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No-cal sodas can trick the brain: sugar-free sweeteners may contribute to obesity risk.

By baffling the brain, saccharin and other sugar-free sweeteners--key weapons in the war on obesity--may paradoxically foster overeating.

At some level, the brain can sense a difference between sugar and no-calorie sweeteners, several studies have demonstrated. Using brain imaging, San Diego researchers now show that the brain also processes sweet flavors differently depending on whether a person regularly consumes diet soft drinks.

"This idea that there could be fundamental differences in how people respond to sweet tastes based on their experience with diet sodas is not something that has gotten much attention," says Susan Swithers of Purdue University in West Lafayette, Ind. A key finding, she says: Brains of diet soda drinkers "don't differentiate very well between sucrose and saccharin."

Erin Green and Claire Murphy of the University of California, San Diego and San Diego State University recruited 24 healthy young adults for a battery of brain-imaging tests. Half reported regularly drinking sugar-free beverages, usually at least once a day. The rest seldom if ever consumed such drinks. While the brain scans were under way, researchers pumped small amounts of saccharin- or sugar-sweetened water in random order into each recruit's mouth.

Both the diet soda drinkers and nondrinkers rated each sweetener about equally pleasant and intense, Green and Murphy report in an upcoming Physiology & Behavior. But which brain regions lit up while making those judgments differed sharply based on past diet drink consumption.

Some affected brain regions are associated with pleasurable feedback or reward in response to desirable sensations. Compared with nondrinkers, diet soda consumers "demonstrated more widespread activation to both saccharin and sucrose in reward-processing brain regions," the scientists say.

One strong link to higher diet soda consumption was reduced activation of the caudate head, an area associated with the food motivation and reward system. Green and Murphy note that decreased activation of this brain region has also been linked to higher risk of obesity.

The new findings may help explain an association between diet soda use and weight gain. Once fooled, the brain's sweet sensors may no longer be able to reliably gauge energy consumption.

Two years ago, Swithers' group showed that rats that always received a saccharin-sweetened yogurt learned to modulate their food intake to account for the sweeteners failure to deliver calories. But rats that alternately got saccharin- and sugar-sweetened yogurts blimped out.


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Title Annotation:Body & Brain
Author:Raloff, Janet
Publication:Science News
Geographic Code:1USA
Date:Jul 14, 2012
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