A two-year clinical trial involving 493 adults with overweight or obesity has found that non-nutritive sweetened beverages perform equivalently to water in supporting long-term weight management, challenging World Health Organization guidance advising against non-sugar sweeteners for weight control.
Adults participating in a structured behavioral weight-management program achieved similar weight loss whether they drank water or commercially available diet beverages over a 104-week period according to University of Liverpool researchers. The findings, published in the British Journal of Nutrition, provide clinical trial evidence examining how low- and no-calorie sugar substitutes compare directly with water in structured weight loss efforts.
How the SWITCH Trial Tested Diet Drinks Against Water
The study, known as the effectS of non-nutritive sWeetened beverages on appetITe during aCtive weigHt loss (SWITCH) trial, began with 493 adult participants aged 18 to 65 who had a body mass index between 27 and 35. Researchers assigned participants randomly to consume at least two 330-milliliter servings daily of either water or non-nutritive sweetened beverages containing aspartame, acesulfame potassium, and sucralose within European Food Safety Authority safety thresholds. Participants in the diet beverage group chose from 20 commercially available products, including diet colas, flavored waters, and diet fruit drinks while water drinkers eliminated all artificially sweetened drinks from their diets, including sweeteners added to coffee or tea.
Everyone in the trial joined a behavioral weight-management program that featured weekly group meetings for the first 12 weeks, followed by monthly sessions for the remaining 40 weeks of the first year.
Participants then entered a voluntary second year involving monitoring without routine coaching sessions.
Professor Jo Harrold, Dean of Psychology at the University of Liverpool, led the peer-reviewed research alongside Professor Jason Halford, Professor of Biological Psychology & Health Behaviours at the University of Leeds and formerly of the University of Liverpool. The primary sweeteners evaluated in the trial included acesulfame potassium, aspartame, and sucralose.
Weight Loss and Body Composition Results at 104 Weeks
Out of the initial 493 participants, 220 completed the full two-year follow-up, leaving 117 in the water group and 103 in the diet-beverage group by week 104. Among those who finished the study, participants assigned diet drinks lost an average of 4.8 kilograms (about 10.6 pounds), compared to an average loss of 3.7 kilograms (about 8.2 pounds) for those assigned water — a difference researchers found statistically insignificant and too small to rule out chance.
Both groups reached their lowest weight during the first year and subsequently experienced a slow weight regain, but participants remained lighter than their starting baseline measurements at the two-year mark. Waist and hip measurements also declined in both groups with no meaningful difference between them.
Further evaluation revealed that overall cholesterol experienced a minor rise among diet drink consumers, whereas HbA1c, a long-term indicator of blood sugar, increased with diet beverages while remaining relatively stable in the water group. Diastolic blood pressure fell more among water drinkers, though authors considered these small health marker differences too minor to matter clinically. Participant-reported hunger increased in both cohorts, yet statistical significance was achieved exclusively by those consuming diet drinks, while inter-group comparisons showed no notable variance.
Challenging World Health Organization Guidance on Sweeteners
The published findings directly address conditional guidance issued by the World Health Organization in 2023 which advised against using non-sugar sweeteners for weight control due to uncertainties derived primarily from observational studies.
By utilizing a randomized controlled trial design, the SWITCH trial aimed to provide the clinical evidence that researchers argue has been missing from international discussions regarding sugar substitutes and metabolic health.
“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.”
Professor Joanne Harrold, lead author of the study and Dean of Psychology at the University of Liverpool
American Beverage Association Funds Study Amid State Soda Bans
The study was funded by the American Beverage Association, though study authors stated that the funder did not design or conduct the trial, analyze data, or interpret results. Researchers acknowledged limitations including a high dropout rate over the two-year period and the study being conducted at a single location in England.
The release of the trial results coincides with policy debates regarding diet beverages in government assistance programs. States including Florida and Indiana have implemented restrictions banning recipients from purchasing diet and regular soda with Supplemental Nutrition Assistance Program benefits under approvals from the U.S. Department of Agriculture. The state lists Diet Coke, Coke Zero, and Diet Pepsi among the drinks recipients cannot buy with their welfare benefits.
Readers concluding their review of this trial should note that these findings apply to adults in a structured behavioral weight management program and do not establish all long-term health effects of artificial sweeteners. For practical next steps regarding personal weight management or dietary choices, individuals should consult qualified healthcare professionals.
The paper is registered with ClinicalTrials.gov under the identifier NCT02591134.
Professor Jason Halford noted that the literature suggests long-term cardiometabolic effects of diet beverages are usually neutral to modest when they replace caloric drinks.