Study author Deborah Tate, associate professor of nutrition and of health behavior and health education at the University of North Carolina at Chapel Hill's Gillings School of Global Public Health, compared weight loss for 318 overweight or obese people drinking different beverages.
People were divided into three groups: those who switched from calorie-laden beverages to diet soft drinks; those who switched to water; and those who were not counseled to change beverages but received general information about healthy choices that could lead to weight loss.
All three groups attended monthly group sessions and had access to a group-specific Web site for six months.
"Substituting non-caloric beverages -- whether it's water, diet soft drinks or something else -- can be a clear and simple change for people who want to lose or maintain weight," Tate said in a statement. "If this were done on a large scale, it could significantly reduce the increasing public health problem of obesity."
All three groups experienced small reductions in weight and waist circumference during the six-month study.
However, people who switched to calorie-free beverages were twice as likely to lose 5 percent or more of their body weight than those who were not counseled to change beverages.
In addition, people in the group who drank mostly water had lower fasting glucose levels and better hydration levels than the control group, the study found.
The study was published in the American Journal of Clinical Nutrition.