A two-year clinical trial involving 493 adults has found that non-nutritive sweetened beverages are as effective as water for long-term weight management. Published in the British Journal of Nutrition, the study suggests that diet drinks do not disrupt appetite or hinder weight loss when consumed as part of a structured program.
The SWITCH Trial: Design and Methodology
Researchers at the University of Liverpool conducted the 104-week study, known as the SWITCH trial, to examine the long-term impact of non-nutritive sweetened (NNS) beverages on weight management. The study followed 493 adults with a body mass index (BMI) between 27 and 35, all of whom participated in a structured behavioral weight-management program. Participants were randomly assigned to consume either water or commercially available diet beverages containing sweeteners such as aspartame, acesulfame potassium, and sucralose. These sweeteners were provided at levels kept well within the safety thresholds established by the European Food Safety Authority (EFSA).
The program included weekly group meetings for the first 12 weeks, followed by monthly sessions for the next 40 weeks. A final 52-week period was voluntary and provided no routine support, allowing researchers to observe if weight loss was maintained without ongoing coaching. The trial results, which were published in the British Journal of Nutrition on August 4, 2026, revealed that weight loss was maintained in both groups even after the formal support sessions concluded. Professor Joanne Harrold, Dean of Psychology at the University of Liverpool, led the research alongside Professor Jason Halford, who serves as Professor of Biological Psychology & Health Behaviours at the University of Leeds.
Weight Loss Outcomes and Metabolic Markers
Among the 220 participants who completed the full two-year trial, those in the diet-beverage group lost an average of 4.8 kg (approximately 10.6 pounds), while those in the water group lost 3.7 kg (approximately 8.2 pounds). Researchers concluded that this difference was not statistically significant. Both groups also experienced reductions in waist and hip circumference, as well as improvements in body composition. By the 104-week mark, 117 participants remained in the water group and 103 participants remained in the diet-beverage group.
While some metabolic measures shifted, the study found no evidence of long-term metabolic harm associated with the consumption of NNS beverages. According to the Medscape report, total cholesterol levels and blood pressure readings remained within healthy ranges for participants in both groups. Although some markers like HbA1c and total cholesterol showed slight variations, they were not considered medically significant. The findings challenge the notion that sugar substitutes necessarily lead to compensatory eating or increased appetite, as the study found no meaningful difference between groups regarding hunger or sugar intake.
“These latest findings show that concerns about sweeteners disrupting appetite or causing weight gain are not supported when tested in a rigorous, long-term randomised trial.”
Professor Joanne Harrold, University of Liverpool
Contextualizing Findings Against WHO Guidance
The SWITCH trial results arrive in contrast to 2023 guidance from the World Health Organization (WHO), which had advised against using non-sugar sweeteners for weight control due to uncertainty in observational studies. As noted by Sci. News, researchers argue that earlier observational data may have been skewed by reverse causality, where individuals already gaining weight switch to diet drinks. The authors hope the new long-term evidence will help inform future international public health policy. Professor Jason Halford noted that this trial provides the long-term clinical evidence that has been missing from previous international discussions.
Despite the positive findings, some experts maintain a cautious perspective. She added that while she still recommends water as the primary hydration choice, the public need not fear that diet drinks will automatically trigger weight gain.
Entities Funded the Study
The trial was funded by the American Beverage Association. However, the study authors stated that the association did not participate in the study design, data analysis, or the interpretation of the results. Outside of this specific study, the authors have reported receiving various forms of support from other entities, including grants from the European Commission, funding from Coca-Cola for a PhD studentship, and honoraria from the International Sweeteners Association. They have also disclosed consultancy or other fees from companies such as Novo Nordisk, AstraZeneca, Boehringer Ingelheim, Dupont, and Mars.
Readers should note that this study compared diet beverages to water rather than sugar-sweetened drinks, testing specifically whether sugar substitutes hinder weight loss.
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