You have probably been told that if you are trying to lose weight, water should be your default drink.
Diet soda, the thinking goes, might subtly trick the body or stall the scale.
For years that advice has been passed around with little solid evidence to back it up.
A new two-year trial published in the British Journal of Nutrition just put the assumption to the test.
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In 493 adults with overweight or obesity, people who drank diet sodas every day lost just as much weight as people who drank only water.
The study, known as the SWITCH trial, was run by researchers at the University of Liverpool and the University of Leeds.
It is one of the longest head-to-head comparisons of diet drinks and water inside a real weight-loss program, according to ScienceDaily.
What the trial actually tested
Participants were randomly assigned to drink either plain water or diet sodas sweetened with aspartame, acesulfame-K or sucralose, three common non-nutritive sweeteners.
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They had to drink at least two 330 ml servings a day of their assigned drink.
The trial ran for 104 weeks and included group coaching sessions in the first year. The final year of the study ran with no coaching at all. That stretch tested whether people could hold their new habits on their own.
What the two years showed
Over the full period, the water group lost about 3.7 kg, roughly 8 pounds.
The diet soda group lost 4.8 kg, roughly 10.5 pounds. The small gap was not statistically meaningful, meaning the two drinks performed about the same.
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Both groups also saw similar drops in waist and hip measurements.
Blood pressure and cholesterol moved in the same direction in both groups. Overall sugar intake fell for everyone, too.
No red flags turned up around appetite, hunger or weight regain over the full two years.
Professor Jo Harrold of the University of Liverpool led the trial and commented on the findings in a statement to News-Medical.
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"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," she said.
What the study did not measure
The trial measured weight, body composition and standard metabolic markers.
It did not look at the gut microbiome, long-term cancer risk or dental health, three areas where other research has raised separate questions about artificial sweeteners.
It is also worth being clear about who paid for it. The study was funded by the American Beverage Association, the trade group for US soft drink makers.
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The authors say the funder had no role in trial design, conduct or analysis, and the trial was pre-registered.
Independent replication by non-industry researchers would strengthen the picture.
For adults in a weight-loss program, swapping sugary sodas for diet versions worked about as well as swapping them for water over the trial's two years.
This article is made and published by Mie Hermansen, who may have used AI in the preparation.