Researchers at the University of Liverpool have finished the longest and most thorough trial of its kind on diet drinks. The study lasted two years. It found that drinks made with low-calorie sweeteners helped people manage their weight just as well as plain water.
Non-nutritive sweeteners are used in place of sugar. They give food and drinks a sweet taste but add very few calories. Diet soft drinks are a common example.
How the Study Worked
The trial is called SWITCH. It involved 493 adults who were living with overweight or obesity. All of them took part in a structured programme designed to help them lose weight. Each person was randomly placed in one of two groups. One group drank water every day. The other group drank ready-made diet drinks every day. These drinks contained aspartame, acesulfame K, or sucralose. The amounts used stayed well below the safety limits set by the European Food Safety Authority.
The work was led by Professor Jo Harrold, Dean of Psychology at the University of Liverpool, together with Professor Jason Halford, who now works at the University of Leeds but was previously at Liverpool. The American Beverage Association paid for the research. The team says it was independent of the funder and had full control over how the data were analysed and reported. The results were peer-reviewed and published in the British Journal of Nutrition, which included the latest findings that add to the existing research.
After 104 weeks, both groups lost weight. The diet drink group lost 4.8 kg on average, while the water group lost 3.7 kg. The gap between the two was not statistically significant, so the two drinks performed equally well.
Other findings were also positive. Both groups kept their weight down across the two years, including the last year, when they received no support. Waist and hip sizes fell in both groups. Body composition improved in both groups, as did several health markers in the blood. Blood pressure and cholesterol showed no meaningful difference between the groups. Sugar intake dropped in both groups, and the fall was slightly larger among diet drink users.
What the Results Mean
The team concluded that diet drinks do not seem to upset appetite control. They also found no sign of long-term harm to the body’s metabolism.
The World Health Organization recently advised people not to use sugar substitutes to control their weight. That advice leaned on observational studies, which cannot prove cause and effect. The authors say a long randomised trial provides stronger evidence. They hope it will shape future global guidelines and influence how health professionals advise people about long-term weight management.
Professor Harrold said the findings show that worries about sweeteners affecting appetite or causing weight gain were not supported in a strict, long-term trial. She added that she hopes the evidence will guide WHO advice and public health policy.
Professor Halford said the study fills a gap in the evidence that international debates have lacked. He said diet drinks are a useful swap for sugary drinks for people who want to manage their weight. Over the course of two years, he noted that the participants who consumed diet drinks performed as well as those who drank water.
The full name of the trial is the Effects of Non-Nutritive Sweetened Beverages on Appetite During Active Weight Loss. It was designed to explore whether replacing sugary drinks with non-nutritive sweetened beverages could help people manage their appetite during weight loss. In particular, researchers examined whether sweeteners influence hunger and appetite when used as a sugar substitute.
The paper builds on earlier findings published in the International Journal of Obesity in 2023. The latest findings were published in the British Journal of Nutrition, adding further evidence to the ongoing debate about sweeteners, appetite, and long-term weight management.
