Passing gas is a normal bodily function that everyone experiences. While flatulence (passing gas or farting) is usually nothing to worry about, particularly smelly farts can sometimes be embarrassing and uncomfortable.

This article explains why farts smell, what causes particularly foul-smelling gas, and when you might need to be concerned.

What Makes Farts Smell?

The unpleasant smell of flatulence primarily comes from sulfur-containing compounds produced when bacteria in your large intestine break down certain foods. These compounds include hydrogen sulfide, which gives farts their characteristic “rotten egg” smell (1).

Most of the gas humans release has little to no odor. Studies have shown that the unpleasant odor comes mainly from sulfur compounds produced by gut bacteria (the helpful microorganisms living in your digestive system). The smell comes from intestinal bacteria that produce compounds containing sulfur (1).

Hydrogen sulfide (which gives a rotten egg smell) is the main culprit behind smelly gas.

Fact
On average, healthy people pass gas about 10 times per day, with up to 20 times still being perfectly normal (2).

Common Causes of Smelly Farts

There are several common reasons why farts might be smelly, from the foods you eat to how your body processes them.

1. High-Fiber Foods

Many high-fiber foods can cause you to pass more gas. It takes longer for these foods to break down in your digestive system, so they ferment over time. High-fiber foods also sometimes smell, which means your farts may smell too. This is especially true with strong-smelling vegetables, such as:

  • Broccoli
  • Cabbage
  • Asparagus

Studies show there is a direct link between how much fiber is in your diet and how much gas you produce. The gas produced from high-fiber foods comes mainly from bacteria breaking down indigestible carbohydrates (parts of food your body can’t absorb) that pass into your colon (3).

Contrary to popular belief, beans may not cause as much flatulence as you might think. A study of bean consumption found that less than half of people reported increased flatulence from eating pinto or baked beans during the first week of adding them to their diet. Only 19% experienced increased gas with black-eyed peas (4).

2. Fermentable Carbohydrates (FODMAPs)

FODMAPs (Fermentable Oligosaccharides, Disaccharides, Monosaccharides, And Polyols) are types of carbohydrates that can be poorly absorbed in the small intestine. These carbohydrates pass into the large intestine where gut bacteria ferment them, producing gas as a byproduct.

A carefully controlled study comparing high and low FODMAP diets found that the high FODMAP diet led to significantly more flatulence and loose stools in participants (5).

Another high-quality study showed that a low-FODMAP diet reduced intestinal hydrogen production compared to a regular diet. This suggests less bacterial fermentation and gas production in the gut (6).

Common high-FODMAP foods that may lead to smelly gas include:

  • Onions and garlic
  • Wheat and rye
  • Beans and lentils
  • Some fruits like apples and pears
  • Artificial sweeteners like sorbitol and mannitol

3. Dairy Products

Lactose intolerance (inability to fully digest the sugar lactose in milk) can lead to excessive gas production when you consume dairy. Research on lactose intolerance has found mixed results, with some studies suggesting that lactose may cause digestive symptoms, though the effects may be modest for normal dairy consumption (7).

Interestingly, the type of milk protein may also affect digestive symptoms. Studies show that milk containing A1 beta-casein protein (a specific type of protein found in most cow’s milk) may lead to more digestive discomfort compared to milk with only A2 beta-casein (8). Flatulence is often associated with digestive discomfort symptoms.

Tip
A 2020 study found that different types of milk can affect digestive comfort differently. Some people experience fewer symptoms with A2 milk compared to conventional milk containing A1 beta-casein (9). If you have trouble with regular milk, you might want to try A2 milk to see if it makes a difference for your symptoms.

4. Small Intestinal Bacterial Overgrowth (SIBO)

Small intestinal bacterial overgrowth (SIBO) occurs when there is an abnormal increase in the overall bacterial population in the small intestine. This means too many bacteria are growing where they shouldn’t be in large numbers. This can cause excessive gas production, including foul-smelling flatulence.

