If you're trying to lose weight, choosing a diet soda instead of water may not sabotage your efforts, according to a new two-year study.
Researchers found that adults assigned to drink beverages with non-nutritive sweeteners maintained about as much weight loss as those assigned to drink water. That's notable because diet drinks have long been caught in a debate over whether their sweetness could increase hunger, encourage overeating or otherwise make weight management harder.
But the results don't mean diet drinks and water are interchangeable in every way. The study, which was published in the British Journal Of Nutrition, looked specifically at weight management in adults with overweight or obesity who were also participating in a behavioral weight-management program. Fewer than half of the original participants completed the full two years, and the research was funded by the American Beverage Association.
The randomized controlled trial began with 493 adults in England. Participants were assigned to drink either water or beverages containing non-nutritive sweeteners, including diet sodas, fruit drinks and flavored waters. The most common sweeteners in the beverages were aspartame, acesulfame potassium and sucralose.
Participants were asked to drink at least two 330-milliliter servings of their assigned beverages each day. Those in the water group were asked to avoid drinks containing non-nutritive sweeteners, while those in the diet-drink group could still drink water.
The drinks weren't the only part of the study. For the first 12 weeks, everyone attended weekly weight-loss sessions with a nutritionist. Those meetings continued monthly for another 40 weeks. During the second year, participants no longer received those regular sessions.
Among those who completed the two-year follow-up, people in the water group were an average of about 8 pounds below their starting weight. Those in the diet-drink group were about 11 pounds below their starting weight. The difference between the groups wasn't statistically significant.
In other words, the study didn't find evidence that regularly drinking diet beverages made it harder to maintain weight loss compared with drinking water.
That's different from saying diet drinks caused weight loss. Everyone in the trial received weight-management support during the first year, including guidance on healthy eating and reducing sugar. The study also wasn't designed to determine whether diet drinks and water are equally healthy overall.
The findings add an interesting piece to a complicated debate over non-nutritive sweeteners.
Some observational studies, which track what people already eat and drink, have linked diet beverages with higher body weight or other health concerns. But those studies can have difficulty untangling cause and effect. Someone who is already concerned about their weight, for example, might be more likely to choose diet drinks.
Randomized trials approach the question differently by assigning people to particular groups. Previous trials have generally found neutral or modest benefits for weight when diet beverages are compared with water or replace sugar-sweetened drinks. What makes this study unusual is its length: Randomized nutrition trials lasting two years are relatively uncommon.
The results are also relevant to a 2023 World Health Organization recommendation against using non-sugar sweeteners as a strategy for long-term weight control. The researchers argue that their findings provide longer-term randomized evidence that wasn't widely available when that guidance was developed.
Still, this single study doesn't settle the broader question of how non-nutritive sweeteners may affect health over many years.
Researchers measured several health markers in addition to weight. There were some differences between the groups in cholesterol, blood sugar and blood pressure, but some of those analyses involved very small numbers of participants. The researchers said the differences they observed in blood sugar and blood pressure were not large enough to be considered clinically meaningful.
The study also didn't find a significant difference between the groups in hunger after two years, which doesn't support the idea that the diet drinks caused people to become noticeably hungrier than water did in this trial.
One major limitation was the number of people who stayed in the study. Although 493 participants started drinking their assigned beverages, 383 completed the first 12 weeks, 262 reached one year and 220 completed the two-year follow-up. The researchers said the COVID-19 pandemic, the length of the study and the voluntary nature of the final year may have contributed to the dropouts.
The trial was also conducted at a single site in England and did not collect information on participants' race or ethnicity, so the results may not apply equally to everyone.
What the study can tell us is considerably narrower, but still useful: For adults with overweight or obesity participating in this particular weight-management program, drinking beverages with non-nutritive sweeteners didn't result in worse weight outcomes than drinking water over two years.
The American Beverage Association funded the study and the beverages used in the trial. The researchers reported that the association was not involved in designing or conducting the study or analyzing or interpreting the results, although a representative was allowed to review the manuscript. The association also funded medical writing assistance. Several researchers reported additional relationships with food, beverage or pharmaceutical organizations.