Are Artificial Sweeteners Bad for Your Gut?

If you type "why does diet soda make me" into Google, the first way it finishes the sentence is "poop." "Bloated," "gassy," and "nauseous" come further down the list, and "is sucralose bad for your gut" and "is stevia bad for your gut" both autocomplete too.

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Nobody has shown that the sweeteners in diet drinks harm a healthy person's gut, and nobody has ruled it out either. The human trials on gut bacteria have split, and only one trial has asked people how their stomachs actually felt.

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What's in a zero-sugar drink?

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Sweeteners like sucralose, aspartame, saccharin, acesulfame potassium, stevia, and monk fruit are so much sweeter than sugar that a drink only needs a tiny amount (Splenda's own site says that sucralose is "over 600 times sweeter than sugar"). A lot of drinks use more than one. Coca-Cola Zero Sugar lists three: aspartame, acesulfame potassium, and stevia extract.

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The packets you stir into coffee are mostly something else. The FDA requires ingredients to be listed "in descending order of predominance by weight" (i.e., the biggest ingredient goes first), and a Splenda packet reads "Dextrose, Maltodextrin, Splenda Sucralose," so the sucralose comes last. Splenda says that the dextrose and maltodextrin (two carbohydrates) "provide volume and texture." Lakanto, a monk fruit brand, says that its formula has "1% monk fruit extract," and its classic sweetener is made with monk fruit and erythritol.

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Erythritol is a sugar alcohol, which is a different family of sweeteners with names ending in "-ol." The NHS says that sugar alcohols "can have a laxative effect if consumed in large amounts," and they're the subject of why sugar-free candy gives you diarrhea. If a "stevia" or "monk fruit" packet bothers your stomach, check whether a sugar alcohol is in it too.

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Do sweeteners change your gut bacteria?

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Some of these sweeteners make it most of the way through you untouched. A 2026 review estimated that about 85% of the sucralose you swallow reaches your colon (the last stretch of your intestines) without being broken down, where it runs into your gut bacteria, and the FDA says that stevia's sweet compounds (steviol glycosides) "are not absorbed through the upper portion of the gastrointestinal tract."

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The worry that sweeteners mess with those bacteria took off with a 2014 study in Nature. Most of it was done in mice. The human part was seven volunteers who took the maximum amount of saccharin that the FDA considers safe for six days, and four of them ended up handling sugar worse, as our energy drink article covers.

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In 2022, the same team ran a much bigger trial. They gave 120 healthy adults saccharin, sucralose, aspartame, or stevia for two weeks, in amounts below the safe daily limit, and compared them with people who got plain glucose or nothing. Each sweetener changed people's gut bacteria as a group, and saccharin and sucralose "significantly impaired glycemic responses" (people's blood sugar rose more after a sugar drink). The changes varied from person to person. When the researchers moved people's gut bacteria into mice raised with no bacteria of their own, the mice's blood sugar mostly followed the pattern of the person the bacteria came from.

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Other trials found much less:

In a trial of 46 healthy adults, the ones given saccharin at the maximum safe daily amount (400 mg a day) for two weeks had no change in their gut bacteria. The authors wrote that it "did not alter microbial diversity or composition at any taxonomic level."
17 adults took aspartame and sucralose in amounts that the researchers compared to about three cans of diet soda a day, and the sweeteners had a "minimal effect" on their gut bacteria.
59 adults drank a stevia-sweetened drink every day for four weeks, and their gut bacteria ended up no different from a group drinking a sugar-sweetened one. Two of that study's authors work at the research center of the food company Cargill.

A 2025 review from the lab of Jotham Suez, the first author on both the 2014 and 2022 studies, tallied the human trials up. Of 14 experimental groups, eight found a significant effect on gut bacteria, one found a limited effect, and five found none. A separate 2026 review looked at ten sucralose trials and found no significant effect on the variety of gut bacteria or on blood sugar, although every trial in it was short and the biggest had 61 people.

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Kim Barrett, a physiology professor at UC Davis, called the 2022 trial "well-designed" and said that it shows "the potential for NNS to have adverse effects in at least some individuals" (NNS stands for non-nutritive sweeteners). Francisco Guarner, who runs a digestive research unit in Barcelona, was less convinced. He pointed out that its saccharin dose was "equivalent to taking 50 tablets or 18 sachets of saccharin every day," and added "I do not know anyone who takes such quantities."

