In fact, for decades scientists have been asking why some people’s droppings usually float in a water-filled toilet bowl, while others sink immediately: an estimated 10 to 15 percent of the population belong to the first group. There has been a thesis on this since 1972; according to her, the swimming properties should be related to the gas content of the digestion products. Previously, this was mainly attributed to the fat content of the stool. Nagarajan Kannan from the Mayo Clinic in Rochester and his team have now finally confirmed the gas-based assumption, as they report in “Scientific Reports”.
As the working group had noticed, mice bred without intestinal flora always produced feces that sank in the water. In contrast, half of all conspecifics with a normal microbiome also produced stool that floated on top. This aroused the interest of the researchers, who therefore injected the germ-free rodents with intestinal bacteria from normal mice and two young women. As a result, the animals treated in this way also produced swimmable faeces.
“Now there’s no doubt as to what causes the stool to float: the gas from gut microbes, air not swallowed, or other causes,” says Kannan. A closer examination of the buoyant faeces also showed that they contained several gas-producing bacteria, such as Bacteroides ovatus and Bacteroides uniformis. Both also lead to increased methane production and more frequent bloating in humans. However, it is still unknown what amounts of gas the different microbes contribute and thus who provides the decisive impetus for the buoyancy. Further tests should determine this.