Diet drinks work as well as water for weight loss, major trial finds

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.

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 behavioral 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 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.