Why are my farts stinky?

Blame it on fiber if you want, but protein is probably the source of the stench.

I’m not sure how many subscribers I just lost with that title, but it was a matter of time before they figured out I possess a child-like curiosity juvenile sense of humor, so now that the adults have left the room, let’s party.

Flatulence impacts us all, some more than others, depending on how often they’re in crowded elevators. But athletes have a special relationship with tooting. They frequently experiment with various diets, consume high or low volumes of food, and push their bodies so hard that their digestion goes wonky.

As a result, they fart a lot.

Most of the time, it’s no big deal. We all pass gas. It’s a normal byproduct of a healthy digestive system. By most reports, twenty farts a day is acceptable. A 2003 study in the European Journal of Surgery claims men fart, on average, 12.7 times per day. Women sound off at a rate of 7.1 times per day. Personal experience causes me to question these numbers, but I’m not about to confront my wife or daughters on the issue, so let’s move on.

Why we fart.

By the time food makes it to the large intestine, we’ve pretty much digested everything we’re going to digest. The leftovers include protein, fat, and miscellaneous stuff that the body can’t process, such as marbles, Legos, and orange chrome Presta valve caps. (I prefer not to discuss that last one. Let’s just say I’ve since learned to better avoid potholes.)

But the bulk of the stuff that ends up in your bowels is fiber—carbohydrates so complex that the human body can’t digest them, so they just pass on through. (That is unless you’re a slow learner and a lousy cyclist, in which case the bulk might feature a high percentage of valve caps.)

There are two kinds of fiber: soluble and insoluble. Insoluble fiber—which appears in wheat, nuts, potatoes, and beans—goes entirely through you, adding bulk to your poop. Soluble fiber turns to a gel as it passes through your gut and soaks up cholesterol. You’ll find soluble fiber in oats, beans (again), carrots, and apples, among other foods.

Soluble fiber doesn’t come out in your poop. Instead, it is eaten by bacteria in your large intestine, along with some protein and fat. This is a good thing.

The term “prebiotic fiber” refers to soluble fiber that healthful intestinal bacteria especially enjoy consuming. Oligosaccharides, which you’ll find in beans and other pulses, are a well-known prebiotic fiber.

When bacteria in your gut (aka your microbiome) eat prebiotic fiber, they produce short-chain fatty acids (SCFAs), which offer a staggering awry of benefits, including anti-inflammatory, immunoregulatory, anti-obesity, anti-diabetes, anticancer, cardiovascular protective, hepatoprotective, and neuroprotective activities.

We can’t really eat SCFAs in a meaningful way, so eating the fiber that allows our gut bacteria to produce them is important.

In addition to SCFAs, this process produces various gases—and this is why we fart.

For what it’s worth, your microbiome becomes more efficient with practice, so if I’ve compelled you to eat more fiber and it gives you excessive gas, tough it out for a few weeks. The volume should decrease.

(Interesting side note: Some of these gases reabsorb into your system and are exhaled through your mouth. That is to say, we breathe out farts, which is basically the opposite of talking out of your ass.)

The bad kind of gas.

99% of the gas in farts is an odorless cocktail of hydrogen, carbon dioxide, and methane. (The methane causes farts to burn blue should you ignite them—not that I’ve experimented with this firsthand.)

(Okay, that was a lie. My friends and I often lit up our farts during my sophomore year at UCSB. We stopped when my roommate Oldberg’s butt somehow inhaled and he burned his anus. It was terrible, but I really wish we had camera phones back then because it was also incredibly funny.)

Anyway, the other 1% of the gas is what you need to worry about from an odor perspective. There are three main stinkers at play here: methanethiol, dimethyl sulphide, and the worst offender, hydrogen sulfide.

In other words, your farts stink when your microbiome produces excessive sulfur.

(Considering that brimstone is an olde timey word for sulfur, Psalms 11:6 literally claims that God will make evil-doers drink a cup filled with fire and farts. I’m not sure how to unpack this except to have mild concern for Oldberg in the afterlife.)

Our bodies have a love/hate relationship with hydrogen sulfide. Its production is normal. At low levels, it can help maintain the mucus lining of your intestines, ward off bad bacteria, fight inflammation, and repair tissue. However, too much of it messes with your gut mucus, causes inflammation, and may contribute to the development of cancer and Parkinson’s Disease.

To be clear, farts never smell great, but that gross, earthy smell is completely normal. What you want to minimize is that pungent, room-clearing rotten egg smell. 

If it’s common for the guy sitting next to you on your commuter flight to fan the air and mumble, “I think I’d prefer snakes on this plane,” it’s time to look at your diet.

Protein makes you poot.

Some fibrous veggies contain sulfur compounds that can result in hydrogen sulfide, most notably allium veggies (garlic, leeks, onion), cruciferous veggies (broccoli, Brussels sprouts, cabbage), and legumes (beans!). However, the primary offenders in most diets are the sulfur-containing amino acids methionine and cysteine. You get these from eating protein—namely meat and eggs.

These two amino acids are broken down in your gut by ironically named sulfate–reducing bacteria. This produces hydrogen sulfide.  

Intolerances to various foods—lactose intolerance, for example—can also produces hydrogen sulfide, but most of the time, too much meat and eggs tend to be the culprit.

Next steps.

If your flatulence only occasionally burns your nose hairs, don’t stress. That’s life—although it wouldn’t hurt to track when it happens in relation to your diet to learn what caused it.

If your farts clear the room on a regular basis, get serious about tracking your diet. Start with dialing down your animal-based protein to see if that helps. You might also experiment with an elimination diet.

If your stinky gas is accompanied by bloating, constipation, pain, or diarrhea, check with your doctor just in case something serious is happening up there.

And if you have any fart questions—or, more importantly, good fart stories, please post them to the comments!