Why Do Farts Smell? A Dietitian Explains #1 Cause & How To Fix

If you’ve done some research into what causes smelly flatulence and how to fix it, you’ve probably seen advice like this…

Now while these sorts of answers are helpful, it left us wondering…where is the scientific research and what does it have to say about all of this?

After all, here at Essential Stacks we pride ourselves on always teaching you how the gut works and how you can improve your digestive health based on studies.

So of course, our research team decided to do some digging for you.

1. Why do most farts have no smell?

Well, that’s because when gas is produced via a healthy process of fermentation, most of it - and we’re talking 99% of it - is simply made up of a mixture of the following 5 gasses:

1. Nitrogen
2. Oxygen
3. Carbon dioxide
4. Hydrogen 
5. Methane.

And since these gasses are more or less odorless, so are your farts. In fact, you could be dropping healthy farts all day long, and no one will know. And best of all the poor dog can go about its day without being wrongly accused!

2. Why do some farts smell?

So to discover why some farts smell, we had to dive deep into the research. And the best study we came across was this 1998 piece published in the BMJ’s Gut Journal. Here's what they found…

Sulfur containing gasses are the major reason behind smelly farts or in their fancy words…“human flatus”.

And out of all the different types of sulfur gasses you might find in farts, they found 3 in particular were the most common, as they concluded…

“...our analyses of human flatus showed that hydrogen sulphide, methanethiol and dimethyl sulfide were present in much higher concentrations than were the other sulfur containing gasses”.

And out of these 3 gasses, hydrogen sulfide was the predominant gas and most important determinant of odor.

3. What do bad farts actually smell like?

Now in terms of what bad farts smell like, well lucky for us the scientists in the 1998 study didn’t stop at simply identifying the gasses. Instead, like carafe-wearing wine snobs from napa valley, they also provided some rather interesting “smelling notes”. As they wrote…

“...the odors of hydrogen sulfide, methanethiol and dimethyl sulfide were respectively described as rotten eggs, decomposing vegetables and sweet”.

And while you’re absorbing that, how about we take a moment to thank the poor scientists who had to sit around collecting this data!

4. What causes these gasses and how to fix?

So now we know that sulfur-based gasses are responsible for our smelly farts, the question becomes: what causes these gasses to occur in the first place? After all, if we can answer this, then we can start to work on ways to reduce them.

And the best starting point is to actually look at what the researchers fed the test subjects in the 1998 study. And so it turns out…

“To ensure flatus output, the diet of the subjects was usually supplemented with 200 grams or 7 ounces of pinto beans”.

Now, what other foods can cause smelly farts?

Well, the researchers noted that hydrogen sulfide, which is the leading cause of rotten egg smelling farts, is a “product of the metabolism of sulfate-reducing bacteria”. And, you don’t need to understand exactly what that means.  All you need to do is just picture these bacteria as magical farting unicorns. Now, how do we get these unicorns to fart less?

Well, researchers said we should try to either reduce these types of bacteria or reduce the availability of their favorite food, which is sulfur rich foods. So what are the sulfur rich foods that they love so much?

Well, the researchers shared some examples such as “cruciferous vegetables like cabbage and broccoli, as well as nuts, and an additive in bread and beer”. Other examples of sulfur rich foods would be garlic, lentils and grains.

And if you want to discover the complete list of foods that might lead to magical unicorn farts…I mean smelly gas…then download our free Flatulence Foods List. It goes through all the different types of foods that cause flatulence…both the smelly and the loud kinds.

5. Our conclusion

So to wrap things up, thankfully most farts don’t smell, since they largely contain odorless gasses like nitrogen, oxygen and methane. But when we do pass wind that has a distinct smell to it, then it’s usually thanks to sulfur containing gasses.

If it smells like rotten eggs, you can thank your friend hydrogen sulfide. And if it smells like decomposing vegetables…well…then you can tip your hat to methanethiol.

In terms of how we can reduce these smelly farts from happening, the main way is to monitor your intake of sulfur rich foods like beans, cruciferous vegetables and nuts.

Of course, since many of these foods are also rich in nutrients and great sources of fiber, you don’t want to cut them out entirely. Instead, you should focus on training your gut to digest these foods better. And we explain how to do this in another article on flatulence.