Researchers at the Weizmann Institute of Science in Israel randomized 120 healthy adults into six groups: saccharin, sucralose, aspartame, stevia, a glucose control, and no supplement. Each group consumed their assigned sweetener in sachets for two weeks at doses below the accepted daily intake.

All four sweeteners distinctly changed the composition of stool and oral bacteria and blood metabolites. Saccharin and sucralose stood out by significantly worsening blood sugar control after meals. Participants split into "responders" whose blood sugar spiked more and "non-responders" who showed little change.

The strongest proof came from a fecal transplant experiment: when stool from human responders was transferred into germ-free mice, those mice developed the same impaired blood sugar handling, confirming the gut bacteria were the direct cause of the metabolic disruption.

This study from the Weizmann Institute was the first major paper to link artificial sweeteners to gut microbiome disruption. Mice given any of the three sweeteners in their drinking water developed significantly worse glucose tolerance compared to mice drinking plain water or sugar water over 11 weeks.

When the researchers treated the mice with two different antibiotic regimens that killed gut bacteria, the glucose intolerance disappeared, directly proving the gut microbiome was responsible. Transplanting feces from sweetener-consuming mice into germ-free mice transferred the metabolic problems.

In a small human follow-up, 7 healthy volunteers who did not normally consume sweeteners took saccharin at 5 milligrams per kilogram of body weight per day for one week. Four of the seven developed notably worse blood sugar responses, and their gut bacteria shifted toward species associated with metabolic disease.

Researchers at Cedars-Sinai Medical Center analyzed the small bowel microbiome of 99 subjects using samples collected during upper endoscopy, a method that captures bacteria in the small intestine rather than just the colon. They compared 35 people who regularly consumed non-aspartame sweeteners, 9 who consumed aspartame, and 55 controls.

People using non-aspartame sweeteners (such as sucralose and saccharin) had significantly reduced bacterial richness in their small intestine compared to controls. In the aspartame group, researchers found enrichment of a pathway that produces cylindrospermopsin, a toxin classified as potentially cancer-causing with known effects on the liver and nervous system.

Both the stool and small bowel microbiomes showed distinct compositional differences in sweetener consumers, along with changes in circulating inflammatory markers in the blood. This was notable because most prior research only examined stool samples, which primarily reflect the large intestine.

Researchers randomized 40 healthy adults aged 18 to 35 with normal weight into two groups: 20 consumed 48 milligrams of sucralose per day (equivalent to roughly 4 packets of Splenda) and 20 drank water as a control for 10 weeks.

By the end of the trial, the sucralose group showed a 3-fold increase in Blautia coccoides bacteria and a 34% decrease in Lactobacillus acidophilus, a species widely considered beneficial for gut health and commonly found in probiotic supplements.

Blood tests revealed that the sucralose group had higher serum insulin levels and a larger area under the glucose curve during oral glucose tolerance testing compared to controls, indicating their bodies were working harder to manage blood sugar. These changes occurred at a dose that is well within normal daily consumption for heavy sweetener users.

Researchers at Mexico''s National Institute of Medical Sciences and Nutrition conducted a triple-blind, placebo-controlled trial in which healthy lean individuals consumed sucralose at 30% of the accepted daily intake or a placebo for 30 days.

The sucralose group showed a significant decrease in insulin sensitivity as measured by the Matsuda index. Their gut bacteria became less diverse, with reduced alpha-diversity scores. Fecal analysis revealed lower levels of butyrate, a short-chain fatty acid produced by beneficial gut bacteria that helps maintain the gut lining and reduce inflammation.

At the same time, participants showed increases in proinflammatory markers, branched-chain amino acids, and acetate in their stool. Curli protein, a bacterial product associated with inflammation, also increased. These changes occurred at a dose well below the maximum acceptable daily intake, suggesting even moderate consumption may affect the gut.

The World Health Organization published a formal guideline in May 2023 based on a systematic review and meta-analysis of all available evidence on non-sugar sweeteners, including aspartame, saccharin, sucralose, stevia, and others.

