The SWITCH trial of 493 adults found no-sugar sweetened soft drinks matched water for losing weight and keeping it off, a challenge to WHO guidance. The beverage industry funded it.

Few ingredients have attracted as much suspicion as artificial sweeteners. They have been blamed for confusing the body’s appetite signals, encouraging a sweet tooth and even promoting weight gain. In 2023, the World Health Organization advised against using non-sugar sweeteners for weight control, citing studies that linked them to poorer long-term outcomes.

The sweeteners at issue are intensely sweet. Aspartame is roughly 200 times sweeter than table sugar, and sucralose about 600 times, which means tiny amounts can replace the sugar in a soft drink while adding almost no calories. Also in 2023, the International Agency for Research on Cancer classified aspartame as “possibly carcinogenic to humans,” its third-highest category, while a separate WHO and FAO expert committee kept its acceptable daily intake unchanged at 40 milligrams per kilogram of body weight. The mixed messages have left many consumers unsure what to believe.

A new two-year randomized trial led by researchers at the University of Liverpool challenges the case against diet drinks. Published in the British Journal of Nutrition, the SWITCH trial found that drinks sweetened with aspartame, acesulfame potassium or sucralose were equivalent to water for losing weight and keeping it off.

“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,” said Professor Joanne Harrold, Dean of Psychology at the University of Liverpool, who co-led the study.

Diet soda versus water

The trial enrolled 493 adults with overweight or obesity who were taking part in a structured behavioural weight management program. Participants were randomly assigned to drink either water or commercially available diet beverages every day. Sweetener levels remained well within the safety thresholds set by the European Food Safety Authority.

The design matters. Much of the evidence behind the WHO’s caution came from observational studies, which track what people choose to drink and how their health changes. Those studies are prone to a problem called reverse causation: people who are already gaining weight or at risk of diabetes may switch to diet drinks precisely because of it, making the drinks look harmful. Randomly assigning participants to one drink or the other removes that bias.

Two years of results

After 104 weeks, participants in the diet-drink group had lost about 4.8 kilograms on average, compared with about 3.7 kilograms in the water group. The difference was not statistically significant, which is why the researchers describe the two as equivalent.

Both groups maintained their weight loss across the full two years, including the final year, when participants received no support from the program. Both saw reductions in waist and hip circumference, improvements in body composition and improvements in several metabolic biomarkers. Blood pressure and cholesterol showed no clinically meaningful differences between the different sweeteners or water. Sugar intake fell in both groups, with a slightly larger drop among those drinking diet beverages.

Keeping weight off is often harder than losing it. Many people regain much of the weight they lose within a few years, so the fact that both groups held their losses through a final year without program support is one of the more encouraging aspects of the study, whichever drink they were assigned.

The authors conclude that the findings do not support concerns that these drinks disrupt appetite regulation or cause long-term metabolic harm. Earlier results from the same trial, covering the first year, were published in 2023 in the International Journal of Obesity.

“This study provides the long-term clinical evidence that has been missing from international discussions,” said co-lead Professor Jason Halford, formerly of Liverpool and now a professor of biological psychology and health behaviours at the University of Leeds. “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.”

Who paid for it

One detail deserves prominence. The trial was funded by the American Beverage Association, the trade group for the soft drink industry. The release states that the research team was independent and controlled the analysis and reporting, and the study was peer reviewed. But industry funding of nutrition research has a long and contested history, and readers are right to weigh it.

Industry funding does not by itself invalidate a well-designed randomized trial, and long, rigorous trials of this kind are expensive and rarely funded by anyone else. Still, independent replication would strengthen the case considerably.

Sugar-sweetened drinks remain one of the largest sources of added sugar in many diets, and policies such as the United Kingdom’s soft drinks levy, introduced in 2018, have pushed manufacturers to reformulate products with sweeteners. How those reformulated drinks affect health is therefore a question of practical, population-wide importance.

What it does and doesn’t show

The trial was set within a structured weight management program, and its participants were adults with overweight or obesity. The results may not apply to people drinking diet soda outside such a program, to children or to other outcomes, such as effects on gut bacteria or cardiovascular events, that a two-year trial is not designed to capture. Concerns raised by other recent research, including observational links between some sweeteners and heart disease risk, remain under study. The release does not detail how many participants dropped out or how randomization was carried out.

What the study does offer is a direct answer to a narrower but practical question: for someone trying to lose weight who would otherwise drink sugary soda, is a diet version a reasonable substitute? In this trial, it worked about as well as water.

Harrold hopes the findings will shape policy. “We hope this evidence informs future WHO guidance and public health policy,” she said.

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Notes

1. “Effect of non-nutritive sweetened beverages versus water on body weight: long-term results of a randomised controlled trial.” British Journal of Nutrition, 4 August 2026. DOI: 10.1017/S0007114526108150.
2. University of Liverpool. “Major trial examines no-sugar sweetened soft drinks.” EurekAlert!, 29 September 2026. https://www.eurekalert.org/news-releases/1145815.

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