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What about stomach trouble?

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Every trial above measured bacteria and blood sugar. The only one that tested stomach symptoms directly was a 2022 trial in Mexico, where 95 volunteers with a median age of 22 were randomly split into two groups for five weeks. One ate a diet with 50 to 100 mg of sweeteners a day (mostly sucralose, with some aspartame, acesulfame potassium, and saccharin), and the other cut sweeteners out. In the sweetener group, the share of people with diarrhea went from 19% to 56%, and constipation went from 30% to 56%. The group that cut sweeteners reported fewer symptoms across the board, with stomach pain in the upper belly dropping from 38% to 3%.

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Everyone in that trial knew which diet they were on, which can shape how people report symptoms, and the results compare each group with where it started instead of comparing the two groups against each other. Nobody has repeated it yet.

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For people with IBS (irritable bowel syndrome), Monash University's FODMAP team (the researchers who built the main diet used to manage IBS) calls sugar alcohols "the most well-documented sweetener that is relevant for those with IBS," and says that research on sweeteners in IBS is "limited to the well-documented effects of polyols" (polyols is the scientific name for sugar alcohols).

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What do health organizations say?

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The World Health Organization advised against using sweeteners for weight control in 2023. Its announcement says that "replacing free sugars with NSS does not help with weight control in the long term" (NSS means non-sugar sweeteners), and it never mentions the gut.

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In 2025, a panel convened by the Academy of Nutrition and Dietetics, the American Academy of Pediatrics, the American Academy of Pediatric Dentistry, and the American Heart Association listed drinks with non-sugar sweeteners as "Not recommended" for kids and teens. Its reasoning mentions "emerging evidence of undesirable effects from shorter-term use, for example on the gut microbiome," and the one source it cites for that line is a 2023 Johns Hopkins graduate thesis. The library lists the thesis as under embargo until 2027. Its abstract describes testing how sweeteners affect the growth of gut bacteria and giving sweetener-exposed bacteria to mice raised without any of their own, and it doesn't describe any human participants. The panel itself found "little conclusive evidence" about sweetened drinks in kids and teens and "concluded that a precautionary approach is prudent."

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The UK's NHS says that "all approved sweeteners are considered a safe and acceptable alternative to using sugar." Splenda's own FAQ asks "Do Splenda® Brand Sweeteners harm the gut?" and answers "No," pointing to "a thorough review in 2018" without naming it. A 2018 review on that question, published in Food and Chemical Toxicology, found no clear evidence of harm to gut bacteria at normal doses, and it was supported by the Calorie Control Council, an association representing makers of low-calorie foods, drinks, and ingredients.

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If you have Crohn's or colitis

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The International Organization for the Study of Inflammatory Bowel Diseases (a group of IBD specialists from around the world) has more specific advice for people who already have Crohn's disease or ulcerative colitis. Its 2020 dietary guidance says that "it may be prudent to limit intake of maltodextrin-containing foods and artificial sweeteners," and it rates the evidence behind that as "very low," drawn from population studies and animal models. It also points out that Splenda is about 1% sucralose and 99% maltodextrin, a starchy filler, and its advice names maltodextrin right alongside the sweeteners.

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One of the animal studies in this area gave Splenda to mice for six weeks, some of them bred to develop a Crohn's-like disease and some healthy. Splenda shifted the gut bacteria in all of the mice, and it didn't make the Crohn's-like damage look any worse under a microscope, but it did raise a marker of inflammation (an enzyme called myeloperoxidase) in the Crohn's-prone mice. Nobody has tested that in people. If you have Crohn's or colitis, sweeteners are worth bringing up with your GI doctor.

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What you can actually do

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If a sweetened drink or packet bothers your stomach, start with the ingredient list. Anything ending in "-ol" is a sugar alcohol, which is the kind of sweetener that the NHS and Monash both single out, and since ingredients go from most to least by weight, the first one listed is the one there's the most of. For everyday drinking, the same 2025 panel recommends plain water for kids and teens, with fluoridated tap water as its first choice.

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