It’s common knowledge that you shouldn’t go grocery shopping when you’re hungry, because you’re more likely to buy junky, fattening foods. Now a new brain scan study shows why: your brain craves junk food more on an empty stomach.
The study led by Yale University and University of Southern California researchers involved 14 men and women — some were of healthy, normal weight, while others were obese. The researchers hooked up the participants to a device that controlled their glucose levels intravenously, in order to approximate hunger and fullness, and then sent them through a functional MRI (fMRI) scanner.
While in the machine, the participants saw pictures of various foods — some high-calorie and some low-calorie — as well as non-food objects. To make sure the volunteers didn’t start out too hungry or too full, the test occurred two hours after they had eaten lunch.
When the researchers kept the participants’ glucose levels near normal, their brains showed more activity in the prefrontal cortex, the area that governs executive functions such as logic, reasoning and planning; this is the region that helps control emotions and impulses, like craving high-calorie junk food.
But when glucose levels were dropped, a deeper area of the brain, which includes the hypothalamus, thalamus and nucleus accumbens, started to light up instead. These areas are related to our emotional or limbic system, and play a key role in motivation, reward and addiction.
This happens because it’s an important survival strategy: when glucose levels wane, it’s a signal that the body is running low on fuel necessary to survive; the response is to remove any barriers to eating. Making food — especially high-calorie goodies rich with glucose — look good is a quick and powerful way to replenish those dwindling reserves.
But while this brain activity occurred only during low-glucose times among normal-weight people, the lack of regulation was the status quo for obese individuals, the study found. Normally, when the body gets calories, the brain’s reward system quiets down and regions related to executive function take over; for normal-weight people, that means enticing foods start to lose their lip-smacking attractiveness. But for some reason, in obese people, the reward switch remains stuck in the on position, leading to constant desire for cheesecake or that juicy burger.
“What happens in lean people, when their blood sugar is not dropping, is that their executive function lights up — the area involved in making decisions,” explains Robert Sherwin, professor medicine at Yale and senior author of the paper, published in the Journal of Clinical Investigation. “This executive function controls the reward system, which is much less activated. But in obese people, that executive control is not activated when their blood sugar isn’t falling. So they have continued activation of their reward system and that system dominates even if they’re not hungry.”
The implication, he says is that there is a biological driver that pushes obese people to continue to eat, even when they’re full. They may not “see” food the same way as lean people do, because their reward and desire signals are consistently signaling that they want to eat, and that they want to eat high-calorie foods in particular.
“We don’t know why, and we don’t know if it’s reversible, but these results imply that there may be a biological difference that when people become obese, their motivation for eating when they see an ad or picture may not be under the same control systems as those of lean people,” says Sherwin.
That may prove to be an important clue to understanding obesity, and, if these findings are confirmed, they could lead to more effective ways to curb weight gain, beginning not on the plate but in the brain.
Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME‘s Facebook page and on Twitter at @TIME.