Excess weight may take a toll on young, developing brains, say researchers who found a link between morbid obesity in toddlers and lower IQ scores later in life.

Obesity is associated with metabolic problems such as diabetes, hypertension and high cholesterol.

"We're postulating that early-onset morbid obesity and these metabolic, biochemical problems can also lead to cognitive impairment," said Dr. Daniel Driscoll, a professor of pediatric genetics at the University of Florida.

Driscoll and his colleagues studied 18 children and adults with early-onset morbid obesity — those who weighed at least 150 per cent of their ideal body weight before age four — to two other groups.

The study, in the August issue of the Journal of Pediatrics, found the second group of 19 children and adults had Prader-Willi syndrome — a genetic disorder that causes people to eat non-stop and become morbidly obese at a young age if not supervised.

A group of 24 normal-weight siblings who shared the same genetic and socio-economic background as the research subjects acted as controls.

Low IQ may be 'complication of obesity'

Children and teens who were obese as toddlers for no known genetic reason fared almost as poorly on IQ tests (average score of 78) as people with Prader-Willi (average score of 63).

"It was surprising to find that they had an average IQ score of 78, whereas their control siblings were 106," Driscoll said.

"We feel this may be another complication of obesity that may not be reversible, so it's very important to watch what children eat even from a very young age. It's not just setting them up for problems later on; it could affect their learning potential now."

MRI brain scans showed lesions in the white matter of many of the patients with Prader-Willi and early-onset morbid obesity.

The lesions are typically found in adults with Alzheimer's or in children with untreated phenylketonuria, a metabolic disorder.

The lesions are likely a result of metabolic changes in the young brain, Driscoll said, adding people who become obese later in life are not at risk for the cognitive impairments since their brains are developed enough to fend off the damage.