Fructose and glucose are both common in the typical American diet. They’re both found naturally in fruits, vegetables, grains, and pretty much anything else that falls into the category of carbohydrates. But one of the most concerning sources of fructose and glucose is table sugar, or sucrose, which contains the two simple sugars bound together. Digestion breaks sugar complexes like sucrose apart, liberating its fructose and glucose units.

So far, the silencing of Roc seemed responsive only to fructose and glucose. When the researchers exposed B. theta to galactose—another simple sugar with a slightly different structure that’s found in milk—the bacteria kept churning out gobs of Roc. But when they next fed B. theta fructose-galactose or glucose-galactose mixtures, Roc disappeared once again. The same held true when galactose was swapped out for other, more complex sugars, including fructo-oligosaccharides, which are often consumed as a prebiotic, or a dietary additive that’s thought to feed populations of beneficial gut microbes (not to be confused with probiotics, or bacteria that are directly ingested in the hopes that they’ll seed the colon). For some reason, Roc seemed highly sensitive to a particular subset of simple sugars—and these sugars were capable of overriding whatever else was around.

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The researchers are still figuring out how exactly Roc is getting put out of commission. But Groisman thinks it’s likely to be at the step at which the bacterial cell’s protein-producing machinery is getting ready to read the genetic instructions for Roc.

In the future, the team also hopes to suss out why Roc and other, similar proteins are so important for B. theta and other gut microbes to settle down in the gut. There’s no guarantee that all species will behave in the same way—but simply knowing that sugar is a signal could be valuable. The researchers have already engineered a version of Roc that’s undeterred by sugar, and B. theta cells with this modification can safely take hold, even in the colons of mice eating high-sugar meals. Taking advantage of these genetic tools could eventually inform the production of future probiotics that are compatible with a multitude of diets.

These results can’t yet be extrapolated to human populations, however, Groisman cautions. The mice used in these studies lacked gut microbes, or were germ-free, before being introduced to B. theta. Also, what’s observed in mice doesn’t always translate over to humans—and several of the mouse diets tested, including one that was mostly table sugar, weren’t realistic representations of even the worst American eating habits.

The researchers also still aren’t certain that fructose and glucose are present in appreciable amounts in the human colon, which is home to the majority of our gut microbes. Not many sugars make it past the small intestine, where most nutrients are absorbed. But Townsend and others have shown that, at least in rodents, fructose is detectable in this far-flung locale—especially in mice feasting on sweets.

Simple sugars like fructose and glucose, which are present in high amounts in food with added sugar, may halt the production of important proteins in beneficial gut microbes. Photo Credit: Rebecca Siegel, flickr

Even if the same phenomenon is observed in humans, Rakoff-Nahoum says, research like this isn’t prescriptive. For one thing, B. theta is already common in human guts—and there’s no evidence that sugars can destroy already-existing populations of beneficial bacteria. Once the microbes have colonized, sugar won’t necessarily jettison them from the gut.

As a whole, though, these findings point to yet another reason simple sugars might not be good for us, Groisman says. Sugar’s never been a superfood—but if it’s actually altering the ability of bacteria to pump out protein, its role may go far beyond nutrition.

“Based on this study alone, I wouldn’t give up fructose and glucose,” says Hannah Wexler, who studies bacteria in the Bacteroides group at the University of California, Los Angeles, but was not involved in the study. “But you have to look at the big picture [of sugar]. So, based on everything we know, it’s a maybe.”

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