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

The enigma of why our feces sometimes float in the toilet bowl while others sink without a trace has been deciphered, and it could provide insight into the health of the bacteria residing in our intestines. Nagarajan Kannan, the director of the Mayo Clinic's stem cell and cancer biology laboratory in Rochester, Minnesota, poses a question: 'Are you a floater or a sinker? ' It's a rather personal inquiry, but it's this line of thought that has sparked a passion project for Kannan. His primary occupation involves studying the cellular and molecular mechanisms that lead to breast cancer. However, during his spare time, Kannan has found himself grappling with another puzzle – why do feces sometimes float?

Most of us have probably experienced it at some point: the feces that stubbornly refuse to flush, bobbing on the water's surface like a damning brown buoyancy aid. At other times, however, our feces simply sink without a trace. A mystery indeed. The solution to this scatological conundrum, however, offers some surprising insights about what is happening inside our bodies and the health of the microbes that inhabit us, Kannan believes. It was initially hypothesized that the occasional floating feces had 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 decided to put it to the test with a series of experiments. They discovered that it was not fat, but gas that caused feces to float.

More specifically, 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. If the gas in a floater was compressed out, the researchers discovered, they would sink. The reason for the difference, they concluded, was excessive methane production, or in other words, excessive flatulence. This is where Kannan's research comes into play. In the intervening years, medical science has revealed the enormous role that our microbiota play in many aspects of our health. Kannan suspected that changes in the composition of the 100 trillion bacteria, fungi, and other microorganisms that inhabit our guts may be responsible for whether our feces is buoyant or not. He and his colleagues at the Mayo Clinic studied the feces of mice raised in sterile conditions. These germ-free rodents have no microbes in their guts. In fecal flotation tests developed by the team, the feces from these mice sank instantly in water, while around 50% of the feces from mice with gut microbes floated, before eventually drifting downwards. When they took a closer look, the reason became clear. The feces from germ-free mice were packed with sub-microscopic undigested food particles and had a higher fecal density than feces loaded with microbes. The team then gave some of the germ-free mice a fecal transplant from the normal mice whose feces had floated – meaning they received their gut bacteria. The formerly germ-free mice then also began to produce feces that floated. Even when the mice were given bacteria from human donors, their feces too floated. Kannan and his colleagues also conducted some broadscale genetic analysis of the bacterial species in the feces 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. Kannan believes the buoyancy of our feces could be an indicator of changes in the different communities of bacteria in our guts. He is now keen to explore just what leads to the gas-producing bacteria in particular flourishing. It's a dirty job, but somebody's got to do it.