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In a two-year behavioral weight-management trial, people assigned non-nutritive-sweetened (NNS) drinks and those assigned water had similar weight loss at the 104-week endpoint. In the study’s primary analysis, the average change was −4.8 kg for NNS drinks and −3.7 kg for water; the difference was not statistically significant, and the paper reported equivalent weight change. This does not show that diet drinks cause weight loss on their own.
What the trial compared
The SWITCH trial was a randomized equivalence study involving 493 adults aged 18–65 with a BMI of 27–35 kg/m². Participants were recruited in the Liverpool area of England and assigned to a water group (246 people) or an NNS-beverage group (247 people). The study was led by Joanne A. Harrold and colleagues and published online in the British Journal of Nutrition on 4 August 2026. Read the published trial.
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Participants were asked to drink at least two 330 ml servings each day of their assigned cold beverage. The NNS category included drinks sweetened with ingredients such as aspartame, acesulfame-K or sucralose; this was not a comparison of particular brands.
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Weight change at 104 weeks
Among participants with weight measurements at both baseline and week 104, mean weight change was −3.7 kg in the water group and −4.8 kg in the NNS group. The between-group difference was not significant, and the authors reported equivalent weight change. Both groups were, on average, below their starting weight.
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| Assigned beverage | Mean change at week 104 | What the number represents |
|---|---|---|
| Water | −3.7 kg | Harrold et al., 2026, primary complete-case analysis of participants with baseline and week-104 measurements. |
| NNS beverages | −4.8 kg | Harrold et al., 2026, primary complete-case analysis of participants with baseline and week-104 measurements. |
The paper also reports similar between-group conclusions in analyses using imputed data and last observation carried forward. The headline result is therefore best read as evidence of similar weight outcomes in this trial—not proof that one beverage is superior.
The drinks were part of a weight-management program
Participants did not simply switch beverages and carry on without support. They attended weekly behavioral weight-loss sessions for the first 12 weeks, then monthly assisted weight-maintenance sessions for 40 weeks. The study’s final 52 weeks were a voluntary, unassisted extension.
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Because both beverage assignment and the behavioral program were part of the trial, its result cannot establish that diet drinks alone produce weight loss. It answers a narrower question: within this program, did assignment to NNS beverages lead to meaningfully different weight change from assignment to water over 104 weeks?
How to interpret the result—and its limits
Only 220 participants reached the 104-week measurement
Of the 493 randomized participants, 220 completed the week-104 time point: 117 assigned to water and 103 to NNS beverages. The primary complete-case result is based on people with the required measurements, so substantial attrition matters when interpreting the averages. The authors discuss the trial’s duration, the voluntary final year and the COVID-19 pandemic as possible contributors to missed follow-up.
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The findings do not settle every health or sweetener question
Both groups had reductions in waist and hip circumference, with no significant differences between them. The paper also reports some differing biomarker results: total cholesterol increased significantly with NNS beverages versus water, while HbA1c and diastolic blood pressure decreased significantly with water versus NNS beverages. The authors note that some parameter samples were small. These results do not establish that diet drinks are healthier than water, or prove long-term safety for every sweetener or product.
The study was conducted at one site in England and did not collect race or ethnicity data. Its results may not apply in the same way to every population, product or individual. The authors reported no discontinuations related to adverse events and no serious adverse events in the trial; that finding is specific to this study and is not a general safety guarantee.
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What this means if you are choosing a drink
If you are following a structured weight-management program, this trial suggests that choosing an NNS beverage instead of water did not meaningfully change average weight loss at two years among participants measured at that endpoint. It does not provide a reason to replace water for health, nor does it show that adding diet drinks without other changes will reduce weight. The direct comparison was between assigned beverages in the context of the program.
A shorter 2014 randomized study compared water and NNS beverages during a 12-week behavioral weight-loss program and reported greater short-term loss in the NNS group. Its shorter duration and different design mean it should not be treated as the same result as SWITCH’s 104-week comparison. Read the 2014 study.
Funding and attribution
The 2026 trial reports funding from the American Beverage Association. The University of Liverpool’s 3 October 2026 release states that the research team operated independently from the funder and retained control over analysis and reporting; that is the university’s characterization. Read the University of Liverpool release carried by ScienceDaily.
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