You have probably heard it from a friend or seen it on a wellness feed. Diet soda is supposed to save you the calories, but somehow the pounds hang on.
Many people quietly give it up, convinced the sweeteners are working against them.
A new two-year trial puts that claim to the test. In a randomized trial of 493 adults in a structured weight-loss program, drinking diet soda produced about the same weight loss as water. Both groups kept the weight off for two years.
The study, published in the British Journal of Nutrition in October 2026, is one of the longest head-to-head tests of diet soda and water inside a supervised weight-management program.
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What the two-year trial actually measured
The SWITCH trial, run by researchers at the University of Liverpool, followed 493 adults with a body mass index of 27 to 35.
Participants were randomly assigned to drink either water or beverages sweetened with aspartame, acesulfame-K and sucralose.
The structure matched a real weight-loss program. Participants attended weekly group sessions for the first 12 weeks.
They moved to monthly sessions through 40 weeks of maintenance. The last 52 weeks they were on their own.
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After 104 weeks, the water drinkers had lost about 3.7 kg, or roughly 8 pounds.
The diet soda drinkers had lost 4.8 kg, close to 10.5 pounds. The gap between the two was not statistically significant, meaning the drinks worked equally well.
Waist and hip measurements shrank in both groups. A smaller subgroup who had a body scan showed matched fat loss. On weight, the diet soda did not sabotage the result.
What the finding does and does not mean
The result pushes back against a widespread wellness belief. Many people assume the sweet taste from zero-calorie drinks tricks the body into craving more food, and that this quietly fuels weight gain in people trying to lose it.
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"Concerns about sweeteners disrupting appetite or causing weight gain are not supported when tested in a rigorous, long-term randomised trial," said Professor Joanne Harrold, who co-led the trial.
The result is narrower than it sounds. Everyone in the trial was part of a supervised program with group sessions and food guidance.
The study does not say what happens if you simply add a diet soda to an unchanged diet.
The trial was also funded by the American Beverage Association, the industry group for soft drink makers. The researchers said they kept full control over the analysis and reporting.
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Where the sweetener debate still sits
Weight loss is not the only sweetener question. In May 2023, the World Health Organization advised against using non-sugar sweeteners for weight control, pointing to possible links to type 2 diabetes and heart disease at the population level.
Francesco Branca, then WHO director of nutrition and food safety, said at the time that replacing sugar with sweeteners does not help with weight control over the long run.
There are other sweetener questions where the evidence is more mixed, including possible effects on the gut microbiome and on blood sugar response in some people.
Those are different questions from the weight outcome measured in the Liverpool trial.
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One other number from SWITCH stands out. Sugar intake dropped in both groups, and dropped a little more in the diet soda group.
For people trying to pull back on sugary drinks, the swap to a diet version lined up with less sugar in the diet overall.
This article is made and published by Mie Hermansen, who may have used AI in the preparation.