Methane vs. Hydrogen: The Two Main Gut Gases
When gut bacteria ferment undigested carbohydrates, they primarily produce two flammable gases: hydrogen (H₂) and methane (CH₄). Both are odorless, both can theoretically burn — but their production, distribution, and effects on the body differ considerably. And not everyone produces both: roughly one third of the population produces barely any methane — a difference rooted deep in the microbiome.
Hydrogen: The Primary Fermentation Gas
Hydrogen is the primary fermentation product in the colon. When bacteria break down fiber, oligosaccharides, and other undigested carbohydrates, hydrogen is always produced first. It is produced by so-called saccharolytic (sugar-digesting) bacteria, including Bacteroides, Clostridia, and Bifidobacterium. Hydrogen is very light and diffuses quickly through the gut wall into the blood, is transported to the lungs, and exhaled — which is why it can be detected in breath tests. Production quantities strongly depend on diet: a fiber-rich meal can increase H₂ production tenfold. Hydrogen accounts for more than half of the total flammable gas content in intestinal winds.
Methane: The Gas of Archaea
Methane in the human gut is produced not by bacteria, but by archaea — a completely different domain of life. The most important methane-producing species in the human gut is Methanobrevibacter smithii. These archaea use hydrogen as a substrate: they 'eat' the hydrogen produced by bacteria and combine it with carbon dioxide to form methane (CO₂ + 4H₂ → CH₄ + 2H₂O). Only about 30–50% of people have enough Methanobrevibacter in their gut to produce measurable methane amounts — the rest excrete barely any methane. This trait appears to be partly genetically determined and runs in families.
Health Differences: Methane Producers vs. Non-Producers
Whether intestinal methane production has health consequences is a subject of active research. Some studies show that methane producers tend more toward constipation: methane slows intestinal transit by inhibiting gut motility. In patients with chronic constipation, elevated methane production was more frequently measured. On the other hand, methane-producing archaea use the hydrogen produced by fermentation bacteria — thus lowering the H₂ partial pressure in the gut, making fermentation by other bacteria more efficient. People without methanogens instead produce more hydrogen and CO₂ — their farts may be more frequent, but are less often associated with constipation.
Does a Fart Really Burn? The Chemistry Behind It
Yes — but under normal conditions no real risk. Igniting intestinal gases requires sufficient concentration of flammable gases (hydrogen and methane), an ignition source, and enough oxygen. In the gut itself there is barely any oxygen, so gases cannot burn there. When released into the air, gas concentrations are strongly diluted. Nevertheless, there are documented cases where during intestinal surgeries with electrocautery, deflagrations occurred when bowel prep had not been performed adequately. In such cases, a critical methane or hydrogen concentration in the gut can actually represent a safety risk — which is why certain surgical preparations completely empty the bowel.
Did you know?
- Only 30–50% of people produce notable methane amounts in their gut — the key is the presence of the archaeon Methanobrevibacter smithii.
- Methane is 28 times more potent as a greenhouse gas than CO₂ over a 100-year horizon — but amounts from human gut gases are climatically irrelevant.
- Hydrogen diffuses so quickly through gut walls that a large portion is exhaled through the lungs even before the fart.
- In surgery there are documented cases of gut deflagrations from excessive gas concentrations during electrocautery procedures.
Fun Fact
The archaeon Methanobrevibacter smithii is not only responsible for methane in the human gut — it is also one of the best-adapted organisms for extreme environments. Related methanogenic archaea are found in hot springs, deep-sea sediments, and Arctic permafrost. The gas-producing creature in your gut belongs to one of the most ancient forms of life on Earth.
Sources
- Levitt, M.D. et al. (1998). Evaluation of an extremely flatulent patient: case report and proposed diagnostic and therapeutic approach. American Journal of Gastroenterology, 93(11), 2276.
- Sahakian, A.B. et al. (2010). Methane and the gastrointestinal tract. Digestive Diseases and Sciences, 55(8), 2135–2143.
- Pimentel, M. et al. (2006). Methane, a gas produced by enteric bacteria, slows intestinal transit and augments small intestinal contractile activity. American Journal of Physiology, 290(6), G1089–G1095.