I just came across this 2010 over-eating study which showed that wide access to sugar-starch created less pleasure as rats ate ever more, and also binged more:
The most recent study to examine the addictive quality of fattening foods was published online March 28 by the journal Nature Neuroscience. For the paper, researchers at the Scripps Research Institute in Jupiter, Fla., examined three groups of lab rats that were fed various diets for 40 days. One group was given typical rat chow only; a second group was offered rat chow, plus a buffet of bacon, sausage, cheesecake, chocolate frosting and other delectable goodies for one hour a day; and a third group was allowed extended access to the fatty buffet for up to 23 hours a day.
The extended-access group began consuming twice as many calories as the other rats, and, not surprisingly, became obese. The limited-access rats, meanwhile, developed a binge pattern of eating, consuming most of their daily calories during the single hour they were allowed in the junk food “cafeteria.”
But what shocked the researchers was that extended-access rats also showed deficits in their “reward threshold.” That is, unrestricted exposure to large quantities of high-sugar, high-fat foods changed the functioning of the rats’ brain circuitry, making it harder and harder for them to register pleasure — in other words, they developed a type of tolerance often seen in addiction — an effect that got progressively worse as the rats gained more weight. “It was quite profound,” says study author Paul Kenny, an associate professor of neuroscience at the Scripps Research Institute. The reward-response effects seen in the fatty-food-eating mice were “very similar to what we see with animals that use cocaine and heroin,” he says.
Kenny’s study did not include rats exposed to drugs, making direct comparison tricky, but other studies have found that chronic cocaine or heroin exposure leads to reductions in reward thresholds of 40% to 50%.
The extended-access rats also showed a lowered level of a certain type of dopamine receptor in the brain, which is thought to contribute to pleasure-seeking behavior in humans. “Human cocaine addicts, people who are obese, alcoholics and heroin addicts also show a down-regulation of this dopamine D2 receptor,” says David Shertleff, director of the division of basic neuroscience and behavioral research at the National Institute on Drug Abuse. “This system is geared toward motivating behavior normally, but what’s happening here is, with chronic exposure to highly fatty and sweet manufactured food, you’re actually getting to a pathological state.”
That is, the down-regulation of D2 receptors seems to turn normal desire into compulsion. In Kenny’s study, the rats that had been given extended access to junk food for 40 days were later willing to continue seeking fatty foods at the risk of getting a painful electric shock to the feet. Limited-access and chow-only rats, however, were significantly put off by the threat of shock, and stayed away from the junk-food buffet.