The number of individuals who are obese and suffer with its associated health problems has continued to rise, even being called an epidemic.

Is it genetics?  The dream of cheap food finally being realized? Or are we slaves to marketing?

Researchers from Yale University School of Medicine and the University of Southern California say they have visualized differences in the way that the brains of obese and non-obese individuals respond to visual cues of high-calorie foods.  They see those foods differently.

The researchers found that in normal weight individuals with low levels of glucose in their blood, certain regions of the brain were triggered in response to food cues and those individuals had a great desire for high-calorie foods. However, if those individuals had a normal level of glucose in their blood, different regions of the brain were triggered by the food cures and the individuals were less interested in high-calorie foods.

But when it came to obese people, glucose levels in the blood did not reduce desire for high-calorie foods.

Brain reward regions are activated when glucose levels drop below normal. (Blue). In lean people, but not in the obese, the prefrontal cortex which is involved in decision making and regulating impulses are activated when glucose levels return to normal. (Red).  Credit: Yale

"Our prefrontal cortex is a sucker for glucose," said Rajita Sinha, the Foundations Fund Professor of Psychiatry, and professor in the Department of Neurobiology and the Yale Child Study Center, one of the senior authors of the research.

When glucose levels drop, an area of the brain called the hypothalamus senses the change. Other regions called the insula and striatum associated with reward are activated, inducing a desire to eat, the study found. The most pronounced reaction to reduced glucose levels was seen in the prefrontal cortex. When glucose is lowered, the prefrontal cortex seemed to lose its ability to put the brakes upon increasingly urgent signals to eat generated in the striatum. This weakened response was particularly striking in the obese when shown high-calorie foods.

"This response was quite specific and more dramatic in the presence of high-calorie foods," Sinha said.

"Our results suggest that obese individuals may have a limited ability to inhibit the impulsive drive to eat, especially when glucose levels drop below normal," commented Kathleen Page, assistant professor of medicine at the University of Southern California and one of the lead authors of the paper.

A similarly robust response to high-calorie food was also seen in the striatum, which became hyperactive when glucose was reduced. However, the levels of the stress hormone cortisol seemed to play a more significant role than glucose in activating the brain's reward centers, note the researchers. Sinha suggests that the stress associated with glucose drops may play a key role in activating the striatum.

"The key seems to be eating healthy foods that maintain glucose levels," Sinha said. "The brain needs its food."

Publishes in the Journal of Clinical Investigation