"The data clearly indicate that consuming a food sweetened with no-calorie saccharin can lead to greater body-weight gain and adiposity (fat tissue) than would consuming the same food sweetened with a high-calorie sugar,"
A second study revealed that the rats fed yogurt sweetened with saccharin were eating more chow.
When people are told to reduce the number of calories they eat, an easy fix is to replace the sugar. Because in a calorie controlled diet you do not want "empty calories". Weight loss on this regime is based on the mechanical view of metabolism that calories in must be less than calories out. So less fuel (energy in) and more excercise (energy out) must lead to weight loss over time.
Except that we are far more complex creatures than this simple model - and that is the reason that most people who have tried them find that diets don't work. Which is why there is huge and highly profitable weight loss industry.