Obese individuals are less able to inhibit the impulsive desire to eat than people of normal weight, especially when their blood-sugar levels go down below normal, researchers from Yale University and the University of Southern California reported in The Journal of Clinical Investigation. They also found that for most people, when the brain becomes hungry, a chocolate bar suddenly looks a lot more desirable.

Rajita Sinha and team used brain imaging scans to see what went on in people’s brains when their blood-glucose (sugar) levels dropped. They found that the prefrontal cortex – a brain area known to regulate impulses and emotions – loses the ability to control desire for high-calorie foods.

Rajita Sinha said:

“Our prefrontal cortex is a
sucker for glucose.”

The scientists manipulated participants’ glucose levels intravenously. They observed variations in blood-glucose levels while the participants looked at photographs of high-calorie and low-calorie foods, as well as some non-foods. They all underwent fMRI (Functional magnetic resonance imaging) scans at the same time.

The hypothalamus, an area of the brain, senses the change when glucose levels fall. The insula and striatum, other brain areas, become active – these areas are associated with reward, and induce a desire to eat, the scientists found.

The prefrontal cortex was found to have the greatest reaction to lower blood-glucose levels. The prefrontal cortex appeared to lose the ability to suppress increasingly urgent signals to eat, signals that were generated in the striatum. This weakened response was particularly noticeable when the obese participants looked at high-calorie foods.

Sinha said:

“This response was quite specific and more dramatic in the presence of high-calorie foods.”

One of the lead authors, Kathleen Page, 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.”

The researchers observed that the striatum became hyperactive when blood-glucose levels went down. However, cortisol appeared to play a greater role in activating the brain’s reward centers than glucose, they added.

Sinha believes that the stress linked to a fall in glucose levels plays a key part in the activation of the striatum.

Sinha said:

“The key seems to be eating healthy foods that maintain glucose levels. The brain needs its food.”

Written by Christian Nordqvist