Farting and Fasting: What Happens to Your Gas When You Stop Eating
When you fast, the food that normally feeds your gut bacteria stops arriving — yet your intestines never fall completely silent. The result is a surprising and often overlooked shift in how, when, and how much you fart.
Why Fasting Changes Your Gas
Intestinal gas is mostly a by-product of bacterial fermentation in the colon. When you eat, undigested carbohydrates such as fibre and certain sugars travel to the large intestine, where trillions of microbes break them down and release hydrogen, carbon dioxide and, in some people, methane. During a fast, that steady supply of fermentable material dwindles, so the raw fuel for gas production drops sharply. Most people notice fewer and less voluminous farts after the first day without food. However, the gut is never truly empty: bacteria continue to feed on shed intestinal cells, mucus and residual material, so a low-level trickle of gas persists. Fasting also triggers a rhythmic housekeeping contraction called the migrating motor complex, which sweeps leftover gas and debris downstream — which is why an empty stomach can rumble loudly.
The Science of Reduced Fermentation
The volume of flatus you produce is closely tied to how much fermentable substrate reaches your colon. Studies of gas physiology show that a diet rich in fibre and fermentable carbohydrates can push daily gas output well above two litres, while a near-fasting or elemental diet drops it dramatically. When fasting, the small intestine finishes absorbing the last meal within hours, and after that little new substrate arrives. Hydrogen and methane production — the gases responsible for most volume — fall accordingly. This is measurable: hydrogen breath tests, which detect fermentation, typically show low baseline readings after an overnight fast, which is exactly why patients are told not to eat before the test. The gases that remain tend to come from swallowed air and from the slow fermentation of endogenous secretions rather than from your last meal.
Refeeding and the Return of Gas
The quietest phase of fasting is often followed by a noisy rebound. When you break a fast, especially with a large or fibre-heavy meal, a flood of fresh substrate suddenly reaches a colon whose bacteria have been comparatively idle. Fermentation ramps up quickly, and many people experience a burst of bloating and flatulence within hours of eating again. Breaking a prolonged fast with high-FODMAP foods — beans, onions, cruciferous vegetables or sugar alcohols — tends to produce the most dramatic gas. Nutritionists recommend easing back in with smaller, gentler meals to give the microbiome and gut motility time to readjust. For people practising intermittent fasting, this cycle repeats daily: a calm fasted window followed by a livelier digestive period once the eating window opens.
Fasting, the Microbiome and Odour
Prolonged fasting does more than reduce gas volume; it gradually reshapes the microbial community itself. With fewer dietary carbohydrates available, fibre-fermenting bacteria become less active while microbes that can use protein and mucus gain ground. This shift can change the smell of the gas you do pass. Foul odour comes mainly from sulphur-containing compounds such as hydrogen sulphide, produced when bacteria break down proteins and sulphur-rich amino acids. Some people report that their occasional fasting farts smell surprisingly strong even though they are few, a hint that protein and sulphur metabolism is dominating over carbohydrate fermentation. Research on fasting and the gut also links these microbial shifts to changes in gut-barrier function and inflammation, an active area of study in nutrition and gastroenterology.
Did You Know?
- Gas production can fall by more than half within a day of fasting because the colon runs low on fermentable fuel.
- The loud rumble of an empty stomach is largely gas and fluid being swept along by the migrating motor complex.
- Hydrogen breath tests require an overnight fast precisely because fasting lowers baseline fermentation gases.
- Breaking a fast with beans or cruciferous vegetables can trigger an outsized burst of flatulence.
Fun Fact
During a long fast your gut essentially switches its bacteria from a carbohydrate buffet to a protein-and-mucus diet — which is why the rare fart you do produce can smell far stronger than usual.
Sources
- Levitt MD, Bond JH, Volume, Composition, and Source of Intestinal Gas, Gastroenterology, 1970
- Suarez FL, Levitt MD, An Understanding of Excessive Intestinal Gas, Current Gastroenterology Reports, 2000
- de Cabo R, Mattson MP, Effects of Intermittent Fasting on Health, Aging, and Disease, New England Journal of Medicine, 2019