Diet Drinks or Water? A 2-Year Study of 493 People Found a Surprising Answer

It is the question millions of people ask with the fridge door open: should I grab a diet soda or just drink water? For two years, 493 adults trying to lose weight in England drank one or the other every day. At the end, a new study finds the difference between diet drinks and water was much smaller than almost anyone expected. Here is what the research really found, who paid for it, and a simple drink plan for your own kitchen..

The trial is called SWITCH, and it was led by researchers at the University of Liverpool, working with the University of Leeds. The results were published in 2026 in the British Journal of Nutrition, and the researchers describe it as the longest trial of its kind for diet soft drinks.

That matters because most of what we hear about sweeteners comes from studies that simply watch what people eat and drink. A randomized trial is different. People are assigned to a group by chance, so it is easier to compare the two drinks fairly.

The Study in Plain Words: 493 People, 2 Years, 2 Drinks a Day

As reported by ScienceDaily and News-Medical, the volunteers were adults aged 18 to 65 with a BMI between 27 and 35, living within 50 miles of Merseyside, England. Half were asked to drink water and half to drink diet drinks sweetened with sucralose, aspartame or acesulfame K. Everyone had at least two 330 ml servings a day, which is about two standard cans.

The trial ran for 104 weeks in three parts. First came 12 weeks of weight loss with weekly support from a nutritionist. Then came 40 weeks of maintenance with monthly meetings. In the final year, people were on their own, with no program support, until a last check-in at week 104.

The Results: Diet Drinks 4.8 kg vs Water 3.7 kg

Among the 220 people who finished the full two years, the diet drinks group lost an average of 4.8 kg, about 10.6 pounds. The water group lost 3.7 kg, about 8.2 pounds. That gap was not statistically significant, which means the study cannot say one drink worked better than the other. In plain words, diet drinks and water performed about the same.

Both groups also kept most of their weight off through the final year without any help from the program, and waist and hip measurements went down in both. Lead researcher Professor Jo Harrold said the results show that concerns about sweeteners “disrupting appetite or causing weight gain are not supported” by this trial.

What Else Changed With Diet Drinks: Sugar, Hunger and Blood Pressure

Sugar intake fell in both groups, a little more in the diet drinks group. Hunger scores went up in both groups, which is common when people cut calories, and there was no real difference between the two drinks.

One small result went the other way. Diastolic blood pressure, the bottom number, dropped a little more in the water group. The researchers said that difference was too small to matter for health, but it is a good reminder that plain water has no downside.

The Fine Print: Who Paid for It and What the WHO Says

Here is the part you should know. The study was funded by the American Beverage Association, the trade group for soft drink makers. The researchers say the funder had no role in running or analyzing the study and that the team kept full control over the results. Still, it is fair to keep the funding in mind when you read the headlines.

The study also had limits. Fewer than half of the people who started finished all two years, COVID-19 restrictions disrupted some check-ins, and it was run at a single center in England.

And the bigger picture has not changed. In 2023 the World Health Organization advised against using non-sugar sweeteners to control body weight. Its review found no long-term benefit for reducing body fat in adults or children, and it suggested people reduce the sweetness of their diet overall, with foods like fruit and unsweetened drinks. The WHO called its advice conditional, and it does not apply to people who already have diabetes.

A Simple Drink Plan: Water First, Smart Swaps Second

So what should you actually pour? Based on this study and the WHO advice, here is a calm, no-guilt plan:

• Make water your everyday drink. It is free, it has no calories, and every health group agrees on it.
• If you drink regular soda, diet drinks may help as a step down. This study finds they performed about as well as water during a weight loss program.
• Do not use diet drinks as a magic fix. The people in this trial also had nutrition support and changed how they ate.
• Make water more fun. Bubbles, fruit and herbs make it easier to choose water over soda.
• Check with your doctor if you have diabetes, high blood pressure or other health conditions.

5 Flavored Waters and Sparkling Sips We Make at Home

These take two minutes and cost pennies a glass. Fill a large pitcher with cold still or sparkling water and add:

• Cucumber mint: half a cucumber thinly sliced and a handful of fresh mint. Cool and spa-like.
• Strawberry basil: 6 sliced strawberries and 4 basil leaves, lightly crushed. Pretty and a little sweet.
• Lemon ginger: 1 sliced lemon and a thumb of fresh ginger, sliced thin. Bright with a gentle kick.
• Orange cinnamon: 1 sliced orange and 1 cinnamon stick. Cozy for fall.
• Lime and berry fizz: sparkling water over ice with a squeeze of lime and a few frozen berries. It feels like a party drink, with no added sugar.

Let the fruit sit in the fridge for 30 minutes to 2 hours for the best flavor, and refill the pitcher with water once or twice before you swap in fresh fruit. For more ideas, try our infused water and healthy drink ideas that make drinking more water easy, or brew a pitcher from our hot and iced tea recipes.

The Bottom Line on Diet Drinks

This two-year trial finds that diet drinks and water performed about the same for people in a weight loss program, and that sweeteners did not seem to raise appetite. But the study was funded by the soft drink industry, many people dropped out, and the WHO still advises against relying on sweeteners for weight control. Water stays the winner for everyday drinking, and diet drinks may help as a stepping stone if you are cutting back on sugary soda.

This article is for general information and is not medical advice. So tell us: are you team diet soda, team sparkling water, or team plain old tap water?