8 Reasons Your Farts Smell So Bad

What makes farts stinky?

To fart is human. People break wind an average of 14 times a day, emitting anywhere from half a liter to more than 2 liters of gas over a 24-hour period. And, believe it or not, 99% of gas is odor-free. But sometimes your farts are just downright funky.

“Silent-but-deadly ones, the really smelly guys, are due to fermentation by bacteria in your colon,” says Patricia Raymond, MD, a Virginia Beach-based gastroenterologist and assistant professor of clinical internal medicine at Eastern Virginia Medical School.

If you’re having wicked gas, it’s probably something you ate–and not necessarily a bad thing. Gas is a healthy, normal byproduct of digestion, after all. While the smell may be embarrassing in social situations, it might mean you’ve fed your gut nutritious, fiber-rich, plant-based foods. However, sometimes a bad odor can signal a more serious health problem requiring a thorough workup by a GI doc.

The bugs in your gut
During digestion, gut bacteria produce sulfur-containing compounds like hydrogen sulfide that create a stench in your gas, Dr. Raymond notes. The foods you eat can influence the population of bacteria that live in your colon, and that can affect your farts, explains Frederick Gandolfo, MD, a gastroenterologist at Precision Digestive Care in Huntington, New York. “Certain people have a certain type of flora inside of them that causes them to produce more gas or smellier gas,” he says.

Sulfur-rich foods
Foods high in sulfur can make your farts reek of rotten eggs. Cruciferous veggies like broccoli, Brussels sprouts, cauliflower, and cabbage are often to blame. Other sulfur-rich foods include garlic, onions, legumes, cheddar cheese, dried fruit, nuts, beer, and wine. Even animal proteins, like eggs, meat, and fish–all high in sulfur–may be problematic.

In a small lab experiment involving seven healthy people, an Australian research team found that mixing poop with cysteine, a sulfur-containing component found in protein sources, resulted in a seven-fold increase in stinky hydrogen sulfide emissions. It may be why bodybuilders–who tend to consume a lot of protein powder–have notoriously foul farts, the lead researcher speculated.

Lactose intolerance
Some people blame milk, ice cream, and cheese for stinking things up, and rightly so. An estimated 30 to 50 million Americans are deficient in the enzyme (lactase) needed to digest lactose, the natural sugar found in dairy products.

In these folks, lactose passes through the small intestine without being absorbed, traveling downstream to the colon, where trillions of bacteria “have a little party,” Dr. Raymond says. The effect is smelly gas, since milk and certain cheeses have a high sulfur content.

People can also be intolerant to (and pass smelly gas from) other sugars, including sucrose (table sugar) and fructose (found in fresh fruit, corn syrup and some processed foods).

Beans, beans. They’re a rich source of protein, antioxidants, vitamins, and minerals. But, whoa, they get a bad rap for making you fart.

Beans contain raffinose family oligosaccharides (RFOs), a group of sugars that are at least partially responsible for the bloating and gas that can occur after a beany meal. So do lentils, legumes, and cruciferous veggies. The human GI tract lacks an important enzyme needed to break down and digest these sugars.

One study found that soaking dried beans in water helps remove RFOs without compromising the nutritional value of the beans. Taking an enzyme-based digestive aid (like Beano, for example) can also help ease symptoms.

Sugar alcohols
Beware of sugar alcohols, like sorbitol and xylitol, which can be found in diet drinks, sugar-free candy, and some chewing gum. These sweeteners cannot be fully absorbed by the body, so they travel to the colon where they can contribute to awful-smelling gas.

Keeping a food diary can help you determine if sugar alcohols are causing your stinky farts. Dr. Raymond recalls a female patient who had bad gas but only on weekdays. It turned out that a big bowl of sugar-free candies at her office was the culprit: The woman would grab some every morning, “and by late afternoon, in a business meeting, she would be passing lots of gas,” Dr. Raymond remembers.

Bagels, rice, and pasta aren’t necessarily gas-producing carbs. But load up on these low-fiber picks, and you may have difficulty moving your bowels. That’s when things get smelly.

When you’re constipated, poop “hangs around your colon for too long, and then bacteria can really go to town and cause lots of gas,” Dr. Gandolfo says.

To keep from getting backed up, drink plenty of water, exercise regularly, and include high-fiber foods in your diet, he says.

Certain prescription drugs, vitamins, and supplements can do a number on your gut, including turning your farts foul.

The reasons they cause flatulence differ by drug, and how much they’ll make you gas (and how badly that gas stinks) varies from one person to the next, explains doctor of pharmacy Oralia Bazaldua, professor of family and community medicine at UT Health San Antonio. “Each person, along with their provider, will have to weigh the risks versus benefits of the offending agent,” she says.

People taking the weight loss medication orlistat commonly experience gas with oily discharge. Iron supplements are also known to cause gas and bloating. Supplements containing fiber can produce gas as a byproduct, and the diabetes drug metformin can also make you gassy.

Diabetes drugs called alpha-glucosidase inhibitors, which slow carbohydrate absorption, can give you gas too. But don’t use Beano with drugs in this class, Bazaldua says, because it can affect how well the medicine works.

Infection or disease
Smelly gas alone is usually nothing to worry about, Dr. Gandolfo says. But if you also have other signs and symptoms, get checked out.

Talk to your doctor if you have fever, weight loss, blood in your stool, or persistent diarrhea alongside your stinky farts, or if you have a history of inflammatory bowel disease or colon cancer. Any change in your bowel habits or abdominal pain that doesn’t go away after you pass gas or poop could signal trouble.

It’s important to rule out (or treat) other conditions that can cause smelly farts, says Dr. Gandolfo, including a bowel obstruction, inflammatory bowel disease (such as ulcerative colitis and Crohn’s disease), colon cancer, celiac disease (an immune reaction to gluten), and C. difficile (a bacterial infection that causes diarrhea).

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