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In a two-year behavioral weight-management trial, participants assigned non-nutritive-sweetened (NNS) beverages lost a similar amount of weight to those assigned water. At week 104, average weight change was −4.8 kg in the NNS group and −3.7 kg in the water group; the difference was not statistically significant, and the researchers reported equivalent weight change. The result describes beverage choices within a structured program—not diet drinks acting as a weight-loss treatment on their own.
What the SWITCH trial compared
The SWITCH trial, published online in the British Journal of Nutrition on 4 August 2026, was a randomized equivalence trial involving 493 adults aged 18–65 with a BMI of 27–35 kg/m². Recruitment and the trial took place in the Liverpool area of England. Participants were assigned to water (246 people) or NNS beverages (247 people) and asked to drink at least two 330 ml servings a day of their assigned cold beverage.
The tested category included commercially available drinks sweetened with aspartame, acesulfame-K or sucralose. This was not a comparison of specific brands. The published trial report describes the design, participants and results.
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Weight change after 104 weeks
The primary complete-case analysis compared participants with relevant measurements at baseline and week 104. It found that both groups remained below their starting weight, with no significant difference between them.
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| Assigned beverage | Mean weight change at week 104 |
|---|---|
| Water | −3.7 kg |
| NNS beverages | −4.8 kg |
These are averages from the trial’s complete-case analysis, not estimates of what any individual should expect. The paper reports similar between-group conclusions in analyses using imputed data and last observation carried forward.
The weight-loss program matters
Participants received behavioral support alongside their assigned drink. The program included weekly weight-loss sessions for the first 12 weeks, monthly assisted weight-maintenance sessions for the next 40 weeks, and then a voluntary 52-week unassisted extension. Because the beverages were tested in that context, the result does not establish that switching to diet drinks alone causes weight loss.
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How to interpret the health findings
The researchers reported reductions in waist and hip circumference in both groups, without significant differences between them. Some biomarker results did differ: total cholesterol increased significantly more with NNS beverages, while HbA1c and diastolic blood pressure fell significantly with water compared with NNS beverages. The authors note that some of these measures had small samples, so they should not be treated as definitive evidence of a broad health advantage for either drink.
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The report recorded no discontinuations due to adverse events and no serious adverse events. Its conclusion that neither assignment was associated with undesirable metabolic or behavioral outcomes applies to this trial; it is not proof that every sweetener or product is safe for every person over the long term.
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What the trial cannot settle
- Attrition: 220 of the 493 randomized participants completed the week-104 time point: 117 in the water group and 103 in the NNS group. That loss of follow-up limits how confidently the two-year result can be generalized.
- Setting and population: The study was conducted at one site in England and did not collect race or ethnicity data.
- Final-year design: The voluntary final year was added after a protocol amendment when recruitment targets were not being met. The authors discuss the pandemic, trial duration and voluntary participation as possible contributors to attrition.
- Scope: The result concerns the assigned beverage categories in this program and population. It does not show that NNS drinks are better than water for health, that every sweetener performs the same way, or that all people will respond similarly.
The trial was funded by the American Beverage Association. A University of Liverpool release said the research team operated independently of the funder and retained full control over analysis and reporting; that statement is the university’s account. The release also includes the researchers’ interpretation of the findings.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How this compares with earlier research
A 2014 randomized study in Obesity compared water and NNS beverages during a 12-week behavioral weight-loss program and reported greater short-term weight loss in the NNS group. It was shorter and differed in design and population, so it should not be read as the same result as SWITCH’s 104-week comparison. The 2014 study provides that earlier context.
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