Food, from the moment it’s in the mouth until it’s excreted as poop, goes through digestion—and this process is a wild journey. Food containing carbohydrates, proteins, or lipids is chewed into a mush in the mouth, where some degree of starch and lipid digestion occurs before it is swallowed. In the stomach, the swallowed ‘ball’ of chewed food is continuously ground and churned until the food particles are small enough to enter the small intestine. Simultaneously, hydrochloric acid (HCl) and pepsin (an enzyme) in the stomach break down complex proteins in the food.
The food, now in a semi-liquid form, reaches the digestive enzyme-rich small intestine, where its fate is sealed for major chemical digestion before being converted into absorbable nutrients. Enzymes in the small intestine break down the partially digested macronutrients—carbohydrates, proteins, and lipids—into smaller molecules that can be absorbed into the bloodstream, along with vitamins and minerals.
What remains is indigestible food, along with leftover digestive fluids and cellular waste, which enters the colon (large intestine), where digestive enzymes are lacking. However, the gut microbiota—the body’s dynamic ‘organ’—in the colon compensates for this dearth by fermenting indigestible food, including dietary fibres and indigestible proteins. Enzymes produced by the gut microbiota break this indigestible food into useful metabolites and gases before the remainder is expelled as poop.
What are intestinal gases?
In humans, intestinal gases include nitrogen (N2), oxygen (O2), carbon dioxide (CO2), hydrogen (H2), methane (CH4), and trace amounts of hydrogen sulphide (H2S)—the gas with the rotten egg smell. While N2 and O2 mostly reflect swallowed air, H2 and CH4 are odourless gases produced by gut microbiota. It is H2S that is primarily responsible for the smell of gas.
The colon—which hosts the highest microbial densities—harbours microbes that prefer indigestible carbohydrates as food, or as an energy source. These indigestible-carbohydrate-loving microbes (carbohydrate-fermenting) flourish in the colon region close to the small intestine (proximal colon), where their energy source availability is higher. Microbes including Bacteroides scavenge (ferment) human-indigestible carbohydrates such as dietary fibres and resistant starch, producing substantial quantities of hydrogen as a major by-product.
Also thriving in this oxygen-poor, hydrogen-rich ecosystem are microbes that compete for, and convert hydrogen to methane, like methanogenic archaea. Likewise, sulphate-reducing bacteria (SRB) in these regions of the colon convert hydrogen into hydrogen sulphide in small amounts. As the food passes through the colon, the availability of indigestible carbohydrates decreases over time. Microbes like SRB in regions of the colon close to the rectum (distal colon) also ferment human-indigestible proteins for energy, producing hydrogen sulphide in the process.
Studies so far suggest that host cells (through distinct mechanisms) and microbial cells utilise intestinal gases. Excess gases are eliminated as exhaled gas (from the lungs) and flatulence.
What do intestinal gases do?
Recent studies suggest intestinal gases are more than just gas. They can, in fact, serve as markers of gastrointestinal (GI) health, and can regulate gut motility and transit time. Gut motility refers to the muscle contractions in the digestive tract that move the food, and gut transit time is the time it takes for food to move through the gastrointestinal tract before being excreted.
Advancing research reveals that hydrogen can influence gut motility and, in experimental models, shorten transit time. Methane, by contrast, has been shown to slow gut motility and increase transit time. However, findings on the effect of hydrogen sulphide on gut motility and transit time have been mixed.
Microbiota research has also pointed to links between imbalances in gut microbiota and intestinal gas levels and gastrointestinal(GI) diseases/symptoms.
A real-world study measured hydrogen, methane, and hydrogen sulphide levels using breath testing in 3,004 participants with GI symptoms, especially excessive bloating and flatulence, from the United States. The study found associations between intestinal gas levels and GI symptoms: higher hydrogen levels (≥20 ppm above baseline) with diarrhoea; higher methane levels (≥10 ppm) with constipation; and higher hydrogen sulphide levels (≥2 ppm) with severe diarrhoea and abdominal pain. The study also suggested dysregulated hydrogen sulphide production was associated with greater diarrhoea, urgency, bloating, and symptom severity.
Animal models and clinical studies have reported similar associations between intestinal gases and GI symptoms. However, these studies lack identical methodologies and comparison with healthy participants as controls.
Monitoring gases for diagnosis
The most commonly adopted method for measuring intestinal gases is breath testing—measuring gases in exhaled breath. Microbial fermentation activity and gut motility are also assessed indirectly with this method.
However, new techniques are being devised for non-invasive, real-time continuous monitoring of intestinal gases. To capture eliminated intestinal gases (exhaled gas and flatus), researchers have designed respiration/fermentation chambers (airtight rooms with gas sensors and continuous air supply) to explore microbial fermentation activity, track intestinal gases, and monitor the effects of therapeutic interventions.
Alternatively, as flatus has relatively higher concentrations of intestinal gases, researchers at the University of Maryland have developed a device that can be attached to underwear. The device, designed to objectively measure hydrogen in flatus, monitors real-time gas profiles and gas-release patterns, providing insights into gut microbial activity and digestive health.
Why this is important
Measuring and monitoring real-time gas profiles could enable researchers to expand gut microbiota research: define ‘normal’ and ‘abnormal’ levels in intestinal gases, establish mechanisms behind gas effects, and study diverse populations, involving both healthy people and people with GI diseases, over long periods. Doctors could also potentially tailor a treatment plan—diet or gas-targeted medication—to rebalance the gut microbial ecosystem and modulate gas production and track GI health progress.
Therefore, the next time someone lets one rip, remember that intestinal gases have uses beyond popular humour or emptying out a room. These gases are part of a dynamic system that shapes the gut microbiota and influences digestive health, including how one poops, and can serve as markers of GI health.
(Smruthi Prabhu is an independent science writer from Mangalore. smruthigp@gmail.com)