Smelly farts are usually not a sign of anything wrong. The odor comes from sulfur-containing gases that gut bacteria produce as a normal part of digestion, and everyone generates them. The smell can vary day to day based on what you eat, how your gut bacteria are behaving, and how long food sits in your colon. That said, a persistent change in smell, especially paired with other symptoms, can occasionally point to a digestive issue worth looking into.
What Makes Farts Smell
Most of the gas you pass is odorless. It’s mostly nitrogen, oxygen, carbon dioxide, hydrogen, and sometimes methane. The smell comes from a tiny fraction of the total volume: sulfur-containing compounds, primarily hydrogen sulfide, the same gas responsible for the rotten egg smell. Gut bacteria produce hydrogen sulfide by breaking down sulfur-containing amino acids (the building blocks of protein) like cysteine and taurine. A separate group of bacteria, particularly species in the Desulfovibrio genus, generate it by processing sulfate, a compound found naturally in many foods and drinking water.
So the smell isn’t a malfunction. It’s a byproduct of bacteria doing exactly what they’re supposed to do. The intensity just depends on how much sulfur-rich material they have to work with and which bacterial populations are most active at any given time.
Foods That Make It Worse
If your gas has gotten noticeably more pungent, your diet is the most likely explanation. Foods high in sulfur compounds give gut bacteria more raw material to convert into hydrogen sulfide. The biggest contributors include:
- Eggs and red meat, which are rich in sulfur-containing amino acids
- Cruciferous vegetables like broccoli, cauliflower, Brussels sprouts, and cabbage
- Alliums like onions and garlic
- Beans and lentils
- Mushrooms
These are all nutritious foods, so the goal isn’t necessarily to eliminate them. But if you recently increased your intake of any of them, that likely explains the change in odor.
The Protein Powder Connection
People on high-protein diets frequently report smellier gas, and there’s a straightforward explanation. More protein reaching the colon means more sulfur-containing amino acids for bacteria to ferment. Meat and eggs are particularly high in sulfur, so diets built around those will tend to produce more hydrogen sulfide.
Protein supplements deserve special attention. Whey protein powders, shakes, and bars are commonly blamed for foul-smelling gas, but the protein itself may not be the main culprit. Many of these products contain additives like sugar alcohols, artificial sweeteners, and lactose (from the whey) that ferment aggressively in the gut. If protein shakes are making your gas unbearable, switching to a product with fewer additives or a non-dairy protein source can make a noticeable difference.
Constipation Intensifies the Smell
When stool moves slowly through your colon, bacteria have more time to ferment it. The longer food waste sits, the more sulfur gas accumulates. Constipation is one of the most common and overlooked reasons for gas that smells distinctly like sulfur. If you’re not having regular bowel movements and your gas has gotten worse, getting things moving again with more water, fiber, and physical activity will often resolve the smell on its own.
How Much Gas Is Normal
Medical textbooks have long cited around 14 times per day as the average, but that number appears to be low. A recent study that used wearable sensors built into underwear to continuously measure gas found that participants passed gas an average of 32 times per day, with a range spanning from 4 to 59 times. The researchers noted that there isn’t actually a widely accepted baseline for what “normal” flatulence looks like. The takeaway: you’re probably gassing more than you think, and so is everyone else.
Frequency matters less than change. If you’ve always been gassy, that’s just your normal. If you’ve suddenly become much gassier or the smell has dramatically shifted, that’s more worth paying attention to.
When Smelly Gas Signals a Problem
In most cases, foul-smelling gas is harmless. But it can occasionally be a symptom of a digestive condition, particularly when it shows up alongside other changes.
Small intestinal bacterial overgrowth (SIBO) happens when bacteria that normally live in the large intestine colonize the small intestine in excessive numbers. These bacteria ferment carbohydrates earlier in the digestive process than they should, producing extra gas, bloating, and often smellier flatulence. SIBO can also interfere with fat absorption, leading to oily or unusually foul-smelling stool. It’s diagnosed with a simple breath test that measures hydrogen and methane levels.
Food intolerances, particularly to lactose or fructose, can also increase gas production and odor because undigested sugars ferment in the colon. Celiac disease, inflammatory bowel conditions, and infections can all alter digestion in ways that change gas patterns.
The NIDDK recommends talking to a doctor if your gas symptoms bother you consistently, if they change suddenly, or if they come with abdominal pain, diarrhea, constipation, or unexplained weight loss. Smelly gas alone, without these accompanying symptoms, is rarely a concern.
How to Reduce the Smell
The most effective approach is dietary. Cutting back on high-sulfur foods for a week or two will usually tell you whether your diet is the driver. You don’t need to eliminate these foods permanently. Just eating smaller portions of them or spreading them across meals rather than loading up at once can help.
For a more direct solution, bismuth subsalicylate (the active ingredient in Pepto-Bismol) is surprisingly effective. In a study published in the journal Gastroenterology, taking bismuth subsalicylate for several days reduced hydrogen sulfide release from stool samples by more than 95%. Bismuth binds directly to sulfide in the gut, neutralizing it before it becomes gas you can smell. This isn’t something most people need daily, but it’s a useful option if you’re dealing with a temporary bout of especially offensive gas.
Probiotics may help by shifting the balance of bacterial populations in your gut, though results vary widely between individuals. Eating more slowly and chewing thoroughly reduces the amount of undigested food reaching the colon, which gives bacteria less to ferment. And staying hydrated and physically active keeps your digestive system moving, reducing the stagnation that lets sulfur gases build up.