Explained: Why obese women have uncontrollable urge to eat

The brain's reward centres in severely obese women continue to respond to food cues even after they have eaten and are no longer hungry, according to a new study which found such women reacted to pictures of food in almost the same way they did when fasting.

The study by researchers from UT Southwestern in the US compared attitudes and the brain activity of 15 severely obese women (those with a body mass index greater than 35) and 15 lean women (those with a BMI under 25). Magnetic resonance imaging (MRI) images of the study participants were taken before and after a meal.

Both groups showed significantly increased activity in the neo- and limbic cortices and midbrain when they were hungry. After eating, however, that brain activity dropped among lean participants while continuing in their obese counterparts.

Even after eating and reporting they were full, the severely obese women continued to react to pictures of food in much the same way they had when fasting, as exhibited in brain scans. "Before or after the meal, they are just as excited about eating. It seems they have an instinctive drive to keep eating," said Nancy Puzziferri from UT Southwestern.

While the appeal of pictured food dropped 15% for the lean women after they ate, the severely obese women showed only a 4% decline, based on brain scans using functional magnetic resonance imaging (fMRI) to measure brain activity, researchers said.

After eating, activity in regions in the prefrontal cortex and posterior cingulate cortex significantly changed in the lean group, but not in the obese group. The obese study participants maintained activation in the midbrain, one of the body's most potent reward centres, they said.

Study participants had fasted for nine hours prior to testing. They were asked to rate their level of hunger or fullness, then given a brain scan as they viewed pictures of food. Again, they were asked to rate their level of hunger. Over the next hour, the women were fed a meal of lean beef or chicken, potatoes or rice, green beans, canned peaches, and iced tea or water.

After eating, the participants went through another battery of hunger/fullness ratings and fMRI scans while exposed to pictures of food. The obese women showed sustained "hungry" brain activation, even though they reported the same increase in satiation as their lean counterparts, researchers said.

"These findings may explain why some people with severe obesity report an underlying drive to eat continually despite not feeling hungry," said Puzziferri. "In contrast, lean women when full will either stop eating or just sample a food they crave. It is just not a level playing field - it is harder for some people to maintain a healthy weight than others," she said.

The findings were published in the journal Obesity.