MONDAY, Feb. 11 (HealthDay News) -- Surprising research suggests a popular artificial sweetener has the unexpected and unwelcome effect of packing on the pounds.

Purdue researchers report that saccharin altered the ability of rats to control their appetites. However, the head of an artificial sweetener trade group scoffed at the findings, saying they don't necessarily translate to humans.

"We found that the rats that were getting artificially sweetened yogurt gained more weight and ate more food," said study author Susan Swithers, an associate professor of psychological sciences at the Ingestive Behavior Research Institute at Purdue University. "The take-home message is that consumption of artificially sweetened products may interfere with an automatic process."

That process, she said, involves the body's ability to detect that it will soon be full. "We often will stop eating before we've been able to absorb all of the calories that come from a meal. One of the reasons we might stop eating is that our experience has taught in the past that, 'After I eat this food, I'll feel this full for this long,' " she explained.

It seems to be a subconscious process based on automatic estimations of how much energy certain foods will provide, she said. For example, a sweet taste might be a sign that "calories are coming, and I should prepare my body for the arrival of those calories." However, when the sweetness is not followed by a lot of calories, the body's digestive system gets confused, and the metabolism rate does not gear up as much the next time sweetness is tasted.

To test this theory, the researchers fed two different types of plain Dannon yogurt to male rats. Some received yogurt sweetened with glucose, a form of sugar, while others ate saccharin-sweetened yogurt. All also ate unsweetened yogurt.

The rats who ate artificially sweetened yogurt consumed more food overall and gained more weight. The body temperatures of those rats also didn't rise as high as the others. "That might be a kind of measure of energy expenditure, suggesting not only are the animals eating more calories, they may be expending or burning up fewer calories," Swithers said.

The findings were published in the February issue of Behavioral Neuroscience.

Essentially, she said, it appears that the bodies of the rats are learning to not expect much in the way of calories from sweet foods. "The artificial sweetener provides the signal that not as many calories are going to come, and the animal responds by consuming more calories."

As for humans, she said, previous research has provided conflicting indications about whether obesity is a bigger problem among people who use artificial sweeteners.

According to her, launching a similar study among people would be difficult, because few have never encountered artificial sweeteners before. The next step, she said, is to do more research in rats.

Lyn Nabors, president of the Calorie Control Council trade group, lambasted the study, saying it has "no basis in science" and "no relation to the human experience whatsoever."

Artificial sweeteners can help people lose weight, she said. "The scientific community firmly believes that calories in, calories out is what makes a difference. The recommendation is that you reduce calories and exercise if you want to lose weight."

More information

Learn about cancer and artificial sweeteners from the National Cancer Institute.

SOURCES: Susan Swithers, Ph.D., associate professor, psychological sciences, Ingestive Behavior Research Center, Purdue University, West Lafayette, Ind.; Lyn Nabors, president, Calorie Control Council, Atlanta; February 2008, Behavioral Neuroscience

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