Trial shows diet drinks match water for weight loss

A two-year randomized trial led by the University of Liverpool found that diet beverages containing sugar substitutes were as effective as water for supporting weight loss in adults with overweight or obesity. The study, published in the British Journal of Nutrition, tracked 493 participants aged 18 to 65 with a body mass index between 27 and 35 over a 104-week period.

Participants were divided into two groups: half received 22.3 ounces of water daily, while the other half consumed the same volume of non-nutritive sweetener (NNS) beverages, equivalent to about two cans. The NNS group could choose from diet sodas, low-calorie fruit drinks, and flavored waters sweetened primarily with aspartame, acesulfame potassium, or sucralose. All participants followed a structured plan that combined dietary advice, behavioral coaching, and exercise guidance.

After two years, individuals drinking diet beverages lost an average of 10.6 pounds, compared to 8.2 pounds for those drinking water. Both groups maintained weight loss throughout the trial, including during the final year when regular weight-management support was withdrawn. Researchers observed that both groups reduced waist and hip circumference and body fat, with no meaningful difference between the beverage groups. Total cholesterol levels remained within healthy ranges for all participants.

Alix Turoff, a registered dietitian in New York City who was not involved in the study, clarified that the results do not indicate that diet beverages independently caused weight loss. She emphasized that the primary benefit lies in substitution rather than a metabolic effect. "If someone normally drinks 250 calories of regular soda and switches to zero-calorie soda, then they’re saving calories," Turoff said. She noted that the study specifically compared diet beverages to water to test whether sugar substitutes hinder weight-loss efforts, rather than comparing them to sugar-sweetened drinks.

Turoff added that the study did not find significant differences in hunger or sugar intake between groups at the trial's end. This addresses concerns that artificial sweeteners might increase appetite or lead to compensatory eating. While she recommended water as the primary hydration choice, she stated that people do not need to fear diet drinks preventing fat loss or causing weight gain.

The study has limitations. It was conducted at a single location in England, and race and ethnicity data were not collected, which restricts the generalizability of the findings. Additionally, only 220 of the 493 participants who began treatment completed the final assessment at week 104. Turoff noted that because the second-year extension was voluntary, those who continued may have been more motivated to adhere to the program. Although the American Beverage Association funded the trial, the authors stated that the association did not participate in the study design, data analysis, or interpretation of results.