Learn About Common Causes of Smelly Poop

Understanding Digestive System Basics and How They Affect Stool Odor

The smell of your stool is a natural part of the digestive process, and understanding why it happens starts with knowing how your body breaks down food. When you eat, your stomach and small intestine work to break down proteins, fats, and carbohydrates into smaller molecules your body can absorb. What doesn't get absorbed moves into your large intestine, or colon, where water is removed and the remaining material becomes stool.

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The odor comes from bacteria in your colon breaking down undigested food, particularly proteins and sulfur-containing compounds. These bacteria are actually beneficial—they help with digestion and produce important nutrients like vitamin K. However, their byproducts create gases and compounds that produce smell. This is why stool always has an odor, but the intensity can vary greatly depending on what you eat and how your digestive system functions.

Research published in the journal Gastroenterology shows that the human colon contains trillions of bacteria representing hundreds of different species. These microorganisms are essential to health, but they also produce volatile organic compounds that create stool odor. The smell gets stronger when certain conditions occur—when food moves through your system too quickly, when you eat more protein than usual, or when your bacterial balance shifts.

Several factors affect how your digestive system processes food. Age plays a role; older adults often experience changes in digestive function. Medications, especially antibiotics, can alter your bacterial population. Stress affects how quickly food moves through your system. Even your genes influence your digestive enzymes and how efficiently you break down certain foods.

Practical takeaway: Normal stool odor is caused by bacteria doing their job during digestion. If you want to track changes in your stool odor, keep notes about what you eat and any other digestive symptoms you experience. This information can help you spot patterns and may be useful if you discuss your health with a healthcare provider.

How Diet Directly Influences Stool Smell and Digestive Function

What you eat has the most immediate impact on stool odor. Certain foods contain compounds that create stronger smells when broken down. Protein-rich foods like meat, eggs, and fish are major contributors because they contain sulfur compounds. When bacteria break down these proteins, they release hydrogen sulfide and other sulfur gases that smell like rotten eggs. A study in the American Journal of Gastroenterology found that people who increased their red meat consumption by 50 percent reported noticeably stronger stool odors within days.

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Cruciferous vegetables including broccoli, cabbage, and cauliflower contain compounds called glucosinolates. Your intestines convert these into sulfurous gases during digestion. While these vegetables are very healthy, eating large amounts can increase gas production and stool odor. Garlic and onions contain similar sulfur compounds. These foods aren't bad for you—they contain beneficial nutrients and fiber—but they do affect odor more than other foods.

Foods high in fat can also increase stool odor, particularly when combined with protein. Fat slows digestion, giving bacteria more time to break down food. This extended time produces more odorous compounds. Fried foods, fatty meats, and heavy dairy products can all contribute to this effect. A clinical observation from Mayo Clinic gastroenterologists noted that patients reducing high-fat intake typically reported reduced stool odor within one to two weeks.

Processed foods and foods with artificial additives can affect stool odor in different ways. Your body may not break down certain additives efficiently, which means they reach your colon partially undigested. This gives bacteria more substrate to work with, increasing odor production. Additionally, foods with high sugar content but low fiber can cause rapid bacterial fermentation, producing more gas.

Other foods that commonly increase stool odor include:

• Beans and legumes (contain complex carbohydrates that bacteria ferment)
• Alcohol, especially beer (increases bacterial fermentation)
• Spicy foods (can speed digestion and alter bacterial activity)
• Dried fruits and nuts (high in compounds bacteria readily ferment)
• Corn and corn products (not fully digestible for many people)

Practical takeaway: Track which foods seem to increase your stool odor. You don't need to avoid these foods, but knowing which ones affect you helps you manage expectations. If you're planning an important day or event, you might eat lighter meals with less protein and fewer cruciferous vegetables the day before. Keep a simple food diary for one to two weeks, noting what you eat and any changes in stool odor.

Bacterial Imbalances and Dysbiosis as Major Odor Contributors

Your colon contains a complex ecosystem of bacteria called the microbiome. When this ecosystem becomes unbalanced—a condition called dysbiosis—stool odor often increases significantly. Dysbiosis means that harmful or odor-producing bacteria have multiplied while beneficial bacteria have decreased. According to research published in Nature Reviews Gastroenterology and Hepatology, dysbiosis affects approximately 10 to 15 percent of people in developed countries.

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Antibiotics are the most common cause of dysbiosis. While antibiotics kill harmful bacteria causing infection, they also kill beneficial bacteria. Even one course of antibiotics can reduce beneficial bacterial populations by 30 to 50 percent. It typically takes several weeks for these populations to recover, during which time stool odor often increases noticeably. Some people take months to fully recover their bacterial balance after a course of strong antibiotics.

Other causes of dysbiosis include excessive alcohol consumption, which creates an environment where certain odor-producing bacteria thrive. Chronic stress affects your gut bacteria through the gut-brain axis—a two-way communication system between your nervous system and your digestive system. High stress levels can shift your bacterial population toward more odor-producing species. A study published in Psychosomatic Medicine found that people under chronic stress had significantly different bacterial compositions than control groups, with increased levels of odor-producing bacteria.

Diet also plays a role in dysbiosis. Eating a diet very high in processed foods and low in fiber can starve beneficial bacteria while feeding harmful ones. Your beneficial bacteria thrive on fiber, so a low-fiber diet essentially starves them. Conversely, a diet rich in whole grains, vegetables, fruits, and legumes feeds beneficial bacteria and keeps them strong enough to outcompete odor-producing species.

Signs that you might have dysbiosis include:

• Noticeably increased stool odor that lasts weeks or months
• Changes in stool consistency or frequency
• Bloating and gas that seems excessive
• Digestive discomfort or cramping
• Changes in appetite
• Fatigue or mood changes

Practical takeaway: If you recently took antibiotics or experience prolonged stool odor changes, focus on eating fiber-rich foods and fermented foods like yogurt, kefir, sauerkraut, or kimchi. These foods support beneficial bacteria recovery. Give your bacterial population time to recover—improvements typically take four to eight weeks. If stool odor changes persist beyond two months or are accompanied by other digestive symptoms, contact a healthcare provider.

Medical Conditions That Cause Changes in Stool Odor

Several medical conditions can cause noticeably increased or changed stool odor. Irritable bowel syndrome (IBS) affects approximately 10 to 15 percent of the U.S. population and frequently causes stool odor changes. IBS alters how quickly food moves through your colon—either too fast or too slow. When stool moves too quickly, undigested food reaches your colon, giving bacteria more material to break down and creating stronger odor. When stool moves too slowly, food sits longer in your colon, also increasing bacterial breakdown time.

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Inflammatory bowel diseases like Crohn's disease and ulcerative colitis can cause malabsorption—your intestines don't absorb nutrients and water properly. Unabsorbed material reaches the colon, feeding bacteria and increasing odor. These conditions also often involve bacterial imbalances. People with IBD typically have different bacterial populations than people without these conditions, with higher levels of odor