Diet Soda vs Water for Weight Loss: What the Two-Year SWITCH Trial Really Shows

Few drinks have been judged as harshly as the diet soda. It has been called a gateway to sugar cravings, a trick on your metabolism and the beverage equivalent of a gym membership you never use. Water, meanwhile, enjoys a reputation so spotless it could run for office. So when a two-year trial concluded that people trying to lose weight did about as well with diet drinks as with water, the headlines practically wrote themselves. Before you stock the fridge with silver cans, it is worth reading past the headline.

What the SWITCH trial actually tested

The study, led by Joanne Harrold at the University of Liverpool, is called SWITCH and was published in the British Journal of Nutrition. It has been making the rounds in health news this month. Researchers recruited 493 adults in the Liverpool area with overweight or mild obesity, defined as a body mass index between 27 and 35. Everyone joined the same behavioural weight management programme. Then they were randomly assigned to one of two groups: one was asked to drink water, the other to drink beverages sweetened with non-nutritive sweeteners such as aspartame, acesulfame potassium and sucralose, at least two 330 ml servings a day. Sweetener intake stayed within the safety limits set by the European Food Safety Authority.

The programme ran in three phases over 104 weeks. First came twelve weeks of weekly group sessions. Then 40 weeks with monthly meetings. In the final year, participants who stayed on received no support at all, which is a realistic test of what happens when the coaching stops and real life resumes.

The results in plain numbers

After two years, the diet drink group had lost an average of about 4.8 kilograms. The water group had lost about 3.7 kilograms. The difference of roughly one kilogram was not statistically significant, which in scientific language means the trial cannot tell whether it reflects a real effect or chance. The researchers concluded that the two approaches were equivalent for weight loss. Both groups also reduced their waist and hip measurements and improved several markers of metabolic health. Both groups were still significantly lighter than at the start after two years, although both regained some weight after hitting their lowest point in the first year.

Harrold has said the findings do not support concerns that sweeteners disrupt appetite or cause weight gain. That is the authors’ interpretation. The press materials do not include the appetite data behind it, so you have to take that part on trust for now.

Why this study is getting so much attention

The timing is not accidental. In May 2023 the World Health Organization advised against using non-sugar sweeteners to control body weight. The WHO said its systematic review found no long-term benefit for reducing body fat and suggested possible links between long-term use and higher risks of type 2 diabetes, cardiovascular disease and mortality in adults. Importantly, the WHO labelled its recommendation as conditional, because much of the evidence came from observational studies where it is hard to separate the effect of sweeteners from everything else about the people who consume them. The guideline does not apply to people with pre-existing diabetes.

That is where a randomised controlled trial is useful. Observational research can be skewed by reverse causation. People who are already gaining weight or worried about their health may switch to diet drinks, which can make the drinks look guilty by association. Randomly assigning people to water or sweetened drinks removes much of that bias. The Liverpool team argues its trial adds the kind of long-term randomised evidence that was largely missing when the WHO wrote its guidance.

The fine print you should not skip

This is where a serious magazine has to spoil the party a little. The trial was funded by the American Beverage Association, the industry group representing soft drink makers. The researchers state that they worked independently and kept full control over analysis and reporting. Industry funding does not automatically invalidate a study, but it is a fact you deserve to know.

Dropout was substantial. According to Sci.News, the analysis covered the 220 participants who completed the trial out of 493 who started, meaning more than half did not make it to the two-year mark. Long weight loss trials often lose many participants, but a high dropout rate can affect results if the people who leave differ from those who stay. The published coverage also does not say whether anyone was blinded, which is understandably hard when one group is literally drinking diet drinks and the other is drinking water.

Finally, the study compared two drinks within a structured weight management programme. It does not tell you whether adding diet drinks to an otherwise unchanged lifestyle helps you lose weight. And it says nothing about long-term health outcomes such as heart disease, which is the territory where the WHO raised its concerns.

So which glass should you pick?

Our reading, which is interpretation rather than proven fact: the trial weakens the claim that diet drinks actively sabotage weight loss, at least in the context of a structured programme. It does not turn them into health food. Water remains the simplest drink there is, with no open questions attached. If diet drinks help someone move away from sugary soft drinks, this study suggests they are unlikely to undo the effort on the scale. The bigger picture, as with almost every nutrition debate, is the whole pattern of eating, moving and sleeping rather than one beverage.

There is something almost nostalgic about the diet soda debate. Coca-Cola launched Tab in 1963, a pink-canned promise that you could have the fizz without the calories. The argument has never really stopped since. Six decades on, the SWITCH trial will not end it either. It does, however, move the conversation from suspicion towards evidence. That alone is a refreshing change.

This article is for general information only and does not replace advice from a doctor, dietitian or other qualified health professional. If you are managing your weight or a health condition, please speak to a professional about what is right for you.