The small study found that food smells activated the parts of their brains related to impulsive behaviour and obsessive-compulsive disorder, which is characterised by recurrent thoughts and repetitive behaviours. This did not occur in children of a normal, healthy weight, researchers reported.
The findings suggest that children with obesity are not able to stop eating, said study author Dr Pilar Dies-Suarez, head of radiology at the Federico Gomez Children's Hospital of Mexico in Mexico City. "Thus, treatment of obesity must focus on the impulsivity problem," she said.
But researchers only saw an association between impulsive brain reactions and children's weight. It's not conclusive which one caused the other.
The findings were to be presented at the Radiological Society of North America's annual meeting in Chicago.
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The researchers used two types of MRI to compared brain imaging results from 30 children, ages 6 to 10. During the MRI, the children smelled chocolate, onion and a non-food smell of diluted acetone, the active ingredient in nail polish remover.
Half the children had a body mass index (BMI) between 19 and 24, considered to be a healthy or normal weight, and half had a BMI over 30, considered obese.
When the children with obesity smelled the chocolate or onion, the researchers saw activity in the part of their brain involved in impulsive decisions but did not see activity in the part of the brain that controls the impulse to eat, Dies-Suarez said. When the children with normal BMI smelled food, the researchers saw activity in parts of the brain related to regulating pleasure, planning and emotional processing or memory.
The brain responses in children with obesity were also much greater when they smelled the chocolate and the onion compared to the responses in the normal-weight kids. When the obese children smelled the acetone, the active parts of their brain were related to memories and risk assessment
"This probably shows the brain processing of a smell which is not that usual," possibly trying to determine whether it's a safe food, Dies-Suarez said. "In contrast, the normal-weight volunteers seemed to be uninterested in this stimulus."
The results point out why it's important to find solutions for people with obesity instead of blaming them for the condition, said Dr Mitchell Roslin, chief of obesity surgery at Lenox Hill Hospital in New York City.
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"This study shows that children who are obese have different wiring in their brain," Roslin said. "The results should make people realise that obesity is not caused by lack of willpower. Instead, there are biological differences that contribute to caloric imbalance."
Obesity treatments should be evidence-based and insurance coverage should be mandatory, said Roslin, who was not involved with the study.
"Obesity is still a condition for which we blame the afflicted," Roslin said. "There is little compassion, and because of our preconceived notions, coverage for effective treatment is frequently not available."
The study is small, however, and it leaves open a number of questions, said Dr Peter LePort, medical director of Memorial Care Centre for Obesity at Orange Coast Memorial Medical Centre in Fountain Valley, California.
"This study does not provide evidence that children are incapable of controlling food cravings," LePort said. "What is not clear is how the parents fed these children to get them to a BMI over 30."
The study would need to be done with newborns, comparing the brain responses of infants when they consume their mother's milk, and then following the children to see which ones become obese, LePort said.
"I have doubts that this effect is inborn except in rare cases," LePort said, explaining that the rapid increase in obesity in the population is occurring too quickly for evolution to be playing a role. One possible problem is that children may be learning from adults to soothe themselves with food during emotional distress.
"Infants and children need a correct model on which to learn how to function," LePort said.
Data and conclusions presented at meetings are usually considered preliminary until published in a peer-reviewed medical journal.