Diet drinks matched water for weight loss in a two-year trial of 493 adults

Participants who drank diet beverages lost an average of 4.8 kilograms over 104 weeks, against 3.7 kilograms for participants assigned plain water, and the 1.1-kilogram gap between the two groups was not statistically significant. The SWITCH trial randomized 493 adults with overweight or obesity, and its two-year results appeared in the British Journal of Nutrition on September 29, 2026. Both groups held on to their losses through a final year in which nobody received weight-management support.

Professor Jo Harrold, Dean of Psychology at the University of Liverpool, and Professor Jason Halford of the University of Leeds led the work. Harrold put the result in unusually plain terms: concerns that sweeteners disrupt appetite or cause weight gain “are not supported when tested in a rigorous, long-term randomised trial.” That sentence is the claim the trial was built to test, and the design explains why it carries more weight than the short experiments that preceded it.

Two servings a day across three phases

According to the study’s registry summary on the NHS Health Research Authority site, the 493 recruits had a body mass index between 27 and 35 and regularly drank cold beverages. Each was randomly assigned to two 330-millilitre servings a day of either a non-nutritive sweetened drink or water. The trial ran in three phases: a 12-week weight-loss period, a 40-week assisted maintenance phase, and a 52-week stretch with no assistance at all.

The diet drinks were commercially available products sweetened with aspartame, acesulfame potassium or sucralose, consumed at levels the European Food Safety Authority treats as acceptable. Everyone, in both arms, took part in a structured behavioral weight-management program, so the comparison is between two beverage choices inside the same diet-and-support package rather than between drinking diet soda and doing nothing.

Net losses of 4.8 and 3.7 kilograms

Coverage of the University of Liverpool announcement reports the final numbers: minus 4.8 kilograms in the diet-beverage group and minus 3.7 kilograms in the water group at 104 weeks, a nonsignificant difference. Waist and hip circumference, body composition and metabolic biomarkers improved in both arms. Sugar intake fell in both groups too, slightly more in the diet-beverage group.

Matching water is the accurate verb only in a statistical sense. The diet group’s average loss was numerically larger, but a trial of this size could not distinguish that edge from chance, so the defensible reading is equivalence rather than superiority. Nothing in the reported results shows diet drinks beating water, and a 1.1-kilogram average gap on top of a loss near four kilograms is the kind of difference the trial’s statistics treated as noise.

Attrition among the 493

Attrition is the caveat that matters most. A summary of the paper counts 220 completers, 117 in the water arm and 103 in the diet-beverage arm, out of the 493 who began, and notes that the trial ran at a single site and did not report race or ethnicity data. Long behavioral trials routinely lose participants, but losing more than half of them leaves room for the people who stayed to differ from the people who left.

The outcome is also narrow. The trial measured weight, waist size, body composition and metabolic markers; it did not track heart attacks, strokes, diabetes diagnoses or deaths. A result that diet drinks do not blunt weight loss says nothing about whether regular sweetener intake carries other risks over decades.

An industry-funded answer to a WHO position

The American Beverage Association paid for the study, and the team says it kept independent control over analysis and reporting. Funding source matters in nutrition science, where industry-sponsored trials have a documented tendency to favor sponsors, and it is the first thing a skeptical reader should weigh. The same group’s 2023 follow-up work, announced by the university at Obesity Week, was funded the same way and reached the same conclusion over a shorter horizon.

The trial lands against a standing recommendation. In May 2023 the World Health Organization advised against using non-sugar sweeteners for weight control, a conditional recommendation based on a systematic review that found no long-term body-fat benefit. Francesco Branca, WHO’s Director for Nutrition and Food Safety, said at the time that such sweeteners “are not essential dietary factors and have no nutritional value.” SWITCH does not contradict the first half of that picture so much as complicate it: inside a supported program, diet drinks cost dieters nothing against water over two years, while the WHO question about long-term health outcomes remains untouched by a weight-only endpoint.

A release repeating the Liverpool figures circulated on October 3, four days after the journal date. The unresolved piece for anyone weighing the evidence is the 273 participants whose two-year weights are missing from the completer count: the reported averages describe the people who finished.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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