Diet soda or water to lose weight? An English study finally delivers its verdict after two years

Diet soda or water to lose weight? An English study finally delivers its verdict after two years
A two-year British study, the longest ever conducted on sugar-sweetened drinks, calls into question the effectiveness of diet sodas in weight management. The results show that these drinks have a similar impact as water on weight loss.

Many overweight people today replace their sugary sodas with sugar-free soft drinks, without really knowing if this choice helps them better manage their weight. A randomized clinical trial conducted in the United Kingdom, billed as the longest ever conducted on these products, followed 493 adults for two years to answer this question. Verdict: in this strict weight management protocol, “light” drinks behave, on the scale, like water.

Baptized SWITCH trialsthe test coordinated by theUniversity of Liverpool and theUniversity of Leeds compared, in overweight and obese adults (BMI 27–35), at least two 330 ml cans of sugar-free sweetened drinks per day to the same amount of water, for 104 weeks. Published in the British Journal of Nutritionthe study shows an average weight loss of
–4.8 kg in the sweetened drinks group versus
–3.7 kg with water, a statistically insignificant difference after two years. This work comes at a time whenWorld Health Organization recommends against these sweeteners for weight control, and it finally brings long-term randomized data to this debate.

Sugar-free soft drinks: results close to water on weight

The 493 participants followed a behavioral weight management program: 12 weeks of active loss, then 40 weeks of assisted maintenance and a final year without formal support. At 104 weeks, those in the water group had lost an average of 3.7 kg, those in the sweetened drinks group 4.8 kg, with waist and hip circumferences reduced in comparable proportions. Analyzes of body composition by DXA absorptiometry, in a subgroup, found a reduction in fat mass in both cases, with no clear difference between water and “zero” sodas.

The researchers did not observe any sign of major appetite disruption: the self-reported feeling of hunger increased slightly in both groups, without a significant difference. Several metabolic biomarkers, such as fasting blood glucose or blood pressure, improved modestly, and small differences between groups did not cross thresholds considered clinically important. Total sugar consumption decreased in both arms of the trial, with a slightly greater decline among drinkers of sweetened drinks.

How the SWITCH trial was conducted, and what “water equivalent” means

Participants in the beverage group consumed soda, flavored water, or sugar-free fruit drinks containing
aspartame, acesulfame-K Or
sucraloseat doses well below the safety thresholds set by the European Food Safety Authority. By “equivalent”, the team means that no clinically relevant weight difference is detected between water and sweetened drinks in this specific context.

Joanne Harrold, professor at the University of Liverpool, said: “These latest findings show that concerns about sweeteners disrupting appetite or causing weight gain are not borne out when tested in a rigorous, long-term randomized trial. We hope this evidence will inform future WHO recommendations and public health policies“.

Jason Halford, professor at the University of Leeds and the University of Liverpool, adds: “This study provides the long-term clinical evidence that has been missing from international discussions. For people trying to manage their weight, drinks sweetened with non-nutritive sweeteners offer an effective alternative to sugary drinks, and they perform equivalently to water over two years“.

What to remember for daily life, and what limits for these sugar-free sodas?

The trial remains marked by several limitations: only 220 of the 493 volunteers were evaluated at 104 weeks, in a single center in England, without data on ethnic origin. Funding is provided by theAmerican Beverage Association
(the American trade association of soft drink manufacturers), although the authors point out that the research team was independent and retained control of the analysis and publication. The results concern only overweight or obese adults engaged in a structured lifestyle change program, and do not constitute a demonstration of absolute safety of sweeteners in all aspects of health.

This caution ties in with broader debates about sweeteners. Since 2023, the WHO has recommended against using sugar-free sweeteners as a strategy for long-term weight control or reducing the risk of non-communicable diseases. Its recommendation is conditional and does not target all individual uses, but it highlights the absence of clearly established lasting benefit on fat mass. To go deeper, several True Medical analyzes return to sweeteners and weight loss, as well as recent questions surrounding their impact.

For someone already looking to lose weight, this work suggests that opting for sugar-free soft drinks based on non-nutritive sweeteners, rather than sugary sodas, does not hinder weight loss or maintenance over two years when the overall diet is accompanied. Water remains the reference for health, but these data provide one more element to decide, with your doctor, if “zero” sodas can have a transitional place in a sugar reduction strategy.

In the long term, the WHO advises above all to gradually reduce the sweet taste in the diet. In other words, the real change is not just replacing sugar with a sweetener, but getting your palate used to seeking less sweet taste. Zero soda may therefore have a transitional place for some, but it does not necessarily become a slimming drink.

