Diet drinks work as well as water for weight loss, trial finds

People trying to lose weight do just as well if they drink diet drinks as if they drink water, according to a two-year trial led by the University of Liverpool.

The findings, published in the British Journal of Nutrition, challenge concerns that artificial sweeteners might increase appetite or cause weight gain.

The trial

The SWITCH trial followed 493 adults living with overweight or obesity for 104 weeks. Participants were randomly assigned to drink either diet drinks containing sweeteners or water, alongside a weight management programme.

The diet drinks group lost an average of 4.8 kilograms, while the water group lost 3.7 kilograms. The difference between the two was not statistically significant, meaning it could have happened by chance.

Both groups kept the weight off throughout the study, including during the final year, when they were no longer receiving support.

Other benefits

Both groups saw reductions in waist and hip measurements, improvements in body composition and better results on measures of metabolic health. There were no clinically meaningful differences in blood pressure or cholesterol between the two groups.

What the researchers said

The trial was led by Professor Jo Harrold of the University of Liverpool and Professor Jason Halford, now at the University of Leeds.

"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," the researchers said.

Who paid for it

The trial was funded by the American Beverage Association, which represents drinks manufacturers. The researchers said they "operated independently from the funder and retained full control over the analysis and reporting".

A contrast with WHO advice

The results contrast with guidance from the World Health Organization, which in 2023 advised against using non-sugar sweeteners to control weight, saying there was no evidence of long-term benefit. The WHO's advice was based on a review of many studies, most of which were observational and could not prove cause and effect.

The trial looked only at drinks sweetened with aspartame, acesulfame K and sucralose, so the results may not apply to other sweeteners.

What it means for you

Health experts say water remains the best drink for most people, but that swapping sugary drinks for diet versions can help cut calories. A standard can of a full-sugar fizzy drink can contain around 35 grams of sugar, which is more than the maximum amount of free sugars adults are advised to have in a whole day.

The sugar tax

The UK's Soft Drinks Industry Levy, introduced in 2018, has led manufacturers to cut the sugar in many drinks. The Government has announced plans to extend the levy to milk-based drinks and to lower the sugar threshold at which it applies.

Obesity in the UK

Around two in three adults in England are living with overweight or obesity. Weight loss medicines have raised hopes in recent years, but experts say diet, activity and the food environment remain central. In Birmingham, the NHS is introducing a new digital weight management service, BRIDGE, for some groups of patients.

Are sweeteners safe?

Sweeteners used in the UK are tested and approved by food safety regulators, who set an acceptable daily intake for each one. Aspartame was classed by an agency of the World Health Organization in 2023 as possibly causing cancer, but the same organisation's food safety experts said the evidence was limited and that the acceptable daily intake remained safe. People with the rare genetic condition phenylketonuria must avoid aspartame.