Over two years, adults with overweight or obesity who drank diet beverages as part of a weight-management programme lost an average of 4.8 kilograms, while those assigned to water lost 3.7. The gap was small enough that the researchers call it statistically non-significant, and the result came from a randomised trial run by the University of Liverpool.
The money behind it matters as much as the scales. The American Beverage Association, the trade body for U.S. soft-drink makers, paid for the work, a fact that sits awkwardly next to a finding that favours its products.
A two-year trial built around real drinks
The study, known as SWITCH, enrolled 493 adults with overweight or obesity and ran for 104 weeks, according to the ScienceDaily summary of the University of Liverpool release. Participants were randomly assigned to drink either water or commercially available diet beverages every day, sweetened with aspartame, acesulfame potassium or sucralose. Intake stayed within the safety thresholds set by the European Food Safety Authority. Everyone also joined a structured behavioural weight-management programme, so the comparison was between two drinks inside the same diet-and-support package rather than between dieting and doing nothing.
A Sci.News account of the paper adds detail on the schedule. Volunteers from the Liverpool area, with a body mass index between 27 and 35, were asked to have at least two 330 millilitre servings a day. They attended weekly group meetings for 12 weeks, then monthly meetings for 40 weeks, then spent a final 52 weeks being monitored without any support sessions.
What 4.8 kilograms against 3.7 actually shows
At 104 weeks the diet-beverage group had lost about 4.8 kilograms and the water group about 3.7 kilograms. The release says both groups held on to their loss through the final year, when support had stopped, and both reduced waist and hip circumference and improved body composition and several metabolic markers. No clinically meaningful differences in blood pressure or cholesterol appeared between the sweetener and water arms. Sugar intake fell in both groups, a little more in the diet-drink group. The peer-reviewed paper, “Effect of non-nutritive sweetened beverages v. water on body weight: long-term results of a randomised controlled trial”, appears in the British Journal of Nutrition.
The sources agree that the 1.1 kilogram edge for diet drinks did not reach significance, and the authors describe the two drinks as performing equivalently to water over two years. Sci.News adds a caveat worth keeping in view: its figures refer to the 220 participants who completed the trial, not all 493 who started, and both groups lost most of their weight in the first 12 weeks before slowly regaining some. The diet-beverage group regained a little more slowly.
An American Beverage Association grant and a stated firewall
The funder is named plainly in the release: the American Beverage Association paid for the research. The authors state that the study team “operated independently from the funder and retained full control over the analysis and reporting.” That is a common arrangement for industry-supported trials, and it is only a statement of process. Readers cannot verify it from the outside, and research on industry-sponsored nutrition studies has long found that sponsorship can shape which questions get asked, even when the analysis is untouched.
Two things can be checked directly. The trial was randomised, which removes the self-selection problem that dogs surveys of people who already choose diet drinks. And the central comparison is against water, a deliberately modest benchmark, rather than against sugary drinks or a no-intervention group. A result of “no worse than water” is a narrower claim than “diet drinks drive weight loss”, and the data support only the former.
The World Health Organization guidance the trial is aimed at
The Liverpool team frames SWITCH as an answer to the World Health Organization. In May 2023 the agency advised against using non-sugar sweeteners for weight control, and the full WHO guideline on non-sugar sweeteners rests largely on observational research linking diet drinks with weight gain and other outcomes. The trial authors argue those associations can be distorted by reverse causality, where people already gaining weight switch to diet products, and by confounding.
Lead author Professor Joanne Harrold, Dean of Psychology at the University of Liverpool, said “concerns about sweeteners disrupting appetite or causing weight gain are not supported.” Her co-author Professor Jason Halford, now at the University of Leeds, called the study evidence that “has been missing from international discussions” and said such beverages offer an effective alternative to sugar-sweetened drinks, as reported by News-Medical. The authors say they hope the evidence will inform future WHO guidance.
Whether it will is an open question. A single trial of 493 people, funded by an industry with an obvious stake, cannot by itself overturn a guideline built on many studies. What it adds is long-term randomised evidence on one narrow question, whether swapping water for diet drinks during a supported weight-loss effort changes the outcome, and its answer was that it did not. The original University of Liverpool announcement is the place to check the institution’s own framing.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
More from Morning Overview
- Long use of a common prostate pill is tied to a higher chance of glaucoma
- Ford is recalling 223,472 F-150 pickups because the fuel tank can leak or detach
- Hybrids have 15% fewer problems than gas cars, while EVs and plug-in hybrids have about 80% more, Consumer Reports finds
- Regulators cleared the first U.S. small modular reactor, 4 months early
