Diet drinks may not help people lose weight, medics have suggested after a study showed that those who drink them tend to get fatter.
Among those with a fondness for the beverages, drinking two or more a day, their waists grew six times as quickly as those who did not consume them.
"Diet", "sugar-free", or "low-calorie" drinks contain artificial sweeteners like aspartame in the place of sucrose or fructose.
However, a study of 474 people over 10 years found that the more of them people drank, the more weight they put on.
Helen Hazuda, professor of clinical epidemiology at the Texas University School of Medicine, warned: "They may be free of calories but not of consequences."
She added: "Data from this and other prospective studies suggest that the promotion of diet sodas and artificial sweeteners as healthy alternatives may be ill-advised."
Researchers looked at waist measurements and weight in the participants, comparing those who didn't consume diet drinks with those who did.
Diet soft drink consumers, as a group, experienced 70pc greater increases in waist circumference compared with non-users.
Heavy consumers experienced a 500pc greater increase - equivalent to a six-fold increase.
The authors of the study, presented to the Scientific Sessions of the American Diabetes Association, said policies to promote diet drinks "may have unintended deleterious effects".
However, the study did not control for differences in the participants' wider diets and lifestyles. It is likely that those who consumed large quantities of diet drinks had a generally poorer diet than those who tended to avoid them.