Why does your poo sometimes float? It might be down to your gut health (2024)

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The mystery of why our number twos sometimes bob to the surface of the toilet water while others sink without trace has been solved – and it could be a window into the health of the bacteria living in our guts.

Before we dive in, Nagarajan Kannan has a question: "Are you a floater or a sinker?" It's a surprisingly intimate thing to ask someone you've only exchanged emails with, but it is exactly this line of thinking that has led to something of a passion project for the director of the Mayo Clinic's stem cell and cancer biology laboratory in Rochester, Minnesota.

Most of his day job is spent studying the cellular and molecular mechanisms that lead to breast cancer. But during rare idle moments, Kannan has found himself wrestling with another puzzle – why do faeces sometimes float.

Most of us have probably experienced it at some point: the poo that simply refuses to flush, bobbing around on the surface of the water like an incriminating brown buoyancy aid. At other times, however, our number twos simply sink without trace. A mystery indeed.

The answer to this scatological conundrum, however, offers some surprising insights about what is going on inside our bodies and the health of the microbes that live there, Kannan believes.

It had initially been thought that the reason for the occasional floating unmentionable was something to do with the levels of fat that made their way into it. But in the early 1970s a couple of gastroenterologists at the University of Minnesota with time on their hands decided to put it to the test with a series of experiments. After subjecting the excrement from 39 volunteers – and some of their own solid waste for good measure – to a barrage of tests, the answer they came up with was that it was not fat, but gas.

More precisely the amount of gas found within a stool can vary to such degrees that they can either float to the surface or sink like a brick (along with a fair bit of mid-bowl drifting in-between). If the gas in a floater was compressed out, the researchers discovered, they would sink.

The reason for the difference, they concluded, was too much methane production. In other words, excessive flatulence.

And this is where Kannan enters the increasingly whiffy arena. In the intervening years, medical science has revealed the enormous role that our microbiota play in many aspects of our health – from obesity to heart disease. Kannan suspected that changes in the make up of the 100 trillion bacteria, fungi and other microorganisms that call our guts their home may be responsible for whether our faeces is buoyant or not.

"The majority of faecal matter primarily comprises transformed food particles forming a bacterial mass," he says.

To put the theory to the test, he and his colleagues at the Mayo Clinic studied the droppings of mice raised in sterile conditions. These germ-free rodents have no microbes in their guts. In faecal floatation tests developed by the team, the poop from these mice sank instantly in water, while around 50% of the droppings from mice with gut microbes floated, before eventually drifting downwards. When they took a closer look, the reason became clear.

"Germ-free faeces are packed with sub-microscopic undigested food particles and have a higher faecal density than microbially loaded faeces," says Kannan. The team then gave some of the germ-free mice a faecal transplant from the normal mice whose poop had floated – meaning they received their gut bacteria. The formerly germ-free mice then also began to produce droppings that floated.

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Even when the mice were given bacteria from human donors, they too floated.

"It seems that once those microbes set up shop, it's a universal "rise to the top" situation for mouse poop, regardless of the donor species," says Kannan.

He and his colleagues also conducted some broadscale genetic analysis of the bacterial species in the poop from mice that were floaters and found they had high levels of 10 bacterial species known to produce gas. Dominant among them was Bacteroides ovatus, which is known to produce gas through the fermentation of carbohydrates and has been linked to excessive flatulence in human patients.

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Although the findings in mice should be treated cautiously – they have yet to be confirmed to bear relevance to human "floaters" and "sinkers" – Kannan believes the buoyancy of our poop could be an indicator of changes in the different communities of bacteria in our guts.

"I have this inkling that a 'floater' might transiently become a 'sinker' while on antibiotics," he says. But he adds he's not seen anyone study this yet. "Securing funding for faecal floatation isn't a walk in the park, unfortunately."

Numerous factors including our diet, whether we smoke, the stress we are under and a wide range of medications we take can all alter the make-up of the bacteria in our guts. Kannan is now keen to explore just what leads to the gas-producing bacteria in particular flourishing.

"Whether you're at a packed social event or on a deep space journey, you probably don't fancy being seated next to someone with a gut full of these gasogenic microbes and a penchant for frequent flatulence."

It's a dirty job, but somebody's got to do it.

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