Dr. Jim Hill of the University of Colorado's Anschutz Health and Wellness Center divided approximately 300 adults into two groups. One group continued to drink diet drinks and the other group replaced artificially sweetened diet drinks with water.
Both groups kept journals on the meals they ate and both groups got intensive coaching on successful techniques for weight loss.
The study, published in the journal Obesity, found study participants who gave up diet drinks for water lost 9 pounds during 12-week study period, while those who drank diet soda lost, on average, 13 pounds in the same study period.
"The results, to us, were not at all surprising," Hill told CNN.
"In fact, those who drank diet beverages lost more weight and reported feeling significantly less hungry than those who drank water alone. This reinforces if you're trying to shed pounds, you can enjoy diet beverages."
The most likely reason the diet soda drinkers lost more weight was they did not have to give up overall calories and a drink they enjoy. Most psychologists say willpower is a limited and it took more willpower to give up the diet drinks and calories at the same time, Hill explained.
Susan Swithers, a professor of Behavioral Neuroscience at Purdue University, last year published a study that found diet soda drinkers have the same health issues as those who drink regular soda over the long term.
"What the prospective studies actually suggest is that if you go out 7 years, 10 years, 15 years, 20 years, the cohorts of individuals who are consuming diet sodas have much worse health outcomes," Swithers told CNN.
The study received funding from the American Beverage Association.