The sensation commonly described as a “spicy fart,” or burning gas, is caused by chemical irritation, not actual heat. While flatus exits the body at internal body temperature, certain compounds carried within the gas physically irritate the highly sensitive mucosal lining of the anus and rectum. This burning feeling is a direct result of the chemical composition of the intestinal gas, which is largely determined by the specific byproducts produced by the gut microbiome during fermentation.
The Chemical Components That Cause Irritation
The primary culprits behind the irritating sensation are volatile sulfur compounds (VSCs), which are highly reactive molecules produced when gut bacteria break down sulfur-containing amino acids from food. Hydrogen sulfide, known for its rotten-egg smell, is the most common of these compounds and can be corrosive to the delicate epithelial cells lining the lower gastrointestinal tract. Other VSCs, such as methyl mercaptan and dimethyl sulfide, also contribute to the strong odor and the chemical irritation.
Although VSCs are the main source of the chemical burn, other volatile organic compounds (VOCs) and short-chain fatty acids (SCFAs) also play a role, especially when present in high concentrations. SCFAs like butyric and valeric acids are normally beneficial by serving as a primary energy source for colon cells. However, excessive production of these acidic and volatile metabolites creates a chemically harsh environment that triggers pain receptors in the anal mucosa as the gas passes.
Common Dietary Sources That Feed the Fire
The raw materials for these irritating volatile compounds are found in specific foods rich in sulfur or hard-to-digest carbohydrates. High-sulfur vegetables, particularly cruciferous types like broccoli, cauliflower, Brussels sprouts, and cabbage, contain sulfur-containing compounds that bacteria readily convert into VSCs. Allium vegetables, such as onions and garlic, also contribute significantly to the sulfur available for gut microbes to metabolize.
A common misconception is that the spiciness comes directly from spicy food, but this sensation is caused by a different mechanism. Capsaicin, the active compound in chili peppers, survives digestion and chemically binds to pain receptors in the anal tissue upon excretion, causing a direct, fiery sensation. While capsaicin causes a hot feeling, the gas-related burn is due to volatile chemicals produced from the fermentation of other foods.
Foods containing high amounts of fermentable carbohydrates and fiber can also lead to excessive gas and irritation. Polyols, which are sugar alcohols often used as artificial sweeteners, are poorly absorbed and fermented rapidly by gut bacteria. The fermentation of resistant starch and certain high-fiber foods can lead to a rapid increase in the production of irritating SCFAs.
When Digestive Issues are the Root Cause
When the burning sensation is persistent, severe, or not easily linked to recent dietary choices, it may signal an underlying digestive issue. Conditions that cause malabsorption, such as undiagnosed food sensitivities like lactose or fructose intolerance, allow high levels of undigested sugars to reach the colon. This significantly increases the amount of fermentable material available to the gut bacteria, leading to an overproduction of irritating gases and SCFAs.
Disorders like Irritable Bowel Syndrome (IBS) or Small Intestinal Bacterial Overgrowth (SIBO) can also alter the balance of the gut microbiome, favoring bacteria that produce more of the problematic volatile compounds. SIBO, for example, involves an abnormal increase in bacteria in the small intestine, leading to fermentation much earlier in the digestive tract. Any persistent or severe burning, especially if accompanied by other symptoms like abdominal pain, significant changes in bowel habits, or the presence of blood, should be discussed with a medical professional.