The Microbiome: Who Is Actually Farting?
Technically speaking, you don't fart alone — at least not entirely. The vast majority of your intestinal gases are produced by about 38 trillion microorganisms living in your colon: bacteria, archaea, and fungi that together are called the gut microbiome. This ecosystem is as individual as a fingerprint, and it determines not only how much you fart, but also how it smells.
38 Trillion Housemates: The Microbiome in Numbers
The human gut microbiome encompasses an estimated 38 trillion microorganisms from more than 1,000 different species. Their total weight is about 200 grams — similar in weight to a large steak. Interestingly, the number of microbes in the gut exceeds the number of human body cells at a ratio of about 1.3:1. This means you are to a significant extent microbial. This community is not evenly distributed — there are barely any microbes in the stomach (too acidic), some in the small intestine, but in the colon there is a mega-density of up to one trillion cells per milliliter of intestinal content. That's exactly where the main fermentation — and thus the main gas production — takes place.
Individual Differences: Why Everyone Farts Differently
The composition of the microbiome is so individual that researchers could theoretically distinguish two people based on their microbiome signature — similar to DNA analysis. This individual composition explains why two people can eat the same meal and react completely differently: one gets severe bloating, the other nothing. People with many Bifidobacterium and Lactobacillus strains produce less foul-smelling gases than people with many Clostridia, which generate particularly many sulfur compounds. The presence of methane-producing archaea (Methanobrevibacter smithii) also varies from person to person and influences whether farts are flammable or not.
How the Microbiome Is Shaped by Diet
The microbiome responds to dietary changes surprisingly quickly — within 24 to 48 hours, the dominant bacterial strains can measurably shift. A fiber-rich diet promotes saccharolytic bacteria (that ferment sugars), leading to more but less foul-smelling gas. A protein-rich diet promotes proteolytic bacteria (that break down proteins), producing more sulfur compounds and more foul-smelling gases. Antibiotics can drastically disrupt the microbiome and alter virtually all gastrointestinal symptoms — including dramatically altered flatulence patterns — for weeks to months. Long-term dietary patterns shape the microbiome more durably than short-term changes: vegans develop a significantly different microbiome than meat eaters.
Fart Profiling: Gut Gas as a Health Indicator
Researchers are increasingly investigating whether a person's gas profile (the composition of their intestinal gases) can serve as a non-invasive health indicator. Elevated H₂S levels are associated with inflammatory bowel diseases. Changes in the methane-hydrogen ratio can indicate SIBO (small intestinal bacterial overgrowth) or constipation. The ratio of short-chain fatty acids — byproducts of fermentation — is considered an indicator of a healthy microbiome. In the future, smart toilets or wearable gas sensors that automatically analyze gas production patterns daily and provide early warning of dysbiosis (microbiome imbalances) could become reality — a concept researchers in Japan and Australia are already developing as prototypes.
Did you know?
- The gut microbiome responds to dietary changes within 24–48 hours — gas composition changes correspondingly quickly.
- Identical twins share nearly identical DNA but their microbiomes can differ considerably — the environment influences it more strongly than genes.
- Antibiotics can disrupt the microbiome so severely that some people fart differently for weeks afterward.
- The combined weight of all gut bacteria is about 200 grams — roughly as much as a large piece of meat.
Fun Fact
The Human Microbiome Project (HMP) of the NIH analyzed the microbiomes of nearly 300 healthy Americans between 2008 and 2016. One of the most surprising findings: there is no universal 'healthy' microbiome. The variability between individuals is so great that what counts as healthy in one person would be atypical in another. This makes it difficult to give blanket dietary recommendations — and explains why beans make one person flatulent and leave another unaffected.
Sources
- Sender, R. et al. (2016). Revised estimates for the number of human and bacteria cells in the body. Cell, 164(3), 337–340.
- David, L.A. et al. (2014). Diet rapidly and reproducibly alters the human gut microbiome. Nature, 505(7484), 559–563.
- Human Microbiome Project Consortium (2012). Structure, function and diversity of the healthy human microbiome. Nature, 486(7402), 207–214.