Seeing a milkshake can activate the same areas of the brain that light up when an addict sees cocaine, American researchers say.

The study helps explain why it can be so hard for some people to maintain a healthy weight, and why it has been so difficult for drugmakers and health experts to find obesity treatments that work.

"If certain foods are addictive, this may partially explain the difficulty people experience in achieving sustainable weight loss," Ashley Gearhardt of Yale University in Connecticut and colleagues wrote in the Archives of General Psychiatry.

Gearhardt's team wanted to see what happens in the brain when young women are tempted by a tasty treat.

They used a type of brain imaging known as functional magnetic resonance imaging, or fMRI, to study brain activity in 48 young women who were offered a chocolate milkshake or a tasteless solution. Women in the study ranged from lean to obese.

The team found that seeing the milkshake triggered brain activity in the anterior cingulate cortex and the medial orbitofrontal cortex - brain areas that have been implicated in an addict's urge to use drugs.

And this activity was higher among women in the study who had high scores on a scale that assessed their eating habits for signs of addictive behavior.

"These findings support the theory that compulsive food consumption may be driven in part by an enhanced anticipation of the rewarding properties of food," Gearhardt and colleagues wrote.

People who are addicted to a substance are more likely to react with physical, psychological and behavioral changes when exposed to that substance. Altering visual "cues" - billboards of tempting treats, for example - might help curb the urge to indulge, they said.

"Ubiquitous food advertising and the availability of inexpensive palatable foods may make it extremely difficult to adhere to healthier food choices because the omnipresent food cues trigger the reward system," they wrote.

The study suggests that advertising might also play a role in the nation's obesity problem, and future studies should look at whether food ads trigger this same kind of brain activity.

Obesity is one of the biggest health challenges facing the United States, and health officials already added a requirement to President Barack Obama's new healthcare law requiring that restaurants add calorie counts to their menus.