Obesity may be a result of a reduced sensitivity to the "rewards" of calorically dense food, fostered by eating too much of the stuff too often, according study published in Nature Neuroscience this week. A group of researchers gave some rats different levels of access to tasty and highly caloric "cafeteria" foods, while training them to respond to aversive stimuli. Rats who had been given the most access to the good stuff ignored any indications of negative consequences and kept right on eating.

During the experiment, researchers used three groups of rats: one given only rat chow, a second given chow and access to highly caloric food for an hour a day, and a third group allowed their choice of chow or the caloric food for 18-23 hours a day. The fatty foods included things like bacon, sausage, cake, and chocolate.

Once the rats had gained some weight, researchers began presenting the sausage links and cake slices to each group while flashing a light and shocking the feet of all the rats. The expectation was that this would link the negative stimulus and the caloric foods in the rats' brains.

Researchers found that, when they flashed the cue light without the shock, the chow-only and restricted access rats would back off the goodies; the extended-access rats were unfazed and continued to gobble their cakes. Obviously, full access to fatty foods causes negative consequences to pale in comparison to the need to keep eating. But the results also suggest that extended access to tasty, highly-caloric foods blunt sensitivity to the "reward" of eating them. 

Researchers compare this study to one where rats were given extended access to intravenous cocaine or heroin, which caused a similarly warped reward sensitivity. Rats in the same experiment that were drug-free or had limited access were also able to keep negative stimuli in perspective, which indicates that there are striking similarities between drug- and caloric food-related behaviors.

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