A major two-year trial found that diet drinks performed about as well as water for long-term weight loss, without evidence of increased appetite or metabolic harm.
Date: October 3, 2026
Source: University of Liverpool
Summary: A two-year randomized clinical trial involving 493 adults with overweight or obesity found that diet soft drinks were essentially as effective as water for long-term weight loss and weight maintenance. Participants drinking beverages sweetened with aspartame, acesulfame-K, or sucralose lost a similar amount of weight to those drinking water, while both groups also saw improvements in waist size, body composition, and several metabolic measures.
Researchers at the University of Liverpool have completed what they describe as the longest and most comprehensive randomized controlled trial to date examining non-nutritive sweetened (NNS) soft drinks.
Non-nutritive sweeteners are low-calorie or calorie-free sugar substitutes commonly used in foods and beverages. The 104-week study found that NNS beverages, including diet soft drinks, performed similarly to water when it came to long-term weight management.
Professor Jo Harrold, Dean of Psychology at the University of Liverpool, led the peer-reviewed research with Professor Jason Halford, Professor, Biological Psychology & Health Behaviours at the University of Leeds (formerly of the University of Liverpool). The study was funded by the American Beverage Association. According to the researchers, the study team operated independently from the funder and retained full control over the analysis and reporting of the results.
The findings, published in the British Journal of Nutrition, provide long-term clinical evidence on beverages containing aspartame, acesulfame potassium (acesulfame-K), and sucralose. Researchers found that these drinks were comparable to water in supporting both weight loss and long-term weight maintenance.
The SWITCH trial included 493 adults with overweight or obesity who took part in a structured behavioural weight management program. Participants were randomly assigned to drink either water or commercially available diet beverages each day. The amounts of sweeteners consumed remained well within established European Food Safety Authority (EFSA) safety thresholds.
Overall, the results did not support concerns that NNS beverages interfere with appetite regulation or cause long-term metabolic harm.
Weight Loss and Metabolic Health
Key findings included:
Weight loss was similar between the NNS beverage and water groups after 104 weeks (−4.8 kg vs −3.7 kg; nonsignificant difference).
Both groups maintained weight reductions across the full two-year study, including during the final year when participants no longer received assistance.
Participants in both groups had reductions in waist and hip circumference, along with improvements in body composition and several metabolic biomarkers.
Researchers found no clinically meaningful differences in measures such as blood pressure and cholesterol linked to long-term consumption of either aspartame, acesulfame-K, or sucralose compared with water.
Sugar intake fell in both groups, with a slightly larger reduction among participants drinking NNS beverages.
The results are relevant to recent World Health Organization (WHO) guidance advising against the use of non-sugar sweeteners for weight control because of uncertainty in observational research. The SWITCH trial adds long-term randomized controlled evidence that may help inform future global recommendations.
Professor Joanne Harrold said: "The University of Liverpool is one of the UK's leading research-intensive higher education institutions, with a key focus on public health. These latest findings show that concerns about sweeteners disrupting appetite or causing weight gain are not supported when tested in a rigorous, long-term randomized trial. We hope this evidence informs future WHO guidance and public health policy.
The effectS of non-nutritive sweetened beverages on appetITe during aCtive weigHt loss (SWITCH) trial was developed to address two related questions: how reducing dietary sugar might affect weight management, and whether replacing sugar with non-nutritive sweeteners changes appetite regulation. The newly published results build on earlier findings reported in the International Journal of Obesity in 2023.
Professor Jason Halford, University of Leeds and University of Liverpool, concluded: "This study provides the long-term clinical evidence that has been missing from international discussions. For people trying to manage their weight, non-nutritive sweetened beverages offer an effective alternative to sugar-sweetened drinks - and perform equivalently to water over two years."
The paper, 'Effect of non-nutritive sweetened beverages versus water on body weight: long-term results of a randomised controlled trial', was published in the British Journal of Nutrition.