Should you drink water or diet soda to lose weight? Scientists in Liverpool, England, tested this for two years. They put 493 adults into two groups.

One group drank water. The other group drank diet drinks with sweeteners and no sugar. Everyone also joined a weight-loss program.

After two years, the water group was 3.7 kilograms lighter on average. The diet drink group was 4.8 kilograms lighter. The scientists said the two results were about the same.

A team at the University of Liverpool wanted to know whether diet drinks help or hurt people trying to lose weight. They asked 493 adults with overweight or obesity to join a group weight-loss program. Half of them were told to drink water, and the other half were given drinks made with artificial sweeteners. In the first year, the groups met with the team weekly and then monthly.

In the second year, they were on their own. At the end, people in the water group had lost 3.7 kilograms on average, and people in the sweetener group had lost 4.8 kilograms. The gap was too small to count as a real difference. The study was paid for by the American Beverage Association, a group that represents drink companies.

Diet sodas have an odd reputation. They contain almost no calories, yet studies that follow large groups of people for years often find that heavy users gain more weight and are more likely to develop diabetes or heart problems. A two-year trial from the University of Liverpool approached the question in a different way.

The researchers randomly assigned 493 adults with overweight or obesity to drink either water or zero-calorie sweetened drinks while taking part in a weight-management program. Meetings were weekly at first, then monthly, and in the second year participants managed alone. After 104 weeks, the water group had kept off 3.7 kilograms and the sweetener group 4.8 kilograms, a gap the authors judged to be statistically equivalent.

Only 220 people completed the final year, so the result rests on fewer than half of those who started. The funding also deserves attention. The American Beverage Association paid for the trial, although the authors state that it played no part in the design or interpretation. And since the main outcome was body weight, the trial says little about long-term health. In 2023 the World Health Organization advised against using sweeteners for weight control.