Nutrition Research Sweeteners
Diet Soda or Water for Weight Loss? A Two-Year Trial Found People Lost About the Same — Here Is What That Does and Does Not Prove
In a randomized study of 493 adults in a weight program, diet-beverage drinkers and water drinkers ended two years with similar weight loss. The findings challenge World Health Organization advice, but the study has real limits.
By The Nutrition DeskFact-checked against the published trial papers, university releases and CDC nutrition dataUpdated November 13, 2026Scope Adults with overweight or obesity trying to lose weight
If you are trying to lose weight, the advice about diet soda can feel like a coin toss. Some experts say swap sugary drinks for zero-calorie versions. Others warn that artificial sweeteners keep cravings alive or confuse your metabolism, and in 2023 the World Health Organization advised against using non-sugar sweeteners for weight control. Most of the evidence behind the warnings came from observational studies, which cannot show cause and effect.
A new two-year randomized trial offers a cleaner test. Researchers at the University of Liverpool assigned 493 adults with overweight or obesity to drink either water or diet beverages daily while following a weight loss program. After 104 weeks, the diet-drink group had lost about 4.8 kilograms, or 10.6 pounds, and the water group about 3.7 kilograms, or 8.2 pounds. The difference was not statistically significant. The results were published in the British Journal of Nutrition this fall.
What We Know
- Both groups lost weight and kept part of it off for two years.
- The two-year difference between diet drinks and water was not statistically significant.
- Waist and hip measurements fell in both groups.
- The trial was funded by the American Beverage Association; authors say they controlled the analysis.
What Is Still Unclear
- Whether results apply to people outside a supervised program or outside England.
- Effects of individual sweeteners, since participants drank mixed products.
- Results for people who rarely drank diet beverages before; 76% already did.
- Very long-term health effects beyond two years.
How the SWITCH Trial Worked
The study’s name stands for the effects of non-nutritive sweetened beverages on appetite during active weight loss. Adults aged 18 to 65 with a body mass index of 27 to 35 who lived near Merseyside, England, were randomly assigned to drink water or commercially available diet beverages. People in the diet-drink group had at least two 330-milliliter servings a day, about 660 milliliters or 22 ounces. Sweeteners included aspartame, acesulfame potassium and sucralose, and intake stayed within European safety limits.
The average participant was about 45 years old, and roughly 70% were women. Their average starting weight was about 90 kilograms, or 198 pounds, with an average BMI of 31.3, according to the 2023 paper on the trial’s first year. Everyone followed the same behavioral weight program in three phases.
- Weeks 1 to 12Weekly group sessions; most weight lost here.
- Weeks 13 to 52Monthly sessions for assisted maintenance.
- Weeks 53 to 104No routine support; voluntary follow-up phase.
Phase widths are proportional to their length in weeks. Source: SWITCH trial design as reported by The Brighter Side of News and Earth.com.
The Results, Year by Year
Most weight came off in the first three months, when the two groups were nearly identical: water drinkers lost 5.6 kilograms and diet-drink users 5.8 kilograms at 12 weeks, according to the trial’s earlier report. At one year, diet-drink users who completed the program had lost 7.5 kilograms versus 6.1 kilograms for water drinkers. That 1.4-kilogram gap was statistically significant in the main analysis, but the authors judged it below the 1.5-kilogram threshold they considered clinically meaningful, and two other statistical methods found no significant difference.
Average Weight Loss Over Two Years
Week 12
Week 52
Week 104
In the final, unsupported year, both groups regained some weight but stayed well below where they started. By week 104, the difference had narrowed to about 1.1 kilograms and was no longer statistically significant. “Concerns about sweeteners disrupting appetite or causing weight gain are not supported when tested in a rigorous, long-term randomised trial,” said trial lead Professor Jo Harrold of the University of Liverpool.
Body Shape and Health Markers
Waist and hip measurements fell in both groups. In a smaller subset that had body composition scans at one year, fat mass dropped by 5.1 kilograms with water and 6.1 kilograms with diet drinks, with no significant difference. Over two years, total cholesterol rose slightly more with diet drinks, while HbA1c, a long-term blood sugar measure, and diastolic blood pressure fell slightly more with water. The researchers judged none of those differences clinically meaningful.
One-Year Changes in Body Measurements and Blood Pressure
Waist, cm
Hip, cm
Systolic blood pressure, mmHg
Fat mass, kg
Did Sweeteners Make People Hungrier?
One of the most common fears about diet drinks is that sweet taste without calories drives hunger and sugar cravings. In SWITCH, self-reported hunger rose slightly in both groups over two years. The increase was statistically significant only in the diet-beverage group, but the difference between groups was not. Sugar intake fell in both groups, slightly more among diet-drink users. Total sweetener intake fell only in the water group, which is expected since they were asked to avoid diet drinks.
Read the Fine Print Before You Stock Up
Every study has limits, and this one has several worth weighing. The American Beverage Association, which represents soft drink makers, funded the research. The authors said the funder did not control the analysis or reporting, though it could review the manuscript, and several authors report ties to food or beverage industry groups, Earth.com reported. Independent replication would strengthen the conclusions.
Who Stayed in the Study
Dropout was high: more than half of participants did not complete two years, and people who stayed may differ from those who left. The study took place at a single site in England, did not record race or ethnicity, and was disrupted by COVID-19, so some measurements were self-reported at home. The final year was added by a protocol amendment. And because participants drank a mix of products, the trial cannot say whether aspartame, sucralose or acesulfame K differ from each other.
