A dog will do anything for a biscuit—over and over again. Most people will, too, because when sugar touches the taste buds it excites reward regions in the brain. A new study shows that people with eating disorders do not react to sweet flavors the way healthy people do, however, lending evidence to the hypothesis that brain differences predispose people toward bulimia and anorexia.

A team of psychiatrists at U.C. San Diego studied 14 recovered anorexic women, 14 recovered bulimic women (who used to binge and purge) and 14 women who had never had an eating disorder, matched by age and weight. None of the women had had any pathological eating-related behaviors in the 12 months preceding the study. After fasting overnight, subjects received a modest breakfast to ensure similar levels of satiety. They were then fed small tastes of sugar every 20 seconds through a syringe pump while their brains were scanned.

The women who had recovered from anorexia—those who formerly starved themselves—showed less activity than the healthy women in a reward center in the brain known as the primary gustatory cortex. The participants who were no longer bulimic showed more activity than the healthy women did. The results were published in October 2013 in the American Journal of Psychiatry.

The researchers believe these abnormal responses to sugar predispose people to eating disorders, adding to a growing body of work suggesting that genetic and biological risk factors underlie most cases, according to study co-author Walter Kaye, director of U.C.S.D.'s Eating Disorders Research and Treatment Program. Kaye acknowledges that the finding could instead reflect a consequence of an eating disorder that persists after recovery, but he thinks it is less likely. Given our culture's fixation on body image and thinness, if nonbiological factors such as social pressure were enough to trigger eating disorders, anorexia would be rampant, Kaye says. Yet only 0.5 percent of women in the U.S. are anorexic, a figure that has held steady for decades.

Whether a cause or effect of eating disorders, the abnormal brain activity has important implications for how we treat patients, according to physician Laura Hill, chief clinical officer of the Center for Balanced Living, a clinic specializing in eating disorders in Columbus, Ohio. “People will say [to anorexics] just be mindful of your eating. They can't be mindful. There is no response in the brain to say, ‘Let me get a sense of how I should eat and when I should eat.’ It's just not firing,” she says. Instead successful therapies use experiential activities that teach patients how to compensate for their brain's irregular responses.