing, sulfide is dispersed across nearly everyone and only tracks symptoms at the extreme, behaving like a protective baseline rather than a binary villain
- Damage comes from production outrunning the gut's capacity to oxidize sulfide, not from the gas being present; the target is balance, not eradication
Your gut is not a list of species to be policed. It is a chemical conversation, and hydrogen sulfide is one of its oldest and most misunderstood words.
Stay curious. Take care of your ecosystem.
References
- Kumar R, Banerjee R. Sulfide dynamics at the gut-microbiota interface: diet, oxygen and redox interplay. Gut Microbes. 2026;18(1):2681720. https://doi.org/10.1080/19490976.2026.2681720 · FREE FULL TEXT
- Pimentel M, Leite G, Joo LJ, et al. Real-world Study of Three-gas Breath Testing Nationwide and the Association With Symptoms. J Clin Gastroenterol. 2026;60(5):406–417. https://doi.org/10.1097/MCG.0000000000002326 · FREE FULL TEXT
- Uchiyama J, Akiyama M, Hase K, Kumagai Y, Kim YG. Gut microbiota reinforce host antioxidant capacity via the generation of reactive sulfur species. Cell Rep. 2022;38(10):110479. https://doi.org/10.1016/j.celrep.2022.110479 · FREE FULL TEXT
- Wolfson SJ, Hitchings R, Peregrina K, et al. Bacterial hydrogen sulfide drives cryptic redox chemistry in gut microbial communities. Nat Metab. 2022;4(10):1260–1270. https://doi.org/10.1038/s42255-022-00656-z · FREE FULL TEXT
- Wallace JL, Caliendo G, Santagada V, Cirino G, Fiorucci S. Gastrointestinal safety and anti-inflammatory effects of a hydrogen sulfide-releasing diclofenac derivative in the rat. Gastroenterology. 2007;132(1):261–271. https://doi.org/10.1053/j.gastro.2006.11.042
- Wallace JL, Caliendo G, Santagada V, Cirino G. Markedly reduced toxicity of a hydrogen sulphide-releasing derivative of naproxen (ATB-346). Br J Pharmacol. 2010;159(6):1236–1246. https://doi.org/10.1111/j.1476-5381.2009.00611.x · FREE FULL TEXT
- Wallace JL, Nagy P, Feener TD, et al. A proof-of-concept, Phase 2 clinical trial of the gastrointestinal safety of a hydrogen sulfide-releasing anti-inflammatory drug. Br J Pharmacol. 2020;177(4):769–777. https://doi.org/10.1111/bph.14641 · FREE FULL TEXT
- Głowacka U, Magierowski M, Śliwowski Z, et al. Hydrogen Sulfide-Releasing Indomethacin-Derivative (ATB-344) Prevents the Development of Oxidative Gastric Mucosal Injuries. Antioxidants (Basel). 2023;12(8):1545. https://doi.org/10.3390/antiox12081545 · FREE FULL TEXT
- Fiorucci S, Santucci L, Distrutti E. NSAIDs, coxibs, CINOD and H2S-releasing NSAIDs: what lies beyond the horizon. Dig Liver Dis. 2007;39(12):1043–1051. https://doi.org/10.1016/j.dld.2007.09.001
- Wallace JL. Prostaglandins, NSAIDs, and gastric mucosal protection: why doesn't the stomach digest itself? Physiol Rev. 2008;88(4):1547–1565. https://doi.org/10.1152/physrev.00004.2008
- Gemici B, Wallace JL. Anti-inflammatory and cytoprotective properties of hydrogen sulfide. Methods Enzymol. 2015;555:169–193. https://doi.org/10.1016/bs.mie.2014.11.034
This post accompanies the Lit Review Friday episode of Learn Something with Thaena.