39 sweeteners tested on gut bacteria: 3 in 4 changed growth.
The catch? It was a lab dish
A new study put 39 sweeteners through a test many people assumed would come back boring. It didn't.
- Claim
- 39 SWEETENERS ONE TEST
- Catch
- LAB DISH ONLY
- Verdict
- WORTH WATCHING NOT PANIC
The study: scientists at the Medical Research Council Toxicology Unit at the University of Cambridge grew 25 strains of human gut bacteria and exposed them to 39 commercially used sweeteners, covering all sweeteners available as pure compounds. They then repeated the work with four things people commonly consume alongside sweeteners — advantame, caffeine, vanillin and duloxetine (an antidepressant) — and added a further 12 sweetener combinations with 8 other common drugs.
The finding: three-quarters of the tested sweeteners individually impacted the growth of at least one bacterial strain. Some slowed the microbes down, some stopped them growing entirely. The effects were not uniform — different sweeteners hit different bacteria, which means they can't be treated as one group. Isosteviol was the most potent on its own, inhibiting three species and promoting a fourth. On top of that, the team identified 30 direct sweetener-bacteria interactions involving 5 strains and 26 sweeteners, none previously reported.
The twist: adding a co-consumed compound changed the effect in 102 separate cases — 68 became weaker (antagonistic) and 34 became stronger (synergistic). The stand-out combination was isosteviol plus duloxetine, which together inhibited Roseburia intestinalis (linked to glucose homeostasis) and Parabacteroides merdae (a prevalent commensal linked to a healthy microbiota). In a synthetic 25-species gut community, that same pair shifted bacterial metabolism, increased toxicity toward HeLa cells, and changed the secretion of the inflammation-regulating cytokines IL-6 and IL-8 by Caco-2 cells.
The catch: every single result is in vitro. No humans were involved, no blood was drawn, nobody swallowed anything. The paper's own title says "in vitro," and the authors' conclusion is that the data "warrant further studies." A lab dish with 25 species is not your gut, which has hundreds to thousands of species, variable pH, transit time, other food and medicines. A finding that a chemical can slow a bacterium in a plate is a reason to run a human trial, not a reason to change your diet. Funding was public (EU Horizon 2020 and the UK Medical Research Council), and the authors declare no competing interests — though that is a small reassurance next to "no human data."
SOURCES
- Blasche S, Periwal V, Beristain Covarrubias N, et al. "Common xenobiotics modulate gut microbial responses to low-calorie sweeteners in vitro." Molecular Systems Biology. 2026;22(9):1453–1473. DOI: 10.1038/s44320-026-00225-6. PMID 42350810. PMCID PMC13538661. Open access.
- University of Cambridge press release, "Scientists tested 39 sweeteners and uncovered surprising gut effects" (via ScienceDaily, 17 October 2026)
Disclaimer: For information only, not medical advice. This was a laboratory study with no human participants, so it does not show that artificial sweeteners harm you, or that they are safe for you. If you have questions about sweeteners and your health, talk to your doctor or a registered dietitian.
