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By Margot Hedlin

It’s a public health crisis: Two-thirds of Americans are overweight or obese, a number that is expected to climb steadily upward if current trends continue. While obesity is attributed to a combination of several different factors — from genetics and junk food to super-sized portions and an inactive lifestyle — experts are now turning their attention to another potential culprit: your brain.

We all joke about being addicted to junk food, but scientists are discovering that it’s no laughing matter: Your cravings for a third slice of chocolate cake have more in common with the urges to smoke a cigarette or polish off that bottle of wine than you may realize. Scientists are trying to understand just what makes these foods so irresistible — and the role our brains play in fueling the nation’s obesity epidemic.

Wired to (Over)Eat

Your brain is fine-tuned to love fat and sugar. For most of our evolutionary history, food was scarce — and people with a taste for nutrient-rich, calorie-dense foods were more likely to survive and pass on their genes. For our hunter-gatherer ancestors, cravings for sugars and fats served a distinct purpose, leading them to seek ripe fruit (a quick source of calories and vitamins) and calorie-laden fats (a hefty source of energy).

These days calories are no longer hard to come by — but our taste buds haven’t changed. We still crave fat and sugar, and few of us reach for foods like nuts and fruit. Instead, we go for processed junk foods. These highly palatable foods contain high levels of artificial fats, salt, and sugar designed to intensify those cravings. These foods overwhelm your neural circuitry, essentially causing some of the same changes in your brain and behavior as drugs — namely, disrupting the parts of the brain that regulate pleasure and self-control.

Gene-Jack Wang, MD, a scientist at Brookhaven National Laboratory who performs brain imaging studies to understand brain disorders and addiction, notes that the brain pathways that make drugs pleasurable are the same pathways that allow you to enjoy food — and these pathways seem to be going haywire in people who overeat.

Brain imaging studies show that the ‘rewards circuits’ of the brain light up more in obese individuals than normal-weight people when subjects look at pictures of sugar- and fat-rich foods or when they expect to receive food. This seems to suggest that people who overeat unconsciously desire junk food more intensely than their svelte counterparts.

But there’s a twist: Studies show that obese people don’t get as much pleasure from food once they actually receive it. For instance, one study performed at the Oregon Research Institute found that obese teens had less activation of those pleasure centers when drinking a milkshake than their skinnier peers had. “The more you overeat, the less ‘rewards circuitry’ is activated when you eat that same food,” said Eric Stice, PhD, a researcher at the Oregon Research Institute who organized the study — meaning that obese people may need to eat more to trigger a feel-good response.

That’s because your brain is re-wiring as you gain weight, dulling your sensitivity to the benefits of dopamine, a brain chemical released after pleasurable activities. Wang and his colleagues have found that obese individuals have fewer dopamine receptors in the striatum (a region of the brain that controls rewards) than normal-weight individuals — and the greater the person’s body mass index (BMI), the fewer receptors they had. A person with fewer dopamine receptors will have a reduced ability to “listen” to dopamine’s signal, so a skinny person, with more dopamine receptors, will enjoy their chocolate cake more than someone who is obese. The bottom line: A skinner person will feel more satisfied after eating less cake than his heftier counterpart.

Those brain changes are directly tied to weight gain — you eat more and more to satisfy your cravings, but have fewer and fewer receptors that allow you to feel pleasure. In one particular study, Dr. Stice performed brain scans on women as they drank milkshakes, then brought them back for a second brain scan six months later and measured how their weight changed in the interim. “When you look at the people who gained weight, you see a reduction in reward region response; the more you gain weight, the more you see a blunted reward circuitry response,” said Dr. Stice.

An Appetite for Addiction

The parallels to drug use are startling. “There’s clear evidence, mostly from really good animal research, that the more you do drugs, the more you have to do to feel the same degree of activation from the drugs,” Stice explained. “That’s called tolerance. If you do cocaine and start off at half a gram, [after a certain period of using the drug] you’re going to have to escalate your use dramatically to get any effect. And it’s looking like that very same thing happens with food.”

And that’s not the only parallel with drug addiction. Food cravings can cause some of the same changes in behavior seen in people addicted to drugs.

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