Diet Drinks Rival Water for Weight Loss in a Two-Year Trial

A can of diet soda may not be sabotaging a diet after all. The longest randomized trial ever run on zero-calorie sweetened beverages found that diet drinks support weight loss about as well as water, according to research from the University of Liverpool published in the British Journal of Nutrition. The 104-week study followed 493 adults with overweight or obesity, making it the most extensive experiment of its kind to date.

The findings cut against years of suspicion around artificial sweeteners, and they arrive at a moment when nutrition debates about diet drinks have rarely been louder. Rather than comparing drinkers with non-drinkers in a survey, the researchers assigned people to drink either water or commercially available diet drinks every day inside a structured behavioral weight management program, then tracked what happened to their weight over time.

How the SWITCH trial worked

The study, known as the SWITCH trial, was led by Professor Jo Harrold, dean of psychology at the University of Liverpool, alongside Professor Jason Halford of the University of Leeds. Participants were randomly assigned to consume water or diet beverages containing common sweeteners, specifically aspartame, acesulfame potassium and sucralose, at levels within established European safety limits. The long follow-up is what sets the trial apart: most research on diet drinks and sweeteners runs for weeks or months, not years, which is why previous guidance has leaned on observational data instead. Independent coverage of the trial summarizes the same peer-reviewed findings.

The research was funded by the American Beverage Association, a point the university addresses head-on. In a statement, the research team said it remained independent of the funder and held full control over the analysis and reporting of the findings, a standard safeguard in industry-funded nutrition science. That independence matters for how the results should be read: the design is randomized and controlled, which is exactly the evidence critics of sweetener guidance have been asking for.

What actually changed on the scale

By the end of the study period, participants drinking diet beverages had lost an average of 4.8 kilograms, compared with 3.7 kilograms in the water group, a difference the researchers described as not statistically significant. The weight reduction held up across the final year of the trial, when participants were no longer receiving the structured program, and both groups also saw reductions in waist and hip circumference alongside improvements in body composition and several metabolic biomarkers.

Crucially, the results did not support the long-running concern that zero-calorie sweeteners scramble appetite regulation or cause hidden metabolic harm. The researchers reported no clinically meaningful differences between the groups on measures such as blood pressure and cholesterol, and sugar intake fell in both groups over the course of the study. For people who have been told that diet drinks stall progress, the data point the other way.

Why the timing matters

The trial lands in the middle of an ongoing dispute about sweetener guidance. The World Health Organization has advised against using non-sugar sweeteners for weight control, a position based largely on observational studies that cannot prove cause and effect. The Liverpool team argues that their randomized, long-duration evidence fills the exact gap those guidelines flagged as uncertain, and health authorities revisiting the topic now have a two-year experiment to weigh against the older data.

The result also complicates the popular story that sweeteners are the real villain in modern diets. With weight loss drugs dominating headlines, it is easy to forget that most people manage their weight through ordinary habits, and for them the evidence is reassuring: swapping sugary drinks for diet drinks remains a reasonable step, one this trial supports better than any experiment before it.

None of this means diet drinks are healthier than water. Water remains a calorie-free option without added sweeteners, and the study does not establish that diet beverages are necessary for losing weight. What it does offer is a clearer answer to a practical question people ask every day: if the choice is between a diet soda and plain water, the evidence now suggests the scale does not care much. For readers following diet drinks news as it develops, see the University of Liverpool announcement for the full breakdown.