Diet Drinks Match Water for Long-Term Weight Loss in Landmark Two-Year Trial

Key Takeaways: 

• The SWITCH trial, described by its authors as the longest and most comprehensive randomised controlled trial of non-nutritive sweetened (NNS) soft drinks to date, found that diet drinks performed similarly to water for weight loss and weight maintenance over 104 weeks.
  
• Among 493 adults living with overweight or obesity, weight loss was comparable between groups (4.8 kg in the NNS beverage group vs 3.7 kg in the water group), with no clinically meaningful differences in blood pressure, cholesterol or other metabolic measures.
  
• The findings challenge concerns that sweeteners disrupt appetite or cause long-term metabolic harm and may help inform future World Health Organization (WHO) guidance on non-sugar sweeteners.
  
  
  

A long-term look at diet drinks and weight management

Researchers at the University of Liverpool have completed what they describe as the longest and most comprehensive randomised controlled trial to date examining non-nutritive sweetened (NNS) soft drinks.

Non-nutritive sweeteners are low-calorie or calorie-free sugar substitutes commonly used in foods and beverages. The 104-week study found that NNS beverages, including diet soft drinks, performed similarly to water when it came to long-term weight management.

The peer-reviewed research was led by Professor Jo Harrold, Dean of Psychology at the University of Liverpool, alongside Professor Jason Halford, Professor of Biological Psychology & Health Behaviours at the University of Leeds (formerly of the University of Liverpool). The study was funded by the American Beverage Association. According to the researchers, the study team operated independently from the funder and retained full control over the analysis and reporting of the results.

Diet drinks compared with water over two years

The findings, published in the British Journal of Nutrition, provide long-term clinical evidence on beverages containing three commonly used sweeteners: aspartame, acesulfame potassium (acesulfame-K) and sucralose. Researchers found that these drinks were comparable to water in supporting both weight loss and long-term weight maintenance.

The SWITCH trial included 493 adults living with overweight or obesity who took part in a structured behavioural weight management programme. Participants were randomly assigned to drink either water or commercially available diet beverages each day. The amounts of sweeteners consumed remained well within established European Food Safety Authority (EFSA) safety thresholds.

Overall, the results did not support concerns that NNS beverages interfere with appetite regulation or cause long-term metabolic harm.

Weight loss and metabolic health outcomes

The trial reported several key findings across the two-year period:

• Comparable weight loss: after 104 weeks, weight loss was similar between the NNS beverage group and the water group (−4.8 kg vs −3.7 kg), with the difference not statistically significant.
  
• Sustained results: both groups maintained their weight reductions across the full two-year study, including during the final year, when participants no longer received assistance.
  
• Body measurements and composition: participants in both groups experienced reductions in waist and hip circumference, along with improvements in body composition and several metabolic biomarkers.
  
• No meaningful metabolic differences: researchers found no clinically meaningful differences in measures such as blood pressure and cholesterol linked to long-term consumption of aspartame, acesulfame-K or sucralose compared with water.
  
• Reduced sugar intake: sugar intake fell in both groups, with a slightly larger reduction among participants drinking NNS beverages.
  

For healthcare professionals supporting people with weight management, these findings reinforce the value of practical, evidence-based dietary swaps within a structured programme. Clinicians looking to strengthen their confidence in this area may find The College of Contemporary Health’s Nutrition & Weight Management Essentials CPD course a useful way to build their knowledge of dietary approaches to weight management.

Findings add to the sweetener debate

The results are relevant to recent WHO guidance advising against the use of non-sugar sweeteners for weight control, which was issued because of uncertainty in observational research. The SWITCH trial adds long-term randomised controlled evidence that may help inform future global recommendations.

Professor Joanne Harrold said: “The University of Liverpool is one of the UK’s leading research-intensive higher education institutions, with a key focus on public health. These latest findings show that concerns about sweeteners disrupting appetite or causing weight gain are not supported when tested in a rigorous, long-term randomized trial. We hope this evidence informs future WHO guidance and public health policy.”

How the SWITCH trial was designed

The effectS of non-nutritive sWeetened beverages on appetITe during aCtive weigHt loss (SWITCH) trial was developed to address two related questions: how reducing dietary sugar might affect weight management, and whether replacing sugar with non-nutritive sweeteners changes appetite regulation. The newly published results build on earlier findings reported in the International Journal of Obesity in 2023.

Professor Jason Halford, of the University of Leeds and the University of Liverpool, concluded: “This study provides the long-term clinical evidence that has been missing from international discussions. 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.”

The paper, ‘Effect of non-nutritive sweetened beverages versus water on body weight: long-term results of a randomised controlled trial’, was published in the British Journal of Nutrition.

CCH insight

Supporting people to make sustainable dietary changes is central to effective weight management. Our Nutrition & Weight Management Essentials CPD course helps healthcare professionals apply current evidence on nutrition and weight management in everyday practice.

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Source: British Journal of Nutrition