You know those no-guilt diet drinks you chug by the gallon, and the fake sugar you dump in your coffee to stay trim? Bad news: a new study suggests that artificial sweeteners may actually make it harder to control your weight.

Psychologists at Purdue University's Ingestive Behavior Research Center report that nine rats given yogurt sweetened with no-cal saccharin ended up eating more and gaining more weight and body fat than eight fellow rodents given yogurt containing plain old glucose (a simple sugar with about 15 calories per teaspoon, the same as table sugar).

Study authors Susan Swithers and Terry Davidson speculate the reason is that the faux sweetener messes with the brain, fooling it into revving up the body's metabolism in anticipation of a never-to-come calorie load.

Typically, they say, the taste buds, sensing something sweet, signal the brain to prep the digestive system to gear up for a caloric onslaught; when the expected sugar jolt (extra calories) fails to materialize, the body gets rattled and has trouble bouncing back and regulating appetite when other food is available. As a result, rats eat more or expend less energy than they would have had they had the real thing.

"The data clearly indicate that consuming a food sweetened with no-calorie saccharin can lead to greater body-weight gain and adiposity [fat] than would consuming the same food sweetened with a higher-calorie sugar," the authors write in the journal Behavioral Neuroscience. They say that other artificial sweeteners—aspartame, sucralose and acesulfame—could have a similar effect.

The researchers note that the findings gibe with other emerging evidence—including a study published last month in the American Heart Association's journal, Circulation—that shows people who down diet drinks are at a higher risk for obesity and metabolic syndrome (a medley of medical problems such as abdominal fat, high blood pressure and insulin resistance that puts people at risk for heart disease and diabetes).

They acknowledge, however, that more research is needed. After all, just because this counterintuitive effect may occur in rats does not necessarily mean it also happens in humans. Still, let it serve as a warning to anyone who may have a false sense of security that artificial sweeteners are all it takes to be fit and healthy.