Why Do Some Poops Float Whilst Others Sink?

Let’s indulge our inner child and ask the question we’ve all asked when we were small

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Now that the Thanksgiving holiday and the huge variety of food overindulgences that accompany it are rapidly approaching, I thought I’d talk about something that we all do: poop. Specifically, I am reviving a question that I remember horrifying the relatives with by asking at the dinner table, and perhaps you do, too: Why do some poops float whilst others sink?

Long ago in the late Pleistocene, medical people thought that the amount of consumed fats in a given meal was the secret ingredient for poop floatiness. But a new study reveals that in fact, this may not be the cause for floaters in an otherwise healthy person. (Note that a gastrointestinal infection or pancreatitis — either of which reduces the body’s ability to digest or absorb fats — can produce floating turds.)

As your inner child always suspected, the ability of a poop to float is related to gas. Basically, gas bubbles trapped inside a given poop is the secret why some float. But this of course gives rise to your inner child’s next question: why do some people have more gas in their guts than others?

A team of scientists accidentally discovered the answers to these related questions whilst investigating a different set of questions. These scientists were studying the gut microbiomes of lab mice to better understand how the presence of specific bacterial species affected digestion and overall health of the mice. To do this, the team sterilized the guts of some of the mice whilst others were allowed to digest their rodent pellets in peace.

During the course of these experiments, the researchers discovered that sterilized mice always produced sinkers. This contrasts sharply with normal mice: half of their poops float (Figure 1).

Well, this was certainly an unexpected surprise. This finding suggested that the bacterial community living within the gut is the source of floaters. To verify this suspicion, the researchers collected floaters from healthy mice who had not participated in this study and injected these poops into the guts of the sterilized mice. (I’ll leave the injection method to your imagination.)

After re-seeding their formerly sterilized mice’s guts with normal mouse floaters, these mice now produced floaters, too. This led to the Eureka! moment when the researchers proposed that it’s actually the gut bacterial community that creates floaters — and this is because some bacterial species produce more gas than others.

Of course, to better establish this relationship, the team tried to isolate precisely which bacterial species is the especially gassy one, but were unable to do so. The team found that there are more than ten gasogenic — gas producing — bacterial species within the typical gut community. These bacteria include the highly prevalent Bacteroides ovatus, an oxygen-intolerant member of the bacterial gut community that, when it overgrows, can be unfriendly to human social interactions because it’s linked with flatulence and intestinal bowel diseases.

Source:

Syed Mohammed Musheer Aalam, Daphne Norma Crasta, Pooja Roy, A. Lee Miller II, Scott I. Gamb, Stephen Johnson, Lisa M. Till, Jun Chen, Purna Kashyap & Nagarajan Kannan (2022). Genesis of fecal floatation is causally linked to gut microbial colonization in mice, Scientific Reports | doi:10.1038/s41598-022-22626-x

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