New research suggests that sweeteners are not as harmless to our bodies as many people think. Scientists say more work is needed to fully understand what this means for human health. But the study is one of the first to look closely at how sweeteners affect gut bacteria, especially when combined with other everyday substances.
Sweeteners appear in many foods and drinks today. You’ll find them in soda, candy, desserts, snacks, and cereal. Companies often promote them as a healthier choice than sugar. Yet growing evidence links sweeteners to serious health problems, including type 2 diabetes, obesity, and cancer.
Despite how common sweeteners are, few studies have examined their direct effect on gut bacteria. These bacteria form a massive community inside our digestive system. They play a major role in keeping our bodies functioning well.
Professor Kiran Patil, from Cambridge University‘s MRC Toxicology Unit, explained that most existing knowledge comes from animal studies or large population surveys. These studies hint that gut bacteria might be involved in how sweeteners affect us. But they don’t clearly show whether sweeteners interact directly with these bacteria.
Dr Sonja Blasche, one of the study’s lead researchers, pointed out another challenge. People almost never consume sweeteners alone. They usually appear alongside other things, like drinks, snacks, or even medications that use sweeteners to hide bitter tastes.
To investigate, Blasche and her team studied how both artificial and natural low-calorie sweeteners affect gut bacteria. They also examined how these effects shift when sweeteners are paired with common substances like caffeine, flavourings, or medications. The findings appeared in the journal Molecular Systems Biology.
The team grew 25 different species of gut bacteria in laboratory conditions. These included helpful bacteria, neutral ones, and some that could be harmful. Each bacterial culture was then exposed to 39 widely used sweeteners. Some were artificial. Others came from natural sources. The researchers tracked how well each bacteria type grew afterward.
The results were striking. Roughly three-quarters of the sweeteners changed the growth of at least one type of bacteria. Some sweeteners slowed or completely stopped the growth of bacteria known to support gut health.
Next, the researchers combined each sweetener with other common substances. These included caffeine, vanilla flavouring, another artificial sweetener called advantame, and eight frequently prescribed medications. They found more than 100 cases where sweeteners behaved differently once combined with something else. In 34 instances, the combined effect grew stronger. In 68 cases, it became weaker.
One combination stood out above all others: isosteviol, a compound made from stevia, mixed with duloxetine, a common antidepressant. Together, they strongly suppressed two beneficial bacteria species, Roseburia intestinalis and Parabacteroides merdae. This matters because more than 4.2 million people in the US were prescribed duloxetine in 2023 alone.
Since bacteria don’t live in isolation but as part of a larger ecosystem, the team built an artificial community using all 25 species together. They let it grow, then tested it against various sweetener-drug pairings. They tracked which species grew or shrank and whether overall diversity changed.
The isosteviol-duloxetine pairing reduced the variety of bacteria in this mini-ecosystem. That matters, because a diverse microbiome is linked to better gut health. This combination also shifted which species dominated the community.
Further testing revealed something concerning. The isosteviol-duloxetine mix appeared to increase harm toward certain human cells. It also disrupted cells involved in immune response and inflammation control.
Dr Blasche noted that sweeteners are often described as having no real metabolic effect. This study challenges that assumption. Sweeteners, she said, can directly influence gut bacteria, particularly when combined with other compounds like medications or additives.
The researchers were careful to note their limits. This was lab-based research, not a human trial. More studies are needed before drawing firm conclusions about real health effects in people.
Professor Patil summarised the significance of the findings. Sweeteners, he said, don’t simply pass through our bodies without effect. They interact with gut microbes, and other substances like medications can strengthen or change those interactions. He hopes these findings will guide future research into how sweeteners might affect health in ways we don’t yet fully understand. The study received funding from the European Union’s Horizon 2020 programme and the UK Medical Research Council.