A thorough review of studies found that SIBO is common in certain conditions and typically presents with symptoms like flatulence and bloating (feeling uncomfortably full or swollen in your abdomen) (10).

Another review found SIBO is prevalent after gastric bypass surgery and is associated with numerous digestive symptoms, including flatulence (11).

5. Digestive Disorders

Certain digestive disorders can increase the production of smelly gas. Conditions like irritable bowel syndrome (IBS), inflammatory bowel disease (IBD), celiac disease, and Crohn’s disease can alter gut function and lead to malabsorption of nutrients (poor absorption of nutrients from food). This can cause excess fermentation in the colon and result in smelly flatulence.

A high-quality study found that both patients with IBS and healthy people had similar physiological responses (gas production) after consuming FODMAPs. However, IBS patients reported significantly more symptoms. This suggests people with IBS may be more sensitive to normal amounts of gas rather than producing excess gas (12).

These findings are further supported by research on the gut-brain axis (the communication system between digestive tract and brain). This research shows that patients with IBS may have altered brain responses to normal intestinal gas, leading to increased symptom reporting (13).

6. Medications

Some medications can affect the digestive process and lead to smelly gas as a side effect. Common culprits include:

  • Antibiotics, which can kill beneficial gut bacteria along with harmful ones
  • Some blood pressure medications
  • Certain cholesterol-lowering drugs
  • Some medications for constipation

For instance, a study found that participants taking lactulose (a medication for constipation) commonly experienced digestive symptoms as side effects. About 45.5% had a combination of abdominal discomfort, bloating, and flatulence (14).

The Gut Microbiome and Smelly Gas

Your gut microbiome, the complex community of microorganisms living in your intestines, plays a crucial role in gas production and odor. Different types of bacteria produce different amounts and types of gas during fermentation.

A carefully controlled study examining the relationship between diet, microbiome, and gas production found that despite relatively minor differences in gut bacteria composition between diets, the microbial metabolism (how the bacteria process food) differed substantially. This led to significant variations in gas production and related symptoms (15).

Interestingly, the study also found that individuals with more diverse microbiomes (greater variety of bacteria in their gut) showed more resilience to dietary changes. These people experienced less dramatic effects from dietary shifts (15).

Fact
Recent research has found that your individual gut microbiome composition can predict your tolerance to certain foods that cause gas. In one study, researchers could predict which people would experience high levels of flatulence based on specific bacteria in their gut (16). This research points to a future where you might get personalized dietary recommendations based on your unique gut bacteria profile.

Is My Smelly Gas Normal or a Health Concern?

Occasional smelly flatulence is entirely normal, especially after eating foods known to cause gas. However, persistent or severe changes in your gas odor, accompanied by other symptoms, might indicate a health issue.

You might want to consult a doctor if smelly gas is accompanied by:

  • Severe abdominal pain or discomfort
  • Blood in stool
  • Unexplained weight loss
  • Chronic diarrhea or constipation
  • Persistent nausea or vomiting

These symptoms, together with changes in flatulence, could indicate underlying conditions that require medical attention (11).

Note
It’s important to understand that some level of gas and occasional odor is completely normal and part of healthy digestion. Most people pass gas about 10 times per day, with up to 20 times per day still considered normal (2). There’s no need to worry about occasional smelly gas – it’s simply part of how your body processes food.

When to See a Doctor About Smelly Flatulence

While occasional smelly gas is normal, you should consult a healthcare provider if:

  • Your gas smells significantly worse than usual and persists for more than a few days
  • You have severe abdominal pain or discomfort
  • You notice blood in your stool
  • You experience unexplained weight loss
  • You have chronic diarrhea or constipation
  • You have symptoms that disrupt your daily activities

These symptoms could indicate an underlying digestive condition that requires medical attention (11).

Why do some farts smell worse than others?

