Demystifying science | Saddles in the hot seat

I don’t know if you will be able to answer my question, but I dare to put it to you: why are the stools always brown? — Nathalie Morin

Because it’s the color food turns when it ferments, says Gwen Falony, a microbiologist at the University of Mainz, Germany, who devotes her career to stool analysis.

“When food ferments in the intestines, under the action of bacteria, it turns brown,” says Mr. Falony. The color may change a little depending on what you eat. With red food it turns reddish. After lots of greens, it gets greener. »

The only medical conclusion that can be drawn from the stools is when they become very pale. “It could be a sign of liver problems,” says Mr. Falony.

A British team which is also working to establish links between stool consistency, transit time and the content of the intestinal microbiota (the bacteria found there) uses another color in its research: cornflower blue.

How can transit time and stool consistency tell us about the intestinal microbiota?

“Stool consistency is closely related to transit time through the intestines,” says Falony. The longer the transit time, the harder the stools. What we want to know is the transit time, which largely determines the composition of the intestinal microbiota. »

If food only spends a few hours in the intestines, only fast-growing bacteria will be present. If it is longer, there is more methane production, and “anaerobic” bacteria, which live in places poor in oxygen. These anaerobic bacteria produce compounds that cause fermentation and inflammation, says Falony.

Some cases of colon cancer may be due to an abnormally long transit time.

Bristol scale

A scale for measuring stool consistency was developed 25 years ago by a university hospital in Bristol. It has seven categories, with images and descriptions ranging from “hard, broken stools, in balls, difficult to pass” to “completely liquid stools.”

On average, the transit time of food from meal to stool is 48 hours, but it can be up to 72 hours. So why do some people go to the bathroom every day? “Digested food can be compacted in the intestines, so there is no direct link between defecation and transit time,” says Falony.

Obese people have, on average, a faster transit time. But a subgroup of obese people suffer from intestinal inflammation usually linked to slower transit time.

“This is one of the important questions that remains to be clarified,” says Mr. Falony. There is also the impact of sex and age on transit time, and its implication in inflammatory bowel diseases. We also want to see if fecal transplants can help treat these inflammatory diseases. »

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