October 21st, 2009
By Laura Sanders
CHICAGO — Junk food elicits addictive behavior in rats similar to the behaviors of rats addicted to heroin, a new study finds. Pleasure centers in the brains of rats addicted to high-fat, high-calorie diets became less responsive as the binging wore on, making the rats consume more and more food. The results, presented October 20 at the Society for Neuroscience’s annual meeting, may help explain the changes in the brain that lead people to overeat.
“This is the most complete evidence to date that suggests obesity and drug addiction have common neurobiological underpinnings,” says study coauthor Paul Johnson of the Scripps Research Institute in Jupiter, Fla.
To see how junk food affects the brain’s natural reward system — the network of nerve cells that release feel-good chemicals — Johnson started at the grocery store. He loaded up on typical Western fare, including Ho Hos, sausage, pound cake, bacon and cheesecake. Johnson fed rats either a standard diet of high-nutrient, low-calorie chow, or unlimited amounts of the palatable junk food. Rats that ate the junk food soon developed compulsive eating habits and became obese. “They’re taking in twice the amount of calories as the control rats,” says Johnson’s coauthor Paul Kenny, also of Scripps.
Johnson and Kenny wanted to know if this overeating affected the pleasure centers of the rats’ brains, the regions responsible for drug addiction. The researchers used electrical stimulations to activate these reward centers and induce pleasure. Rats could control the amount of feel-good stimulation by running on a wheel — the more they ran, the more stimulation they got. The rats fed junk food ran more, indicating that they needed more brain stimulation to feel good.
After just five days on the junk food diet, rats showed “profound reductions” in the sensitivity of their brains’ pleasure centers, suggesting that the animals quickly became habituated to the food. As a result, the rats ate more food to get the same amount of pleasure. Just as heroin addicts require more and more of the drug to feel good, rats needed more and more of the junk food. “They lose control,” Kenny says. “This is the hallmark of addiction.”
To see how strong the drive to eat junk food was, the researchers exposed the rats to a foot shock when they ate the high-fat food. Rats that had not been constantly exposed to the junk food quickly stopped eating. But the foot shock didn’t faze rats accustomed to the junk food — they continued to eat, even though they knew the shock was coming.
“What we have are these core features of addiction, and these animals are hitting each one of these features,” Kenny says.
These reward pathway deficits persisted for weeks after the rats stopped eating the junk food, the researchers found. “It’s almost as if you break these things, it’s very, very hard to go back to the way things were before,” Kenny says. When the junk food was taken away and the rats had access only to nutritious chow (what Kenny calls the “salad option”), the obese rats refused to eat. “They starve themselves for two weeks afterward,” Kenny says. “Their dietary preferences are dramatically shifted.”
Scientists are interested in determining the long-term effect of altering the reward system. “We might not see it when we look at the animal,” says obesity expert Ralph DiLeone of Yale University School of Medicine. “They might be a normal weight, but how they respond to food in the future may be permanently altered.”