Image credits: Daniel Kraus.
We all know the problem with too much sugar. So, millions of people reach for substitutes. Whether it’s diet drinks, protein bars, chewing gum, or countless other “sugar-free” foods, sugar substitutes have become a massive industry.
But how well do we actually understand what those replacements do once they reach the gut? In particular, how do they interact with our healthy gut bacteria?
In a new laboratory study, scientists tested 39 commercially used sweeteners against 25 species of human gut bacteria. They found that roughly three-quarters affected the growth of at least one bacterial strain. But the bigger surprise came when the researchers mixed sweeteners with other compounds people commonly consume: more than 100 new interactions appeared, including one in which a sweetener component and a widely used antidepressant became disruptive together.
Not Sugar
Sugar replacements can come in many different forms. They include intensely sweet compounds such as aspartame and sucralose, plant-derived stevia compounds, and sugar alcohols such as erythritol and xylitol. Some can deliver sweetness with far fewer calories than sugar, and many are hundreds or even thousands of times sweeter, meaning manufacturers need only tiny amounts. That’s why they’re so appealing.
But replacing sugar does not automatically make a food healthier.
The World Health Organization advises against relying on non-sugar sweeteners for long-term weight control, saying the available evidence does not show lasting benefits for reducing body fat. Plus, there’s another question researchers are only beginning to untangle: What happens when these compounds reach the gut alongside everything else we consume?
That is where the new Cambridge study comes in.
Rather than looking only at individual sweeteners, the researchers tested 39 sweeteners and sugar substitutes against 25 gut bacterial strains — and then began mixing them with compounds such as caffeine, vanillin and the antidepressant duloxetine. The combinations produced effects that the individual ingredients alone could not always predict.
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Researchers first tested the sweeteners one by one. Across 975 sweetener-bacteria pairings, they confirmed 30 cases in which a sweetener changed bacterial growth. Isosteviol stood out: it inhibited three bacterial strains while encouraging another.
Then, the researchers started combining things.
They combined the sweeteners with four compounds people may encounter alongside them: caffeine, the vanilla flavoring vanillin, the sweetener advantame and duloxetine, a commonly prescribed antidepressant. That expanded the experiment to 3,900 sweetener-bacteria-compound combinations.
They found 102 significant interactions involving nine bacterial species. Of them, 68 combinations weakened one another’s effects. But 34 did the opposite: together, the compounds produced a stronger effect than researchers would have expected from either alone.
One combination stood out.
A Sweetener Compound and an Antidepressant
Isosteviol, a compound associated with stevia sweeteners, became particularly potent when researchers paired it with the antidepressant duloxetine.
Together, the two strongly inhibited Roseburia intestinalis, a gut bacterium that previous research has linked to glucose regulation and protection against intestinal inflammation. They also affected Parabacteroides merdae, another common member of the healthy gut microbiome. The University of Cambridge highlighted the pairing when announcing the study in July.
Basically, the sweetener compound and the antidepressant, when used together in the lab, made it much harder for certain normally beneficial gut bacteria to grow.
The main takeaway isn’t that people should stop using sweeteners or stop taking duloxetine. This was a laboratory study, not a human trial. What it does show is that compounds we consume together can affect gut bacteria in ways that would be hard to predict by studying each one alone.
The broader message is that our gut is exposed to mixtures — food additives, medicines, caffeine, flavorings and sweeteners all at once. The study suggests those combinations deserve more attention. Researchers now need to test whether the same interactions happen in real people, at real-world doses, and whether they actually influence health.
Journal Reference: Sonja Blasche et al. Common xenobiotics modulate gut microbial responses to low‑calorie sweeteners in vitro. Molecular Systems Biology, 2026; DOI: 10.1038/s44320-026-00225-6