Obese children may suffer irreparable damage to crucial regions of their brains, a study suggests.
Researchers found severely overweight teenagers had visible signs of destruction in areas responsible for controlling appetite, emotion and pleasure.
MRI scans also revealed damage in regions responsible for thinking speed, suggesting youngsters' school grades may take a hit if they are obese.
The researchers believe constant eating causes an inflammatory reaction in the brain which causes the body to kill off healthy tissue.
And they say being so overweight triggers an overproduction of the 'hunger hormone' leptin, which further fuels their weight gain.
One in five British children are overweight when they start primary school, with that figure rising to one in three by the time they start secondary school.
In the US, almost a third of children and teenagers are either overweight or obese, with one in five obese.
The Royal College of Paediatrics and Child Health estimates half of all children will be overweight or obese by 2020.
Obesity can trigger a slew of potentially-fatal health conditions later in life, including heart disease and strokes, high blood pressure, type 2 diabetes and cancer.
The latest study, by the Sao Paulo University in Brazil, took brain scans of 120 children aged 12 to 16. Fifty-nine of participants were obese, while 61 were a healthy weight.
The MRI's showed a decrease in white matter located in the middle orbitofrontal gyrus - a brain region related to emotional control and the reward circuit.
They also revealed brain cells had been destroyed in the corpus callosum, a bundle of nerve fibers that connects the left and right part of the brain and is responsible for cognitive function.
The team say this could lead to a reduced attention span and trouble concentrating.
The study also found obese children overproduced leptin - a hormone in the brain that controls appetite.
In some obese people, the brain does not respond to leptin, causing them to keep eating despite adequate or excessive fat stores.
This condition, known as leptin resistance, makes the brain produce even more of the hunger hormone.
Co-author Dr Pamela Bertolazzi, a biomedical scientist at Sao Paulo University in Brazil, said: 'Brain changes found in obese adolescents related to important regions responsible for control of appetite, emotions and cognitive functions.
'Our maps showed a positive correlation between brain changes and hormones such as leptin and insulin.
'Furthermore, we found a positive association with inflammatory markers, which leads us to believe in a process of neuro-inflammation besides insulin and leptin resistance.'
She called for more studies to establish if this inflammation in young obese people is a consequence of the structural changes in the brain.
Dr Bertolazzi added: 'In the future, we would like to repeat brain MRI in these adolescents after multi-professional treatment for weight loss to assess if the brain changes are reversible or not.'
The findings are being presented next week at the annual meeting of the Radiological Society of North America (RSNA) in Chicago.
Last month a study of 2,700 children aged between nine and 11 found they were more likely to do worse in problem-solving and memory tests if they were obese or overweight.
They had a thinner cortex - the brain region important for reasoning. Similar structural brain changes are seen in middle-aged people as they get older.
The Cambridge University team said a fatty diet may cause damaging inflammation at a time when a child's brain is still developing.
Another theory is they had different brains before they became obese, which made fatty and sugary foods more rewarding.