Diet soda matches water for weight loss in two-year study, researchers find

A two-year clinical trial from the University of Liverpool has found that diet sodas sweetened with aspartame, sucralose, and acesulfame potassium perform essentially as well as plain water when it comes to losing weight. The study enrolled 493 adults between 18 and 65 years old who were overweight or obese, with body mass indexes ranging from 27 to 35. Half received 22.3 ounces of water daily for 104 weeks; the other half received the same volume of diet beverages—which included diet sodas, low-calorie fruit drinks, and flavored waters—drawn from the same sweetener family.

After two years, those assigned to diet drinks had shed approximately 10.6 pounds, while the water group lost about 8.2 pounds. Both groups also reduced their waist and hip measurements and lowered their body fat percentages. Total cholesterol stayed within healthy ranges for everyone. The difference between the two groups was not statistically meaningful, meaning the researchers found no evidence that the artificial sweeteners themselves hindered weight loss.

But here is the critical caveat: both groups were enrolled in a structured weight-management program that included nutrition counseling, behavioral coaching, and physical activity guidance. Alix Turoff, a registered dietitian in New York City who reviewed the findings, emphasized that the study does not demonstrate that diet beverages caused the weight loss. Rather, the program itself—the counseling, the support, the exercise recommendations—appears to have driven the results. The real value of diet soda, Turoff explained, lies in simple substitution: if someone normally drinks 250 calories of regular soda and switches to a zero-calorie alternative, they save those calories. The beverages have no magical metabolic effect.

The study also found no meaningful difference in hunger levels or sugar intake between the two groups at the trial's end, which matters because a common concern about artificial sweeteners is that they might increase appetite or trigger compensatory eating. Turoff noted that these findings do not mean water and diet soda are nutritionally equivalent, but they do suggest that people who prefer diet drinks need not fear they will automatically sabotage weight loss efforts.

The research does carry significant limitations. It was conducted at a single location in England, and the researchers did not collect data on race and ethnicity, which restricts how broadly the findings can be applied to other populations. Of the 493 participants who began the trial, only 220 completed the full 104-week assessment. The second year was voluntary, meaning those who stayed may have been more motivated or disciplined than those who dropped out—a pattern that could skew the results. The American Beverage Association funded the trial, though the study authors stated the association played no role in study design, data analysis, or interpretation.

The findings were published in the British Journal of Nutrition. Turoff's bottom line: water remains the primary hydration choice, but the evidence from this trial does not support the idea that diet beverages are inherently harmful to weight loss goals.