Which foods make the smelliest farts?

Nothing clears a room like a smelly fart. Even though this gas, known scientifically as flatus, is a part of our daily lives, we still know surprisingly little about what goes into it. So which foods make us the gassiest, and which ones make for the stinkiest farts?

A smelly fart usually starts with carbohydrates, especially insoluble ones that make it through the stomach and upper intestinal tract without being absorbed, Dr. Ali Rezaie, a gastroenterologist at Cedars Sinai in Los Angeles, told Live Science. Bacteria populating the colon thrive on these unabsorbed sugars, which are "like high-octane fuel for them," Rezaie said.

Although these carbs like fiber and starch weren't absorbed higher in the gut because our bodies lack the enzymes to break them down, bacteria, like those in the phylum Firmicutes in the colon, digest them easily. In feasting on these carbs, the bacteria produce gas, which can turn into farts.

However, not all of the gas that bacteria produce from food become smelly farts. A person produces about 30 to 91 cubic inches (500 to 1,500 milliliters) of flatus every day regardless of their diet, and over 99% of those gases are odorless, Rezaie said. Unscented gases — like methane, carbon dioxide and hydrogen — all contribute to farts, but gassiness doesn't necessarily correlate with stench.

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Offensive gases include hydrogen sulfide, known for its rotten-egg smell; indoles; and skatoles, "which, as the name implies, smells like poo," Dr. Eric Goldstein, a gastroenterologist at Mount Sinai Medical Center in New York City, told Live Science. But, when it comes to converting food into gas, it's not a one-to-one conversion rate in the gut.

"You can eat a ton of sulfur-containing compounds and have bacteria present that are making hydrogen sulfide," but "your flatulence will not smell like hydrogen sulfide," Goldstein said. Instead, the hydrogen-sulfide-producing bacteria may be counterbalanced by other bacteria feasting on that very compound. Sulfur-rich foods include legumes (such as lentils, beans and peas) and brassicas (such as broccoli and cabbage). These fibrous veggies also contain insoluble carbs that bacteria in the colon may convert into stinky gas.

Goldstein and Rezaie emphasized that many factors affect the volume and smell of flatulence. While we can assess common compounds in both foods and farts, some people have unique food sensitivities based on their gut microbiome. Insoluble sugars generally provide the basis for noxious gas, but there are no universal foods that become a polluting toot from every rear end.

"The gas production of bacteria in our gut is not just dependent on what you eat," Rezaie said. "It's all dependent on what other gas-producing bacteria in the gut that are feeding them other gases." Other factors — like gut motility, changes in bacterial composition, and how long it takes food to move through the gut — also influence how a fart might stew. What's more, farts also comprise swallowed air and gases diffused from the bloodstream, which are also odorless. These factors mean that a fart is more likely to be heard than smelled.

Disorders and intolerances also affect how bacteria produce gas. For example, in people who are lactose intolerant, the carbohydrate lactose makes it all the way to bacteria in the colon, which may leave someone farting up a storm that may or may not be smelly. Goldstein pointed to the FODMAP diet, a temporary elimination diet, as one way to reduce extreme cases of flatulence in those who are prone to it.

Still, Goldstein underscored that "there's really no clear one size fits all" when it comes to diet and farts, as myriad factors in each person's body interfere with all flatus. The measure of whether to seek help depends on how much  flatulence disrupts someone's everyday life. On the other hand, farts — even stinky ones — are inevitable, and we can learn to forgive others and ourselves.