Olivia: Have you ever looked down after doing your business and noticed that your poop is floating? Maybe this comes and goes, or maybe your poop is always buoyant. You might have even worried that this was a problem.

Like, if you look it up online, a lot of reputable websites will mention that one reason your poop floats is because of fat. Fat is less dense than water, and so, the thinking goes, if you have enough of it in your poop, that’s probably why it floats.

But fatty stools—what doctors like to call steatorrheic stools— happen because you aren’t absorbing the fat you eat. And that’s usually a sign of a really serious problem, like pancreatic cancer or cystic fibrosis.

Back in the early ‘70s, Michael Levitt and William Duane, a pair of researchers at the University of Minnesota, were annoyed by this assumption that fat is why feces float. They suspected that trapped gas was the more likely culprit. After all, about 15 percent of perfectly healthy people—including Duane— consistently had bobbers.

So, they decided to investigate, and kindly wrote up their results in the prestigious New England Journal of Medicine in a paper called, “Floating Stools — Flatus versus Fat.” 33 healthy volunteers, some with floaters, donated samples, along with 6 patients with steatorrhea.

The scientists put the specimens in flasks with water to see if they sank. If not, they added pressure to squeeze out any gas and checked to see if anything still floated. But even the fattiest poops sank to the bottom when degassed. In fact, the scientists determined that it’s almost impossible for a bowel movement to float just because of its fat content. You would need it to be half fat.

So gas creates most of the uplift. But what is it, and where is it coming from? Luckily, our gutsy duo also analyzed the poo samples with gas chromatography to separate out the compounds and identify the mystery gas. It turns out that a big part is methane, the same stuff in natural gas. It’s made by special bacteria in the colon called methanogens.

Everybody has some of these bugs, but some people seem to have way more— like William Duane, who admitted in his paper that he produced methane at ‘near record proportions.’ Methanogens run on hydrogen, and can ferment fiber in the gut, a bit like a cow can. That means you get more energy out of fiber-rich foods like beans.

So, don’t believe everything you read online about your poop. If you have floaters, you could be just gassy. And if you have fatty poop that happens to float? Well, that’s because of gas, too.

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