SUNDAY, March 28 (HealthDay News) -- Obese people often say they'd like to eat less but feel almost powerless to stop indulging, and now new research suggests that explanation might be all too true.
The theory stems from a study in rats. When researchers gave the rats unlimited access to a calorie-laden diet of bacon, pound cake, candy bars and other junk food, the rats quickly gained lots of weight. As they plumped up, eating became such a compulsion that they kept chowing down even when they knew they would receive an unpleasant electric shock to their foot if they did so.
Meanwhile, rats fed the human equivalent of a well-balanced, healthy diet -- and given only limited access to the junk food -- didn't gain much weight and knew enough to stop eating when they received the cue that a foot shock was imminent.
Even more startling, the researchers report, is that when they took away the junk food from the obese rats and replaced it with healthier chow, the obese rats went on something of a hunger strike. For two weeks, they refused to eat hardly anything at all.
When researchers looked at the obese animals' brains, they noted there were declines in the dopamine D2 receptor that previous research has implicated in addiction to cocaine and heroine.
"A hallmark of drug addiction is that it leads to changes in how the brain's reward system works," Kenny said. "Addiction is a loaded term, but in this case, there is evidence of addiction-like adaptations."
When researchers artificially suppressed the receptor using a virus in the brains of other rats, those rats starting eating junk food compulsively.
"What we think is happening is that, as you become obese over a period of time, the D2 receptors go down, which plays a major role in becoming a compulsive eater," Kenny said, noting there are almost certainly other factors at play as well.
"They went into voluntary starvation," said study author Paul Kenny, an associate professor at Scripps Research Institute in Jupiter, Fla.
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