Diet soda or water to lose weight? An English study finally delivers its verdict after two years

Diet soda or water to lose weight? An English study finally delivers its verdict after two years
A two-year British study, the longest ever conducted on sugar-sweetened drinks, calls into question the effectiveness of diet sodas in weight management. The results show that these drinks have a similar impact as water on weight loss.

Many overweight people today replace their sugary sodas with sugar-free soft drinks, without really knowing if this choice helps them better manage their weight. A randomized clinical trial conducted in the United Kingdom, billed as the longest ever conducted on these products, followed 493 adults for two years to answer this question. Verdict: in this strict weight management protocol, “light” drinks behave, on the scale, like water.

Baptized SWITCH trialsthe test coordinated by theUniversity of Liverpool and theUniversity of Leeds compared, in overweight and obese adults (BMI 27–35), at least two 330 ml cans of sugar-free sweetened drinks per day to the same amount of water, for 104 weeks. Published in the British Journal of Nutritionthe study shows an average weight loss of
–4.8 kg in the sweetened drinks group versus
–3.7 kg with water, a statistically insignificant difference after two years. This work comes at a time whenWorld Health Organization recommends against these sweeteners for weight control, and it finally brings long-term randomized data to this debate.

Sugar-free soft drinks: results close to water on weight

The 493 participants followed a behavioral weight management program: 12 weeks of active loss, then 40 weeks of assisted maintenance and a final year without formal support. At 104 weeks, those in the water group had lost an average of 3.7 kg, those in the sweetened drinks group 4.8 kg, with waist and hip circumferences reduced in comparable proportions. Analyzes of body composition by DXA absorptiometry, in a subgroup, found a reduction in fat mass in both cases, with no clear difference between water and “zero” sodas.

The researchers did not observe any sign of major appetite disruption: the self-reported feeling of hunger increased slightly in both groups, without a significant difference. Several metabolic biomarkers, such as fasting blood glucose or blood pressure, improved modestly, and small differences between groups did not cross thresholds considered clinically important. Total sugar consumption decreased in both arms of the trial, with a slightly greater decline among drinkers of sweetened drinks.

How the SWITCH trial was conducted, and what “water equivalent” means

Participants in the beverage group consumed soda, flavored water, or sugar-free fruit drinks containing
aspartame, acesulfame-K Or
sucraloseat doses well below the safety thresholds set by the European Food Safety Authority. By “equivalent”, the team means that no clinically relevant weight difference is detected between water and sweetened drinks in this specific context.

Joanne Harrold, professor at the University of Liverpool, said: “These latest findings show that concerns about sweeteners disrupting appetite or causing weight gain are not borne out when tested in a rigorous, long-term randomized trial. We hope this evidence will inform future WHO recommendations and public health policies“.

Jason Halford, professor at the University of Leeds and the University of Liverpool, adds: “This study provides the long-term clinical evidence that has been missing from international discussions. For people trying to manage their weight, drinks sweetened with non-nutritive sweeteners offer an effective alternative to sugary drinks, and they perform equivalently to water over two years“.

What to remember for daily life, and what limits for these sugar-free sodas?

The trial remains marked by several limitations: only 220 of the 493 volunteers were evaluated at 104 weeks, in a single center in England, without data on ethnic origin. Funding is provided by theAmerican Beverage Association
(the American trade association of soft drink manufacturers), although the authors point out that the research team was independent and retained control of the analysis and publication. The results concern only overweight or obese adults engaged in a structured lifestyle change program, and do not constitute a demonstration of absolute safety of sweeteners in all aspects of health.

This caution is consistent with broader debates about sweeteners. Since 2023, the WHO has recommended against using sugar-free sweeteners as a strategy for long-term weight control or reducing the risk of non-communicable diseases. Its recommendation is conditional and does not target all individual uses, but it highlights the absence of clearly established lasting benefit on fat mass. To go deeper, several True Medical analyzes return to sweeteners and weight loss, as well as recent questions surrounding their impact.

For someone already looking to lose weight, this work suggests that opting for sugar-free soft drinks based on non-nutritive sweeteners, rather than sugary sodas, does not hinder weight loss or maintenance over two years when the overall diet is accompanied. Water remains the reference for health, but these data provide one more element to decide, with your doctor, if “zero” sodas can have a transitional place in a sugar reduction strategy.

In the long term, the WHO advises above all to gradually reduce the sweet taste in the diet. In other words, the real change is not just replacing sugar with a sweetener, but getting your palate used to seeking less sweet taste. Zero soda can therefore have a transitional place for some, but it does not necessarily become a slimming drink.