5 Reasons Why Your Farts Smell (Really) Bad, According to GI Doctors

Woman clutching her stomach from stomach pain and gas

Constantly asking yourself, “Why do my farts smell so bad?” GI doctors break down what causes smelly gas and how to get rid of it.

So you’ve passed some gas and it smells, er… funky. First off, there’s nothing to be embarrassed about. Everyone has gas, and it’s completely normal for your farts to smell. In fact, having gas is a byproduct of digestion and the air you swallow, hence why you pass gas several times a day.

But if you’re asking yourself why your farts smell really bad, there’s usually a perfectly reasonable explanation. Turns out, the food you eat can affect the way your gas smells, causing a metallic or rotten egg odor. Below, gastroenterologists share everything you need to know about what causes smelly gas and how to avoid it.

What Causes Gas?

Gas is produced when the foods we eat are broken down and processed, but also by the good and bad bacteria we have in our colon and small intestine, explains board-certified gastroenterologist Niket Sonpal, MD. “In normal circumstances, everybody is going to produce a little bit of gas,” he says.

Why Do My Farts Smell So Bad?

It’s mainly the bacteria in your large intestine that emit sulfur that gives the gas you pass a stinky smell, according to Cedars Sinai.

Eating something out of your norm or that’s heavy or spicy is a likely cause of smelly gas. However, if your gas smells really bad on a consistent basis, you’re probably wondering if it’s something more serious. Here are some of the possible causes behind ultra smelly farts.

what causes smelly gas

1. You’re Eating Foods With Sulfur 

Foods that contain sulfur double down on smelly gas. When the gut breaks them down, they trigger sulfur-containing gases such as methanethiol, dimethyl sulfide, and hydrogen sulfide.

Foods high in sulfur include:

  • Cruciferous vegetables (such as cauliflower, broccoli, kale, and cabbage)
  • Alliaceous vegetables (like onions, leeks, and garlic)
  • Minced beef
  • Fish
  • Chicken
  • Rice
  • Pasta
  • Nuts 
  • Eggs

“It is the sulfur compounds that cause gas to smell, and the formation of sulfur occurs by metabolism of food in the colon, principally by gut bacteria,” says Jacqueline Wolf, MD, a gastroenterologist at Beth Israel Deaconess Medical Center, associate professor of medicine at Harvard Medical School, and author of A Woman’s Guide to a Healthy Stomach: Taking Control of Your Digestive Health

But the different compounds in foods can cause different smells. For example, garlic can cause metallic-smelling gas and cruciferous vegetables can cause a rotten egg smell, Dr. Wolf says. 

So if you’re asking yourself, “Why does my gas smell like metal?” it’s likely due to certain foods in your diet. “A paleo diet that is high in meat is said to cause a metallic odor,” Dr. Wolf explains. “Fatty meats contain large amounts of methionine, which contains sulfur and is broken down in the gut to hydrogen sulfide to cause a rotten egg smell.”

That said, the benefits of eating fiber-rich vegetables and fruits outweigh the foul-smelling gas they may cause, Dr. Sonpal notes. 

2. You Have a Food Intolerance

When food isn’t absorbed properly, it sits in your GI tract and decomposes, which causes foul-smelling gas, Dr. Sonpal says. He adds that lactose intolerance—the inability to digest lactose, the sugar in dairy—is one of the most common food intolerances that may cause smelly gas. “The body loses the ability to break down lactose, so the milk sugars rot and give people very foul-smelling gas and diarrhea,” he explains. 

In addition to foul-smelling gas, people with lactose intolerance may also experience nausea or vomiting, stomach cramps, and bloating.

If you’re lactose intolerant, some milk-based foods you may want to avoid or choose non-dairy alternatives or lower-lactose options for are yogurt, ice cream, and cheese. Other lactose-containing foods you’ll want to be aware of are lunch meats, coffee creamers, and salad dressings. It’s also in some birth control pills, so make sure to read the ingredients labels of foods and supplements carefully.

3. You Have Celiac Disease

Celiac disease is another reason you might have foul-smelling farts, Dr. Sonpal says. Celiac disease is an autoimmune disorder that damages your small intestine after eating foods that contain gluten—a type of protein naturally found in wheat, barley, and rye.

