Why do some stools float and others sink?

A team of Mayo Clinic researchers has solved the mystery of why some people find their bowel movements floating while others find theirs sinking to the bottom of the toilet bowl. In his article published in Scientific reportsthe group described their serendipitous discovery of the answer.

Before the 1970s, scientists believed that faeces either sink or float depending on the amount of fat they contain. Experiments have shown that this is not the case. Instead, experiments with healthy people show that the difference is due to the amount of gas in a given fecal sample. But the question still remains: Why do the feces of some people tend to have more gas and therefore more buoyancy than that of others?

In this new effort, the researchers studied the microbiome of several labs miceand sterilized the guts of some of them as a way to isolate the differences in digestion and overall health associated with different bacteria. As the experiments continued, the researchers noticed that none of the fecal samples produced by the sterilized mice floated. In mice, usually half of the samples are floating.

This suggests that floating fecal matter is related to the composition of the gut microbiome. The researchers then collected stool samples from healthy mice that were not part of the original study but produced floating mice and injected the material into the intestines of the sterile mice. They found that all the test mice started producing floaters. This, the researchers say, suggests that the reason for some fecal matter floating is due to the nature of the bacteria in the gut – some produce more gas than others.

The researchers were unable to isolate the bacteria that produced more gas, but noted that Bacteroides ovatus had previously been found to cause more gas in human patients. Logic suggests that it is probably one of the culprits responsible for the formation of floaters in humans and perhaps in lab mice. The team suggests that more work will need to be done to confirm their suspicions and find other bacteria involved in producing more gas and therefore buoyant matter.

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