why does my fart stink so bad

Gas – specifically, the kind that makes a room empty quickly – is a topic many people find themselves pondering. And you’re not alone. That stuff that comes out of your rear end (medically referred to as flatus) is created by the combination of multiple factors, mainly: what you ate and how much, and what type of bacteria lives in your gut.

The Science Behind A Stinky Fart

Most of the air expelled from the anus contains various combinations of gases. Many of the gases you exhale were swallowed; they’re mostly nitrogen and oxygen, so they usually aren’t offensive. There are however gases generated while your gut bacteria digest food that wasn’t fully digested in your stomach. Those are the ones that create the really bad smells.

Hydrogen Sulfide: The Bad Boy Gas

Of all the compounds associated with the strong smells of passing gas, hydrogen sulfide (H2S) may be the most well-known. H2S is the same gas that produces the pungent odor of rotten eggs. Your gut bacteria, especially those living in low-oxygen (anaerobic) conditions, generate H2S as a by-product when they metabolize certain sulfur-containing compounds. These compounds are present in certain foods.

Odorous Compounds

H2S is frequently the strongest odor generating gas, but there are others. Methanethiol (methyl mercaptan) and dimethyl sulfide are two more sulfur containing compounds that will give your gas a similar smell to cabbage or just plain stink. Both indole and skatole are also created from tryptophan metabolism and add a distinctly fecal smell. Finally, ammonia can also contribute to a lesser extent to the total odor.

The Food Connection

What you eat affects the types of gases your gut bacteria will produce. Some foods are high in sulfur compounds or resistant starches, which will increase the generation of odorous gases.

Sulfur-Containing Foods

The following are common sources of sulfur-containing compounds and thus can be considered among the primary causes of pungent flatulence.

Cruciferous Vegetables: Broccoli, Cauliflower, Cabbage, Kale, etc. These vegetables are notorious for their sulfur content.
Onions & Garlic: Members of the Allium family of plants.
Beans: Legumes contain a significant amount of sulfur compounds and are generally know for being gassy.
Eggs: Yolk is particularly high in sulfur.
Meats & Fish: Meat is high in protein and fish can be high in sulfur containing amino acids.
Dairy Products: Lactose intolerant individuals may notice an increase in volume and odor with their stools due to an inability to digest lactose effectively.

Resistant Starches

Fiber and resistant starches are great for your gut. They feed your beneficial bacteria. But their consumption results in a greater VOLUME of gas. So, although they don’t introduce additional sulfur-containing compounds into your system, they DO increase the chances that smelly components will accumulate. Whole grains, potatoes, and green bananas are good examples of resistant starches.

The Bacteria In Your Gut

As stated earlier, each person’s gut microbiome is unique, much like fingerprints. As such, two people eating the same meal can have vastly different flatulent experiences based upon their individual gut microbe populations.

Sulfate Reducing Bacteria (SRBs)

Specific groups of bacteria within your colon specialize in converting sulfur-based compounds into hydrogen sulfide. SRBs can be more active depending upon your diet and have been shown to produce varying amounts of H2S. Studies have identified links between hydrogen sulfide levels in flatus and specific microbial populations. One study demonstrated Desulfovibrio piger is a SRB commonly linked to hydrogen sulfide production.

Gastric Fermentation

Fermentation occurs when your gut bacteria consume undigested carbohydrates that escaped digestion in the upper part of your digestive system (the small intestine). The fermentation process itself generates gas. Although some of this gas is odorless (such as hydrogen and methane), its sheer volume contributes to the overall quantity of gas you release through passing gas.

Factors Beyond Diet & Gut Bacteria

There are other factors outside of your diet and the bacteria that live inside your gut that affect how “bad” your passing gas will smell.

Digestion Rate & Digestive System Function

How fast food passes through your digestive system will determine how long your gut bacteria have to work on breaking it down. Faster transits can result in less complete digestion. Slower transit times give more time for bacteria to break down more nutrients resulting in more gas (and therefore more potential odor).

Health Issues & Medications

Medical problems affecting digestion can result in altered rates of passage and/or disrupted microbial balances. Thusly affecting gas volume and/or odor. Common examples include antibiotics (which kill off helpful bacteria allowing bad bacteria to overgrow), iron supplements (which disrupts digestive function), and certain medications used to treat IBS.

Hormones & Menstrual Cycle

Hormone swings throughout menstruation have been reported to alter digestion speed and gut motility indirectly causing changes in gas volume and odor. However this connection is anecdotal.

Determining When To Seek Medical Attention

In most cases, having stinky farts every now and then is completely normal. However, occasionally it might be indicative of an underlying health concern.

Look for signs such as:

Sudden or persistent changes in gas volume/odor.
Severe abdominal cramps/pain.
Bloating.
Unintended weight loss.
Diarrhea or constipation without explanation.
Blood in stool.

In these instances it would be wise to seek advice from a medical physician to ensure no underlying health issue exists.

Controlling Flatulence Odors & Volume

You won’t get rid of flatulence entirely (you shouldn’t want to either; it indicates that your digestive system is functioning!). You’ll probably need to adjust your diet to control odor/frequency. Keep track of when you eat specific foods by using a food diary to see which foods elicit the worst smells. Once you’ve established a list of possible culprits consider limiting portion sizes or decreasing how often you consume them.

Take longer when you chew food: Chewing food better initially breaks-down food further reducing the burden placed upon bacteria in your colon.
Eat slower: Swallowing less air means less non-stinky gas is introduced into the mix.
Use Probiotics & Prebiotics: Balance can be achieved between your microbiome and your diet utilizing pre/probiotics. Before adding anything new please discuss with your healthcare provider.
Use Over-The-Counter Remedies: Activated Charcoal or Bismuth Subsalicylate (Devrom) may help neutralize some odors by absorbing gas, however results vary.

Overall, how smelly your farts are depends upon how efficiently you digest food; what you eat; and what tiny organisms reside in your gut. Understanding these factors will allow you to begin making educated decisions regarding what you eat and take care of your own digestive health.