The WHO issued a conditional recommendation against the use of non-sugar sweeteners as a means of achieving weight control or reducing the risk of noncommunicable diseases. Their review of observational data found that long-term use of these sweeteners was associated with increased risk of type 2 diabetes, cardiovascular disease, and all-cause mortality in adults.

The guideline noted that non-sugar sweeteners have no nutritional value and are not essential dietary factors. The WHO stated that replacing free sugars with sweeteners does not help with long-term weight control and that people should consider other ways to reduce sugar intake, such as consuming food with naturally occurring sugars like fruit, or unsweetened foods and beverages.

Researchers at Yale University randomized 45 healthy adults into three groups that each consumed seven beverages over two weeks: one group drank sucralose alone, one drank sucrose (table sugar), and one drank sucralose combined with maltodextrin (a carbohydrate).

The group consuming sucralose with carbohydrate showed decreased insulin sensitivity measured by oral glucose tolerance tests. Brain imaging using functional MRI revealed that this group also had reduced neural responses to sweet taste in the midbrain, insular cortex, and cingulate cortex — brain regions involved in processing reward and metabolic signals from the gut.

The sucralose-alone group did not show these changes, suggesting the disruption specifically occurs when the sweet taste signal from sucralose is paired with calories from carbohydrate, potentially confusing the gut-brain axis that normally calibrates metabolic responses to food.

Researchers studied 97 adults in two parallel 12-week randomized controlled trials: 49 with type 2 diabetes and 48 with overweight or obesity but without diabetes. Participants either replaced their daily sugar in coffee and tea with sucralose or continued using sugar.

In the type 2 diabetes group, sucralose consumption led to a significant decrease in alpha-diversity (Shannon index p=0.02, Simpson index p=0.03) and a significant increase in beta-diversity (p=0.001), meaning the overall mix of gut bacteria shifted. Of 185 bacterial types tested, 14 were reduced — primarily sugar-fermenting and short-chain fatty acid-producing bacteria from the Firmicutes group and Lachnospiraceae family that are considered beneficial for gut health. Meanwhile, Enterococcus and Pediococcus, bacteria sometimes associated with infections, increased.

Notably, people with overweight but without diabetes showed no such changes, suggesting that individuals with existing metabolic conditions may be especially vulnerable to sweetener-driven microbiome disruption.

Researchers at the University of Granada reviewed all available experimental studies and clinical trials examining the effects of different sweetener classes on gut microbiota.

Among non-nutritive sweeteners, saccharin showed the most consistent evidence of microbiome disruption across studies, followed by sucralose and stevia. The review found that synthetic sweeteners reduced levels of Akkermansia muciniphila, a bacterium that helps maintain the mucus layer protecting the gut lining, and Faecalibacterium prausnitzii, one of the most important producers of butyrate, a compound that nourishes intestinal cells and fights inflammation.

These bacterial changes led to decreased production of short-chain fatty acids, the main energy source for cells lining the colon, and compromised gut barrier integrity. The authors concluded that while the evidence was growing, there remained a critical need for well-designed, long-term randomized controlled trials with adequate sample sizes in humans to confirm effects seen in animal studies.

Published in the Canadian Medical Association Journal, this systematic review and meta-analysis examined 30 studies — 7 randomized controlled trials with over 1,000 participants and 23 cohort studies with over 400,000 participants — on the long-term health effects of non-nutritive sweetener consumption.

The meta-analysis of cohort studies found that routine intake of artificial sweeteners was associated with a 14% higher risk of type 2 diabetes for each daily serving of artificially sweetened beverage, a 13% higher risk of cardiovascular events, and modest but significant increases in BMI, body weight, and waist circumference.

While this review did not directly measure gut bacteria, the authors noted that emerging evidence of microbiome disruption offered a plausible biological mechanism for these associations. The randomized controlled trials did not show weight loss benefits from artificial sweeteners, challenging the primary reason most people use them.