Add this to the list of modern life's ironies: the artificial sweetener used to keep the calories out of some of the tastiest foods could be making you fat. As Americans have steadily gained weight over the past two decades, scientists have developed a slew of theories to explain why and what to do about it. Now, psychologists at Purdue University are throwing in their two cents: They think it may have something to do with how the body learns to intuitively count calories—and how some foods throw that natural scale off balance.

What the researchers wanted to know: Do artificial sweeteners and high-calorie drinks inhibit the body's ability to intuitively count calories?

What they did: The scientists devised two experiments to see if foods that confuse the body's instincts about the number of calories, like drinks with artificial sweeteners and watery beverages with lots of calories, could cause people to overeat. To test the effect of artificial sweeteners, the scientists divided 20 rats into two groups. For 10 days, one group was given sugar drinks and the other group was given one drink with sugar and another flavored with saccharin, a zero-calorie sweetener. After 10 days, all of the rats wolfed down a chocolate snack, and then were given a regular meal of rat food. The scientists measured the difference in how much of their regular meal each group of rats ate, to see if they realized the chocolate had a lot of calories in it. In the second experiment, to test how the body dealt with thick versus watery liquids, eight rats ate a mixture with the consistency of pudding for a month, and eight more drank a mixture that was equal in calories but had the consistency of milk. The scientists measured the weight gain for rats in both groups.

What they found: In the first experiment, after both groups finished off their chocolate snack, the group that had been given the saccharin solution ate about three times as much of their regular meal as those that had been given solutions with regular sugars. The scientists think that the rats that were used to saccharin didn't associate sweetness with calories, so they couldn't instinctively guess the number of calories in their pre-meal chocolate snack and they overate at their next meal. In the second experiment, the rats that drank the thick liquid gained an average of 11 grams over a month, while those drinking the thin liquid gained 17 grams. The authors wrote that the rats drinking the thin liquid probably did not interpret it as having as many calories as those with the thicker substance.

What it means to you: Low-calorie drinks and watery, sugary drinks could be throwing off your body's natural ability to regulate calories. The percentage of Americans who say they consume artificial sweeteners increased from 29 percent in 1987 to 58 percent in 2000, and the Purdue psychologists think that may be linked to our rising obesity. Also, they say that you learn intuitively that thick liquids contain more calories so that the popularity of watery but calorie-laden juice drinks could also be causing Americans to put on extra pounds.

Caveats: These studies were done on rats, which really aren't used to eating any sugar, much less artificial sweetener, so the results are not conclusive. Other studies done with humans have shown that liquid consistency can affect calorie consumption and that different properties of foods tip the body's internal calorie counter in different ways.

Find out more: Everything you ever wanted to know, including information about low-calorie sweeteners and tools to count calories can be found at the Calorie Control Council's website (yes, there is such an organization).

Read the article: Davidson, T.L. and S.E. Swithers. "A Pavlovian Approach to the Problem of Obesity." International Journal of Obesity. July 2004, Vol. 28, No. 7, pp. 933–935.

Abstract online: http://www.nature.com/