Diet Soda Matches Water for Weight Loss, but a High-Profile Study Has a Sponsor Catch

The belief that dieters should drink only plain water has been called into question. The British Journal of Nutrition published the final results of the marathon two-year SWITCH experiment. Researchers concluded that drinks with artificial sweeteners maintain lost weight just as effectively as plain water. However, the source of the funding behind the tests makes it hard to trust these numbers unconditionally.

The clinical trial began with 493 overweight adults. They were divided into two groups: one was instructed to drink at least two 330 ml servings of water daily, while the other drank the same volume of zero-calorie diet beverages. The experiment was split into three phases: 12 weeks of active weight loss, 40 weeks of supervised maintenance, and a full year of completely independent control.

Weight Loss and a Mass Dropout of Participants

After 104 weeks, patients in the “diet” group had maintained an average weight loss of 4.8 kg from their initial body weight. The water-only group showed a result of minus 3.7 kg. The one-kilogram difference was statistically insignificant, so the researchers deemed both approaches equivalent for long-term dieting.

Interim results from the same trial, taken at the one-year mark, showed a slightly larger gap: minus 7.5 kg for sweetener fans versus 6.1 kg for water advocates, but even then the difference fell short of a clinically significant level. The experiment’s main problem was a mass dropout of participants — only 220 people out of nearly five hundred who started made it to the end of the two-year observation.

Who Paid for the Party and What the WHO Thinks

The most piquant detail is hidden in the conflict-of-interest section. The study was fully funded by the American Beverage Association, a trade group representing the interests of major American soft drink manufacturers. And although the authors state that the sponsors did not interfere with the tests themselves or the data analysis, association representatives read the manuscript before publication, and the work of the hired medical editor was also paid for by the corporations.

Such conclusions run counter to the global recommendations of the World Health Organization. In 2023, the WHO issued a direct guideline not to use non-caloric sweeteners for weight control. The organization’s experts found the evidence for their long-term benefits unconvincing, and long-term observational studies pointed to a potential risk of developing type 2 diabetes with regular use of such additives.

The published experiment does not prove that sugar-free soda helps burn fat on its own. Both groups lost weight through a full behavioral program, dietary changes, and physical activity. The study only shows that replacing water with diet lemonade will not drastically ruin your diet if you are already actively restricting calories.