“Unlike drug-taking behavior, however, exercise has an element of punishment to it. This punishing aspect can be seen in descriptions of exercise even from people without eating disorders. People describe an upcoming exercise session as penance for eating too much. I meted out the most unpleasant and grueling workouts after I over-indulged or bombed a test at school. Even as I actively sought to marinate my brain in endorphins, I was attempting to punish myself.

“[Melbourne-based neuroscientist Charlotte] Keating calls this curious blend of pain and pleasure ‘reward contamination.’ The brain processes both reward and punishment in an area of the brain called the anterior cingulate cortex (ACC). The ACC is filled with dopamine receptors, which is what makes it so sensitive to both reward and punishment. People find something rewarding when they experience an unexpected burst of dopamine. Dopamine and the ACC are also involved in punishment, only it involves a drop in dopamine instead of a surge (Schultz, 2007). The neurons responsive to reward and punishment in the brain are very close together — so close, Keating says, that their messages can sometimes be confused. This neurological confusion, Keating believes, may help explain why so many sufferers cling to their illness even as it destroys their lives.”