Common sweeteners can directly change the growth of bacteria in the human gut, according to a large laboratory study. The strongest effects appeared when sweeteners were tested beside substances consumed at the same time. One pairing involving a stevia-related compound and an antidepressant produced notable changes.
The research does not show that sweeteners cause illness in people.
Scientists tested 39 commercially used sweeteners against 25 bacterial strains and species representing the healthy human gut microbiome. The group included common residents, probiotic organisms, and bacteria that can sometimes become harmful. Researchers examined 975 individual sweetener-and-bacterium pairings.
The study was published in Molecular Systems Biology by researchers in Britain and at the European Molecular Biology Laboratory in Germany. Each compound was tested at 50 micromoles per liter. The researchers estimated that level was within the range that could reach the colon.
The bacteria were grown under controlled laboratory conditions. Researchers measured growth after exposure to each sweetener and compared it with untreated controls. A result had to meet statistical requirements, exceed a 20% growth change, and be confirmed in another experiment.
About three-quarters of the sweeteners changed the growth of at least one bacterial strain. Some slowed bacterial multiplication, while others stopped the growth of certain organisms under the test conditions. The findings suggest that low-calorie sweeteners are not always passive substances when they encounter gut microbes.
The University of Cambridge reported that earlier evidence came largely from animal experiments and population studies. Those methods can identify possible links between sweeteners, the microbiome, and health. However, they cannot easily show whether a sweetener acts directly on bacteria or produces changes through another process.
The researchers also examined a practical problem. People rarely consume a purified sweetener alone. Sweeteners are present in drinks, snacks, desserts, cereals, flavorings, and medications used to hide bitter tastes.
The team combined the 39 sweeteners with caffeine, vanillin, advantame, and duloxetine. Advantame is another low-calorie sweetener. Vanillin is a flavoring, while duloxetine is an antidepressant also prescribed for anxiety and several types of chronic pain.
This part of the work produced 102 significant interactions involving nine bacterial species. In 34 cases, a combination created a stronger effect than expected from the individual compounds. In 68 cases, one compound weakened the effect of the other.
The clearest interaction involved isosteviol and duloxetine.
Isosteviol is related to compounds derived from the stevia plant and used in food production. Combined with duloxetine, it strongly suppressed Roseburia intestinalis and Parabacteroides merdae. Roseburia intestinalis has been linked to glucose regulation, while Parabacteroides merdae is associated with a healthy microbial community.
News-Medical reported that more than 4.2 million patients in the United States were prescribed duloxetine in 2023. That wide use makes the laboratory interaction worth further study. It does not establish that patients taking the medication are being harmed by foods or drinks containing stevia-related sweeteners.
Researchers next assembled all 25 bacteria into a synthetic community. They grew the organisms together through five passages while exposing them to isosteviol, duloxetine, both compounds, or a control. This showed how the mixture affected a simplified microbial ecosystem rather than one isolated species.
The combined treatment reduced microbial diversity and changed which bacteria survived or became more abundant. Diversity is generally considered an important feature of a healthy microbiome. Still, a community containing 25 laboratory-grown organisms is much simpler than the microbiome inside a person.
Technology Networks reported that the mixture also changed bacterial metabolism. In Roseburia intestinalis, butyric acid production fell by more than 25%, while glutamine secretion rose by about 50%. Butyrate is a short-chain fatty acid produced by gut bacteria and used as an energy source by cells lining the colon.
Protein, metabolite, and genetic analyses pointed to changes in small-molecule transport as a possible explanation. Surviving bacteria appeared to rely on cell-wall functions and magnesium transport systems. These findings suggest a mechanism, but they do not prove what happens in a living digestive tract.
Researchers collected liquid from the treated bacterial communities and applied it to human cell lines. Fluid from communities exposed to isosteviol and duloxetine caused greater toxicity in HeLa cells than control fluid. Tests separated changes caused by bacterial products from direct exposure to the compounds.
The same community fluid did not produce excess toxicity in Caco-2 intestinal cells. However, it reduced secretion of the immune-signaling proteins IL-6 and IL-8 by more than 75%. Another signaling protein, known as CXCL10 or IP-10, was not significantly affected.
Those immune findings are difficult to interpret.
IL-6 and IL-8 can have inflammatory and protective roles depending on circumstances. Lower levels might appear beneficial, but they could also weaken responses needed to control pathogens. The researchers said interactions among gut cells, bacteria, metabolites, and immune signals require testing in living systems.
StudyFinds emphasized several limits. The experiments used one concentration, one growth medium, a restricted set of bacterial strains, and four compounds in the main combination screen. People have more diverse microbiomes, different diets, varying medication doses, and changing exposures.
The laboratory results should not be treated as evidence that a sweetener, food, or medication is unsafe. They also do not support stopping duloxetine or changing treatment without medical guidance. The study identifies questions for future animal studies and human trials.
The main message is not that every sweetener harms every gut bacterium. Responses differed sharply by sweetener, bacterial species, and accompanying compound. Some combinations strengthened an effect, while more combinations weakened it.
The research shows why testing ingredients one at a time may miss important interactions. Foods, beverages, additives, and medicines meet inside the same digestive system. Their combined effects on microbes may differ from what scientists observe when each substance is examined alone.
For now, the findings provide a detailed laboratory map rather than a clinical warning. They show that commonly used sweeteners can influence bacterial growth directly and that other compounds can change those effects. Human research will be needed to determine whether the observed changes affect digestion, metabolism, immunity, or disease risk in different groups of people and settings.