Swap your daily water for diet soda and you will stall your weight loss, right? A two-year randomized trial of 493 adults says that fear did not show up. People who drank diet drinks in a structured weight-loss program lost 4.8 kg on average, versus 3.7 kg for the water group, and the researchers reported that gap was not statistically significant.
The results, published in the British Journal of Nutrition and widely shared this week, land in the middle of a long-running argument about sweeteners. There is a big catch, though: the trial was paid for by the beverage industry. Here is what the study did, what it did not do, and how to read it.
Quick note: this article is general information, not medical advice. Talk to your doctor before changing your diet or weight-loss plan.
What the Diet Drinks Trial Found
The SWITCH trial (short for the effects of non-nutritive sweetened beverages on appetite during active weight loss) was led by Professor Jo Harrold of the University of Liverpool and Professor Jason Halford of the University of Leeds. It ran for 104 weeks, according to the ScienceDaily write-up of the paper.
The 493 adults all had overweight or obesity (a BMI of 27 to 35, according to the paper’s abstract). Everyone joined a behavioral weight-management program. The only planned difference was the drink: half were assigned to water, half to commercially available diet beverages sweetened with aspartame, acesulfame-K or sucralose.
At the two-year mark, the diet-drink group had lost 4.8 kg and the water group 3.7 kg. The authors wrote that “the between-group difference was not significant.” Both groups also improved waist and hip measurements, body composition and some metabolic markers, and there were no clinically meaningful differences in blood pressure or cholesterol.
How the Two-Year Study Was Built
The trial ran in three phases, as laid out in the published paper:
- Weeks 1-12: active weight loss
- Weeks 13-52: assisted maintenance
- Weeks 53-104: a final year with no program support
That last year matters. Plenty of weight-loss studies end when the support ends. Here, both groups held on to their losses even without the program, which is the part many dieters struggle with most.
The participants were split almost evenly: 246 to water and 247 to diet drinks. But only 220 people completed the full 104 weeks (117 in the water group, 103 in the diet-drink group). That is fewer than half the starting group, and it is a normal but important limit in long behavioral trials.
Do Diet Drinks Make You Hungrier? What Researchers Say
One big worry about sweeteners is that intense sweetness without calories might push appetite up or drive people toward sugary food later. This trial was designed to test that idea directly.
The university’s summary says sugar intake fell in both groups, with a slightly larger drop among people drinking diet beverages. That fits a simple explanation: if one of the diet drinks replaces a sugary one in someone’s day, sugar intake goes down.
In a statement reported by News-Medical, the team said: “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.”
That is a strong claim, and it comes from the researchers themselves. Read it as a conclusion about this group of adults in this program, not a verdict on every person who drinks diet soda.
The Industry Funding Question
The American Beverage Association funded the trial. The authors say the funder had no role in the design, conduct or interpretation of the study, and that the researchers kept independent control over analysis and reporting. The paper also notes that several authors have disclosed industry ties beyond this project.
Does that mean the results are wrong? No. A randomized design is a real safeguard, and the numbers are laid out in the paper for anyone to check. But funding source is a standard reason to want other teams to repeat the work. Independent trials with similar results would make the finding much more convincing.
It also helps to keep the question narrow. This study asked whether diet drinks and water were equivalent for weight change. It did not set out to test long-term disease outcomes.
How This Fits With the WHO Guidance on Sweeteners
In 2023, the World Health Organization advised against using non-sugar sweeteners to control body weight. The WHO said its review found no long-term benefit for reducing body fat in adults or children, and flagged possible links to type 2 diabetes, heart disease and death in long-term observational data.
Two details are easy to miss. First, the WHO called its recommendation conditional, partly because of possible confounding in the studies it relied on. Second, observational links show association, not proof that sweeteners caused the outcomes. People who already have weight or blood sugar concerns may be more likely to choose diet products in the first place.
WHO’s Francesco Branca said in the agency’s announcement: “NSS are not essential dietary factors and have no nutritional value. People should reduce the sweetness of the diet altogether, starting early in life, to improve their health.”
Notice that this is not in direct conflict with SWITCH. The trial says diet drinks did not hurt weight loss compared with water. The WHO says they are not a nutritional tool and that cutting back on sweetness overall is the better goal. Both can be true at once.
What This Means for Diet Drinks in Your Routine
Here is a practical way to read the results:
- Water still wins on simplicity. In this trial, water performed just as well as diet drinks. It costs less and has no sweeteners to debate.
- Diet drinks may be a reasonable bridge. If you are moving away from regular soda, switching to a diet version is not likely to sabotage your effort, based on this evidence.
- The program did the heavy lifting. Both groups were in a structured behavioral program. The drink was one small piece.
- It is one trial. It involved adults with a BMI of 27 to 35, and it does not cover children, people with diabetes or other groups.
The same logic shows up in other recent coverage. Our report on how sugar may deepen gut damage after antibiotics shows why cutting added sugar is worth the effort, and our look at a type 2 diabetes remission trial is another example of structured support doing the heavy lifting.
What We Still Do Not Know
The study cannot tell us whether years of daily diet soda affect the gut microbiome, taste preferences, heart health or cancer risk. It was not built to answer those questions.
It also cannot say what happens in people who are not in a weight-management program, where habits are looser and drinks are only one part of the day. Dropout is another unknown: we do not know whether the 273 people who left the study were different from the 220 who stayed in ways that could affect the result.
For now, the honest summary is modest: in this trial, diet drinks did not hold weight loss back compared with water. Whether diet drinks are good for you in other ways is a separate question that this study leaves open, and it is one researchers will keep testing. Until more independent trials arrive, treat diet drinks as an option, not a health food.
Your Takeaway
If you drink diet soda and are trying to lose weight, this study gives you a little less to worry about on the weight front. If you drink water, there is no need to switch. The bigger levers, whether you choose diet drinks or water, are still the usual ones: the overall pattern of what you eat, how much you move, and having support that lasts.
Not sure what is right for your body? Ask your doctor or a registered dietitian, especially if you have diabetes, are pregnant, or take medication. And keep an eye on our look at the iterative mindset for goals and setbacks for habit ideas that pair well with any plan. Follow USA One News for more plain-English takes on the latest health research.
