People who drank diet soft drinks sweetened with aspartame, acesulfame-K or sucralose lost about as much weight over two years as people who drank water, according to what its authors describe as the longest randomized controlled trial of its kind, published in the British Journal of Nutrition.
The 104-week SWITCH trial, run by researchers at the University of Liverpool and the University of Leeds, followed 493 adults with overweight or obesity enrolled in a structured behavioral weight management program. Participants were randomly assigned to drink either water or commercially available diet beverages each day, with sweetener intake kept well within safety thresholds set by the European Food Safety Authority.
After two years, the diet drink group had lost an average of 4.8 kilograms, against 3.7 kilograms in the water group, a difference that was not statistically significant. Both groups held onto their weight reductions through the full study, including the final year, when participants no longer received assistance. Both also saw smaller waist and hip measurements, improvements in body composition and gains on several metabolic biomarkers, with no clinically meaningful differences in blood pressure or cholesterol between the two.
Sugar intake dropped in both groups, with a slightly larger reduction among those drinking the sweetened beverages. Overall, the researchers found no support for concerns that the sweeteners interfere with appetite regulation or cause long-term metabolic harm.
The trial was funded by the American Beverage Association, the soft drink industry’s trade group. The researchers said the team worked independently of the funder and kept full control over the analysis and reporting of the results.
The findings land in the middle of a running dispute over sweeteners. The World Health Organization has advised against using non-sugar sweeteners for weight control, citing uncertainty in observational research. The Liverpool-led team said its randomized, long-term evidence could inform whatever global guidance comes next.
Professor Joanne Harrold, dean of psychology at the University of Liverpool, who led the research, said the concerns did not hold up under testing. “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,” she said. “We hope this evidence informs future WHO guidance and public health policy.”
The new results build on earlier SWITCH findings reported in the International Journal of Obesity in 2023. Professor Jason Halford of the University of Leeds and the University of Liverpool, the co-lead, said the study fills a gap. “This study provides the long-term clinical evidence that has been missing from international discussions,” he said. “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.”