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Scientists Accidentally Discover Why Some Poops Float And Others Sink

We finally know why some poops are seaworthy.

James Felton

James Felton

Senior Staff Writer

clockNov 16 2022, 10:53 UTC
A man, in visible distress, cleaning a toilet.
Unfortunately, knowing why they float will not stop them floating. Image credit: Odua Images/

How did life spring into existence? Are we alone in the universe? Why does some poop float and bob around in the toilet while other poop sinks? These are the big questions that keep us up at night, and we finally have an answer to one of them by accident. Yes, it's the one about poop.

For years, we haven't really known precisely why some poop floats and others sink like an anchor. Theories have suggested that increased fat content in the stool can lead to the increased seaworthiness found in floaters. 


However, when one 1972 study – published in The New England Journal of Medicine – looked at the stools of 33 healthy subjects (nine with floating stools, 24 with sinking stools, and six patients with fatty stools) they found that all floaters sank when the gas within their stools was "compressed by positive pressure" (smushing up the poop).

"After degassing, previously floating and sinking stools had similar specific gravities, indicating that the floating or sinking propensity of such stools depends upon differences in gas rather than fat content," the team wrote, adding that the fatty poops were less dense than the others, though this was due to an increase in water rather than fat content. 

"Thus, stools float because of an increased content of gas or water (or both); the floating stool should not be considered a sign of steatorrhea [increased fat content in poop]."

All of that, while interesting, doesn't really explain the cause behind the difference in gas and water content. But recently, a team studying mice that had been made to be germ-free noticed something unusual about the mouse poop. While around 10 percent of healthy humans produce floaters consistently, this percentage is far higher in mice at around 50 percent. The team, who published their work in Scientific Reports, noticed that the poop of germ-free mice tended to sink.


"Our serendipitous finding of ‘sinker’ and ‘floater’ feces in TFS [Trump’s fixative phosphate-buffered solution] in germ-free and gut-colonized mice, respectively, led to the question of whether gut colonizers were fundamentally linked to the genesis of fecal floatation phenomenon," the team write in their study.

Investigating further, the team took gut bacteria from healthy mice and put them into the stomachs of germ-free mice. Sure enough, their poops began to float too.

"By introducing microorganisms into the gut of germ-free mice, we have conclusively demonstrated that gut colonization of microbiota is a pre-requisite for feces to float."

Though they stress that more study is needed to determine which gut bacteria causes the floatation – by introducing them individually to germ-free mice – and analysis of human poop is also needed, they did identify a few species of bacteria that were associated with floating poops.


"In fact, we identified Bacteroides ovatus to be the most enriched species in our analysis which has been positively correlated with flatulence and anal gas evacuation in human patients. Further, we also identified Bacteroides fragilis which is known to produce hydrogen gas in the gut."

The study is published in Scientific Reports.

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