The smell of flatulence mainly comes from sulfur-containing compounds produced when gut bacteria break down certain foods. Some farts smell worse than others depending on what you’ve eaten (particularly sulfur-rich foods like eggs, meat, cruciferous vegetables, and some dairy products), your gut bacteria composition, how long gas remains in your intestines, and individual factors like digestive conditions. Most of the gas you pass is actually odorless, while a small percentage contains odor-causing sulfur compounds (1).

Can probiotics help with smelly gas?

Some research suggests certain probiotic strains may help reduce smelly gas. A high-quality study found that a specific type of beneficial bacteria (Lactobacillus paracasei F19) significantly reduced flatulence compared to a placebo (dummy treatment) (17).

However, responses to probiotics vary widely between individuals, and not all probiotic strains have the same effects. If you want to try probiotics for gas symptoms, look for products with research supporting their effects on digestive health and consider consulting a healthcare provider for recommendations.

Do beans really cause as much gas as people think?

While beans do contain fermentable carbohydrates that can cause gas, research suggests their reputation for causing excessive flatulence may be exaggerated. A carefully controlled study found that less than half of people reported increased flatulence from eating pinto or baked beans during the first week of consumption, and only 19% reported increased gas with black-eyed peas (4).

Your body also tends to adapt to regular bean consumption over time, often resulting in reduced gas production. If you want to add more beans to your diet, try starting with small amounts and gradually increasing your intake to give your digestive system time to adjust.

How can I tell if my smelly gas is caused by a food intolerance?

To identify if food intolerance is causing your smelly gas, pay attention to which foods consistently trigger symptoms. Common culprits include dairy products (lactose intolerance) and certain proteins like A1 casein (8).

You can try keeping a food diary for a few weeks, noting what you eat and when gas symptoms occur. You might also try an elimination diet under healthcare supervision, where you temporarily remove suspected trigger foods and then reintroduce them one at a time to observe effects. This methodical approach can help you identify your personal food triggers.

Can stress or anxiety cause smelly flatulence?

Stress and anxiety can affect digestive function. Research has found that cognitive therapy, which addresses stress and anxiety, improved flatulence and other digestive symptoms in patients with irritable bowel syndrome (18).

When you’re stressed, your digestive system can slow down or speed up, potentially changing how food is processed and leading to excess gas or odor. Managing stress through techniques like deep breathing, meditation, or physical activity may help improve overall digestive comfort, including gas-related symptoms.

How long after eating do foods typically cause gas?

The timing of gas production after eating varies depending on several factors, including the specific food consumed, your digestive transit time (how quickly food moves through your system), and individual metabolism. A study investigating sugar intolerance found that symptoms like flatulence originated from the colon during food processing (19).

In general, gas related to food typically occurs within a few hours after eating, as food moves from your small intestine to your large intestine where most gas-producing fermentation happens. However, this can vary widely among individuals and depend on the specific foods eaten.

Conclusion

Smelly flatulence is a common and typically harmless bodily function, primarily caused by the breakdown of certain foods by gut bacteria. The sulfur-containing compounds produced during this process are mainly responsible for the unpleasant odor.

Understanding the common causes of smelly gas—including high-fiber foods, FODMAPs, dairy products, small intestinal bacterial overgrowth, and certain digestive disorders—can help you identify your personal triggers. Your individual gut microbiome plays a crucial role in determining how your body responds to different foods and how much gas you produce.

It’s important to remember that passing gas is a normal part of digestion, and occasional odorous flatulence is nothing to be embarrassed about. Most adults pass gas between 10-20 times per day, and most of this gas has little or no odor.

If you’re concerned about smelly gas, try keeping a food diary to identify trigger foods, gradually introduce high-fiber foods to your diet, stay well-hydrated, and consider speaking with a healthcare provider if you have persistent symptoms or other concerning signs.

If smelly gas is persistent, severe, or accompanied by other concerning symptoms, it’s important to consult a healthcare provider to rule out underlying conditions. Otherwise, dietary adjustments, allowing your body time to adapt to new foods, and understanding your individual triggers can help manage this common and natural bodily function.

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