We all know that stuffing our faces with sweet treats is not good for us. In part because it’s bad for the health-promoting bacteria that inhabit our intestines. Now, researchers have figured out how simple sugars wipe out a particular strain of beneficial gut microbes.

"The underlying assumption that existed in the literature was that simple sugars such as fructose and sucrose which are prevalent in Western diet are not good for humans.”

Yale professor of Microbial Pathogenesis Eduardo Groisman, who led the study.

Simple sugars…like those in high fructose corn syrup or the table sugar formally known as sucrose…were thought to be absorbed in the small intestine, so a lot of our gut bacteria would never actually be exposed to them.

Because fiber and complex carbs, made of long chains of sugar molecules, are harder to digest…they make it all the way to the large intestine, where they promote the growth of good bugs like Bacteroides thetaiotaomicron…a microbe found in individuals who are healthy and lean.

“But now what our work actually shows is that both fructose and sucrose do make it to the colon where the microbiota exist. And second that these sugars impact a good bacterium even though nutrition is not involved.”

In other words, the bacteria are not using fructose and sucrose as food. Instead, the sugars serve as signals that shut down production of a protein that beneficial Bacteroides need to colonize the intestine. The findings appear in the Proceedings of the National Academy of Sciences. [Guy E. Townsend II et al., Dietary sugar silences a colonization factor in a mammalian gut symbiont]

Groisman says he’d like to explore whether complex polysaccharides can save Bacteroides from this sugary death sentence. Because then maybe we can eat cake and have our gut bugs too.

—Karen Hopkin 

[The above text is a transcript of this podcast.]