Ftrain, by Paul Ford

"Junk food turns rats into addicts," Laura Sanders, Science News:

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.