Why do some poops float and others sink?

A study sheds light on the question

It’s a mystery that comes from afar. I’m talking about why sometimes poop floats and other times sinks. Theories told that a higher percentage of fat may contribute to making poop float.

According to this article, one 1972 study published in The New England Journal of Medicine, however, revealed that all poops sank when the gas inside was “compressed by positive pressure” when it examined the stools of 33 healthy individuals (9 with floating stools, 24 with sinking stools, and 6 patients with fatty stools).

“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 fatty poops were less thick than the others, but this was attributable to an increase in water content rather than fat content, the authors noted.

“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 this is fascinating, but it doesn’t really explain why the gas and water contents differ. But lately, a team investigating germ-free mice found something peculiar in the mouse poo. Compared to mice, who make floating poop at a rate of about 50%, healthy humans only consistently produce about 10% of them. The team discovered, after publishing their findings in Scientific Reports, that the stools of germ-free mice tended to sink.

“Our serendipitous finding of ‘sinker’ and ‘floater’ feces in TFS 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 writes in their study.

The scientists conducted additional research by introducing healthy mouse gut flora into the stomachs of germ-free mice confirming that their poop started to float as well.

“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”.

While they emphasize that additional research is necessary to determine which gut bacteria are responsible for the floatation and that analysis of human poop is also required, they were able to identify a few species of bacteria that are connected to floating poop by introducing them individually to germ-free mice.

“In fact, we identified Bacteroides ovatus as 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”.