- A new study found that diet soda and water caused similar levels of weight loss.
- Diet soda can help reduce sugar intake, but doesn’t outperform water when it comes to hydration.
- Questions about the long-term safety of diet soda remain unanswered.
Diet drinks have long been a topic of debate in the nutrition world. Some say they’re helpful for cutting calories and added sugar, while others believe they have hidden drawbacks that may outweigh the weight loss benefits. Previous research has supported both sides of the argument: A 2025 study presented at a conference suggested that diet drinks were linked to a higher risk of liver cancer, while another found that diet drinks were better than sugary drinks for stomach cancer risk.
Complementing the literature, a new two-year large-scale randomized controlled trial published in the British Journal of Nutrition found that overweight or obese adults who drank diet soda daily lost 4.8 kg over two years, compared to 3.7 kg among those who drank water. They also saw improvements in waist circumference, blood pressure, cholesterol and other metabolic markers. Here’s what you need to know.
How was the study conducted?
Researchers at the University of Liverpool conducted the SWITCH trial (the effects of sugary non-nutritive drinks on appetite during active weight loss), recruiting 493 overweight or obese adults into a structured behavioral weight management program. Participants were randomized to drink water or commercially available diet sodas sweetened with aspartame, acesulfame-K, or sucralose each day.
The trial lasted 104 weeks, making it the longest randomized controlled trial of its type. The first year included active support through the weight management program. The second year took place without assistance, which allowed the researchers to observe whether the effects persisted without continued behavioral support. Researchers assessed participants on their weight, waist and hip circumferences, body composition, blood pressure, cholesterol levels, sugar intake and appetite.
What did the study reveal?
Both the water and diet soda groups lost weight over both years. However, the research team found that the difference between them was not statistically significant. Here’s how it breaks down:
- THE diet drink group lost 4.8 kg on average.
- THE water group lost 3.7 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 consumption decreased in both groups, with a slightly greater reduction in the diet soda group. Researchers suggest this could reflect the replacement of sugary drinks with sugary alternatives.
“These results show that concerns about sweeteners disrupting appetite or causing weight gain are not supported when tested in a rigorous, long-term randomized trial,” Professor Joanne Harrold of the University of Liverpool said in a statement. “We hope this data will inform future WHO public health guidance and policies.” »
Boundaries
The study received industry funding from the American Beverage Association. Although the researchers say this had no impact on their results, industry-funded trials in nutrition science have consistently produced results favorable to the funders’ products.
Beyond funding, all participants were enrolled in a structured behavioral weight management program, which limits the generalizability of the results to non-sustained daily consumption of diet drinks. The study also did not examine all the potential health effects of long-term use of artificial sweeteners, only the outcomes measured in the specific setting of the trial.
Additionally, the study only looked at weight loss and metabolic markers over two years. It was not designed to evaluate other health outcomes related to diet beverage consumption in observational research, such as effects on the gut microbiome, liver health, and cardiovascular risk over long periods of time.
How does this apply to real life?
“This study provides the long-term clinical evidence that has been missing in international discussions,” Professor Jason Halford of the University of Leeds said in a statement. “For people trying to manage their weight, non-nutritive sugary drinks offer an effective alternative to sugary drinks and perform equivalently to water over two years.”
That said, this trial should not be taken as a green light to replace your water intake with diet soda. Water remains the gold standard for hydration and presents none of the uncertainties that still surround the long-term use of artificial sweeteners. Other research has raised questions about potential links between diet drinks and liver health as well as associations with diabetes risk.
If you currently drink regular soda and are looking to reduce added sugar and calories, switching to a diet version may be helpful. But if you already drink mostly water, this study doesn’t give you reason to change course.
Our expert opinion
The longest randomized controlled trial of diet drinks to date found that people who drank diet soda daily for two years lost as much weight as those who drank water, with similar improvements in waist circumference, blood pressure and cholesterol. The results suggest that artificially sweetened drinks do not hinder weight loss efforts. However, its industry funding and narrow scope mean it does not resolve all questions regarding the safety of diet drinks. Ultimately, diet soda seems to be a better choice than sugary drinks, but water is still the best option for hydration and overall health.
Gn Health