“People with celiac disease are unable to tolerate wheat (or anything with gluten, for that matter), so then that allergy to wheat causes an antibody to be produced. That antibody ends up attacking the lining of the GI tract and leads to malabsorption [of food], which causes it to rot and creates gas, diarrhea, and abdominal pain,” Dr. Sonpal explains. 

So if you experience foul-smelling gas after eating foods made of wheat, barley, rye (or any combination of those), then you’ll want to check in with your doctor about getting tested for celiac disease.

Pasta, crackers, and baked goods are obvious sources of gluten. However, you may also find it in seasonings for French fries and potato chips, as well as thickeners for sauces, marinades, and soups.

Man chopping garlic and onions, both of which contain sulfur and FODMAPs

4. You’re Eating High FODMAP Foods

People with irritable bowel syndrome (IBS) or small intestinal bacterial overgrowth (SIBO) have trouble digesting high FODMAP foods, which can cause more stinky gas, Dr. Wolf says. However, people who don’t have IBS can also experience smelly gas from eating high FODMAP foods. 

FODMAPs are short-chain carbohydrates (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols) in foods that aren’t completely digested in our intestines. The smelly gas from high FODMAP foods is due to bacteria in the gut breaking down the undigested food, as well as the sulfur compounds in the foods, Dr. Wolf explains. 

“Most of the odor is from sulfur compounds. However, odor can also come from short-chain fatty acids (metabolites from the fermentation of dietary fiber), skatoles (from the decomposition of tryptophan), indoles (a chemical found in vegetables), volatile amines, and ammonia. They all have a distinctive odor,” she says.

For people who have SIBO, this means that the balance of good and bad bacteria in your small intestine is off. Having an excess of bad bacteria can disrupt the digestive process and cause gastrointestinal distress via gas, bloating nausea, and diarrhea.

“A number of things can cause SIBO, including developing gastroenteritis (stomach virus) and taking antibiotics,” Dr. Sonpal explains. 

Foods high in FODMAPs include:

  • Apples
  • Peaches
  • Cherries
  • Blackberries
  • Garlic
  • Onions
  • Leeks
  • Cauliflower
  • Bread
  • Pasta
  • Legumes and pulses
  • Dairy
  • Processed meats
  • Sweeteners, such as sugar alcohols

If you have IBS or SIBO, following a low FODMAP diet may help you find some relief from your symptoms. That said, Dr. Wolf recommends working with a registered dietitian who specializes in the diet and treats patients with IBS. 

5. You’re Taking Certain Medications

Metallic-smelling gas might also come from B vitamins, iron, dyes, and preservatives in supplements, as well as NSAIDs, laxatives, and statins, Dr. Wolf says. 

“Women sometimes take fluconazole (a medication to treat yeast infections), which throws off the balance and destroys all the good yeast. You have to look at that possibility as well as to why you have foul-smelling gas,” Dr. Sonpal says. “Moreover, a lot of medications are encased with sorbitol, a sugar alcohol that can cause GI distress.”

why does my gas smell so bad gut instinct product picture

How to Get Rid of Smelly Gas

Treatment for smelly gas will depend on the underlying issue you have. That’s why it’s important to see a doctor if you notice that your farts are chronically foul and you’re producing a lot of them.

So if your doctor determines that you have lactose intolerance or celiac disease, for instance, you would eliminate lactose or gluten from your diet, and that should help relieve your symptoms, Dr. Sonpal says. 

“If you have IBS or SIBO, you may supplement your foods with probiotics, like yogurt, sauerkraut, and kombucha, and prebiotics (fiber that feeds probiotics),” Dr. Sonpal says. Looking for a probiotic supplement to add to your routine instead? Try HUM’s Gut Instinct, made of a vegan blend of 10 strains to support a balanced gut microbiome and healthy digestion.

You can also eat more foods that contain prebiotics, as well as take a supplement that contains them, such as HUM’s Beauty zzZz Gummies.

Consulting a registered dietitian who specializes in IBS and SIBO may also help you figure out a meal plan that will identify which foods may trigger your symptoms.

Final Thoughts

“Foul-smelling gas once in a while is okay. But if it’s chronic, we have to get to the bottom of why it’s happening,” Dr. Sonpal says.

If your smelly gas persists, see your doctor to get a proper diagnosis—especially if you’re experiencing additional symptoms. “If it’s accompanied by diarrhea or a lot of bloating, or if you’re starting to become anemic and there’s blood in the stool, these are all signs that something is wrong with the lining of the GI tract,” Dr. Sonpal concludes.

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