Why Habit May Have Mattered
One detail shapes how to read the results: about 76% of participants already drank diet beverages regularly before the trial. For them, being randomized to water meant giving up a familiar habit, while those assigned to diet drinks simply kept going. That could make the diet-drink arm easier to stick with, which may partly explain why that group did slightly better in the first year. It also means the findings say less about people who have never used sweeteners. The 2023 report noted that only 24.1% of participants were new to non-nutritive sweeteners, too small a group to analyze separately with confidence.
Where This Leaves the WHO Guidance
In 2023, the World Health Organization advised against non-sugar sweeteners for weight control, based largely on observational studies. “Replacing free sugars with NSS does not help with weight control in the long term,” WHO’s Francesco Branca said at the time. The SWITCH authors argue their randomized trial fills a gap. “We hope this evidence informs future WHO guidance and public health policy,” Harrold said. Co-author Professor Jason Halford, now at the University of Leeds, said the study “provides the long-term clinical evidence that has been missing.”
What It Means for Your Drink Choices
For Americans, the bigger issue is often sugary drinks. CDC data show about half of U.S. adults, 49.3%, drink at least one sugar-sweetened beverage on a given day, averaging 145 calories a day, or 6.5% of total calories. Younger men drink the most.
Calories From Sugary Drinks Among U.S. Adults
Who Should Think Twice
The trial included healthy adults, not children, pregnant people or those with specific medical conditions. People with the rare inherited disorder phenylketonuria must avoid aspartame, which is why products containing it carry a warning label. People with diabetes can generally use diet drinks to cut sugar but should discuss their overall plan with a clinician. And if you notice that sweet drinks of any kind send you looking for snacks, that personal pattern matters more than any average from a study. Parents should note that U.S. dietary guidance discourages added sweeteners for young children, and plain water and milk remain the standard drinks for kids.
| Common claim | What SWITCH found | Evidence strength | Key caveat | Practical takeaway |
|---|---|---|---|---|
| Diet drinks block weight loss | 4.8 kg vs 3.7 kg lost at 2 years, no significant gap | Randomized trial | Industry funded; 55% dropout | Diet drinks did not hinder loss |
| Diet drinks make you hungrier | Hunger rose slightly in both; no significant gap | Moderate | Self-reported scales | Watch your own cravings |
| Sweeteners raise sugar cravings | Sugar intake fell in both, slightly more with diet drinks | Moderate | Food questionnaires | No sign of rebound sugar eating |
| Water is better for blood sugar | HbA1c fell slightly more with water, not clinically meaningful | Moderate | Few people had diabetes | Water remains a solid default |
| Sweeteners are unsafe long term | Not tested beyond 2 years | Not addressed | Trial not designed for this | Follow regulatory limits |
| Results apply to everyone | Adults with BMI 27 to 35 in England | Limited | Single site; no children | Talk to your doctor if unsure |
- Replace sugary drinks firstThe biggest calorie win comes from cutting regular soda, sweet tea and juice drinks.
- Choose what you will stick withIf diet drinks help you avoid sugary ones, SWITCH suggests they will not undercut your weight loss.
- Make water the defaultIt is free, has no sweeteners and was just as effective in the trial.
- Notice your appetiteSome people find sweet tastes trigger snacking; adjust if that is you.
- Pair drinks with a planBoth groups lost weight inside a structured program with regular support.
- 2-year loss, diet drinks4.8 kg or 10.6 lb
- 2-year loss, water3.7 kg or 8.2 lb
- Most lossFirst 12 weeks
- Diet drink doseAt least 660 mL a day
- U.S. sugary drinks145 kcal a day average
- FundingAmerican Beverage Association
Frequently Asked Questions
Is diet soda as good as water for weight loss?
In the two-year SWITCH trial, people who drank diet beverages lost about as much weight as water drinkers, 4.8 kg versus 3.7 kg, a difference that was not statistically significant.
Do artificial sweeteners cause weight gain?
This randomized trial found no evidence that diet beverages caused weight gain or blocked weight loss during a structured program. Some observational studies have suggested links, but they cannot prove cause.
Do diet drinks increase hunger?
Hunger rose slightly in both groups in the SWITCH trial, with no significant difference between diet drinks and water.
Who funded the diet drink and water study?
The American Beverage Association. The researchers say the funder did not control the analysis or reporting, though it could review the manuscript.
Which sweeteners were in the study drinks?
Mainly aspartame, acesulfame potassium and sucralose, in commercially available beverages, within European safety limits.
Why does the WHO advise against sweeteners for weight loss?
In 2023, the WHO said non-sugar sweeteners do not help with long-term weight control, based largely on observational studies. The SWITCH authors hope their trial informs future guidance.
How much diet soda did people drink in the study?
At least two 330 mL servings a day, or about 22 ounces.
Should I switch from diet soda to water?
Water is a healthy default, but this study suggests diet drinks will not undermine weight loss for most adults. The most important step is replacing sugary drinks.
The Short Version
A two-year randomized trial of 493 adults in a weight program found diet-beverage drinkers lost about 4.8 kg and water drinkers about 3.7 kg, a difference that was not statistically significant. Waist size, body fat and most health markers improved in both groups, and hunger did not differ meaningfully. The study was funded by the beverage industry, more than half of participants dropped out, and it ran at one site in England. The practical takeaway: replacing sugary drinks matters most, and water or a diet drink can both help.
Omisha is a health writer passionate about turning complex medical research into clear, actionable content readers can trust. She covers everything from nutrition and mental wellness to chronic disease management, always grounding her work in credible science and real-world relevance. When she's not writing, she's usually reading up on the latest health studies or exploring new wellness trends to write about next.
