Scientists Studied Water, Diet Soda and Weight Loss—Here’s What They Found

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A new study found diet soda and water led to similar levels of weight loss.

Diet sodas may help reduce sugar intake but don’t outperform water for hydration.

Questions about long-term diet soda safety remain unanswered.

Diet drinks have long been a topic of debate in the nutrition world. Some say they’re helpful for cutting back on calories and added sugar, while others believe they have hidden downsides that can offset any weight loss benefits. Past research has supported both sides of the argument—a 2025 study presented at conference suggested that diet drinks are connected to a higher risk of liver cancer, while another found that diet drinks were better than sugar-sweetened drinks for stomach cancer risk.

Adding to the literature, a new large-scale two-year randomized controlled trial published in the British Journal of Nutrition found that adults with overweight or obesity who drank diet soda daily lost 4.8 kg over two years, compared to 3.7 kg in those drinking water. They also found improvements in waist circumference, blood pressure, cholesterol and other metabolic markers. Here’s what you need to know.

Researchers at the University of Liverpool conducted the SWITCH trial (the effectS of non-nutritive sWeetened beverages on appetITe during aCtive weigHt loss), enrolling 493 adults with overweight or obesity in a structured behavioral weight management program. Participants were randomized to drink either water or commercially available diet sodas sweetened with aspartame, acesulfame-K or sucralose each day.

The trial ran for 104 weeks, which makes it the longest randomized controlled trial of its kind. The first year included active support through the weight management program. The second year was unassisted, allowing researchers to observe whether effects held up without ongoing behavioral support. Researchers assessed participants based on body weight, waist and hip circumference, body composition, blood pressure, cholesterol, sugar intake and appetite.

Both the water and diet soda groups lost weight over the two years. However, the research team found the difference between them was not statistically significant—here’s how it broke down:

The diet beverage group lost 4.8 kg on average.

Both groups experienced reductions in waist and hip circumference, improvements in body composition, and similar improvements in blood pressure and cholesterol. Additionally, both groups lost weight during the second year.

Sugar intake went down in both groups, with a slightly greater reduction in the diet soda group. The researchers suggest this may reflect the substitution of sweetened drinks for sugar-sweetened alternatives.

“These findings show that concerns about sweeteners disrupting appetite or causing weight gain are not supported when tested in a rigorous, long-term randomized trial,” said Professor Joanne Harrold of the University of Liverpool in a statement. “We hope this evidence informs future WHO guidance and public health policy.”

The study received industry funding from the American Beverage Association. While the researchers say this had no impact on their findings, industry-funded trials in nutrition science have a history of producing results favorable to the funder’s products.

Beyond funding, all participants were enrolled in a structured behavioral weight management program, which limits how well the findings generalize to everyday unsupported use of diet drinks. The study also didn’t examine all potential health effects of long-term artificial sweetener use, only the outcomes measured within the trial’s specific scope.

Additionally, the study only looked at weight loss and metabolic markers over two years. It wasn’t designed to assess other health outcomes that have been linked to diet drink consumption in observational research, such as effects on gut microbiome, liver health and cardiovascular risk over longer periods.

“This study provides the long-term clinical evidence that has been missing from international discussions,” said Professor Jason Halford of the University of Leeds in a statement. “For people trying to manage their weight, non-nutritive sweetened beverages offer an effective alternative to sugar-sweetened drinks—and perform equivalently to water over two years.”

That said, this trial shouldn’t be considered a green light to swap out your water intake for diet soda. Water remains the gold standard for hydration and comes without any of the uncertainty that still surrounds long-term artificial sweetener use. Other research has raised questions about potential links between diet drinks and liver health and associations with diabetes risk.

If you currently drink regular soda and are looking to cut added sugar and calories, switching to a diet version may be helpful. But if you’re already drinking mostly water, this study doesn’t give you a reason to change course.

The longest randomized controlled trial of diet beverages to date found that people who drank diet sodas daily for two years lost as much weight as those drinking water, with similar improvements in waist circumference, blood pressure and cholesterol. The results suggest that artificially sweetened drinks don’t hinder weight loss efforts. However, its industry funding and narrow outcome scope mean it doesn’t settle all questions about diet drink safety. Ultimately, diet sodas appear to be a better choice than sugar-sweetened beverages, but water is still the best option for hydration and overall health.