We now know why some poos float and others sink

Whether your poo floats or sinks depends on the types of bacteria in your gut and how much gas they produce, a new study suggests.

About 10 to 15 per cent of people consistently do poos that float in toilet water – so-called “floaters”, while the rest typically produce poos that sink to the bottom, or “sinkers”.

In 1972, Michael Levitt, a gastroenterologist at University of Minnesota Hospitals, and his student William Duane showed this was largely to do with the gas content of faeces, not fat content, as was previously assumed. They collected floaters from 13 people and found they all sank when the gas inside was removed by increased pressurisation, even if they had high fat content.

Their research was prompted by Duane revealing to Levitt that his poos always floated. “About 2 hours after our discussion, he passed a stool, we put it in a flask, pressurised the flask and watched the stool sink, demonstrating the stool floated because of its gas content,” says Levitt.

Levitt and Duane believed this gas must have come from gut bacteria that became incorporated in the faeces, because two floaters they tested contained high levels of methane gas, which is made by bacteria that ferment carbohydrates as they pass through the large intestine. However, they couldn’t tell for sure.

Now, Nagarajan Kannan at the Mayo Clinic in Rochester, Minnesota, and his colleagues have confirmed this hunch after noticing that mice bred so they don’t have any gut bacteria – known as germ-free mice – always produce sinkers, whereas nearly half of standard mice produce floaters.

To explore further, the researchers injected gut bacteria collected from the faeces of standard mice and from two seemingly healthy young women into the stomachs of the germ-free mice, and found that it caused many of their poos to start floating.

“Now, there’s no confusion as to what makes stool float, it is gas from gut microbes, not from swallowed air or other sources,” says Kannan. Closer analysis of the mouse floaters revealed they contained multiple gas-producing bacteria, including Bacteroides ovatus and Bacteroides uniformis, which are known to increase methane production and the frequency of flatulence in people.

To work out which of these species produce enough gas to make faeces float, the next step will be to individually introduce each of them into the guts of germ-free mice, says Kannan. Microbial analysis of human faeces could also reveal which gas-producing bacteria are more common in floaters, he says.

Whether people produce floaters or sinkers may depend on their diet, genetics, how they were delivered at birth and their environment, since all of these factors are known to influence the mix of bacteria found in the gut, says Kannan.

At this stage, we can’t say whether it is healthier to do floaters or sinkers, he says. “It probably depends on exactly which gut bacteria are producing the gas.”

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