The findings are preliminary, but, "It just raises a red flag," Daniel J. Driscoll, M.D., Ph.D., tells WebMD.
Driscoll is a professor of pediatrics and the John T. and Winifred M. Hayward professor of genetics research at the University of Florida.
He notes "a sense of urgency to really address the obesity problem — the younger the better.
"It's right to worry about heart disease in 20-30 years, or hypertension in 20 years, and diabetes in 10 years," Driscoll says. "But there could be consequences now."
The study by Driscoll and colleagues appears in the August edition of the Journal of Pediatrics.
Driscoll's team studied 18 people who had been morbidly obese by age 4. That means they were more than 150 percent of the ideal weight for their height.
"We're not talking about a little baby fat," Driscoll says. "We're talking about a very select group."
At the time of the study, participants were 4-22 years old (average age: nearly 11). First, they were screened to make sure their early obesity wasn't due to known genetic disorders. Next, they got their IQ and cognitive skills tested.
For comparison, 24 of their brothers and sisters who hadn't been obese at an early age were also studied. So were 19 children with Prader-Willi syndrome from other families.
Prader-Willi syndrome is "the most commonly-recognized genetic cause of childhood obesity" and is linked to mental retardation, the researchers write.
The children with Prader-Willi syndrome and those who had been morbidly obese for unknown reasons by age 4 had the lowest test scores.
Average IQ scores were as follows:
The lower scores are "not in the mentally retarded range, but it's cognitively impaired, getting borderline," Driscoll says.
The gap between the morbidly obese children and those of more normal weight was about 25-30 points, Driscoll says, adding that IQ scores varied.
Most participants also got brain scans using magnetic resonance imaging (MRI).
Those who were at least 11 years old and had been morbidly obese by age 4 tended to have spots seen by MRI in different parts of their brains. But their siblings didn't.
Those spots seen on the MRIs aren't fully understood. But they're "not a good thing," Driscoll says. "We're trying to further investigate that with other techniques," he says.
Similar spots were also seen in the brain scans of Prader-Willi patients who were older than 18.
"We're really not talking about older children, someone who gets obese at 12, or 23, or in their 40s or 50s," Driscoll says. "I mean, there are some really brilliant people" who are obese, he notes. "But that's much different. Their brains are much more fully developed [by the time they become obese]."
The first few years of life are a critical time for brain development.
Very young children "are full of potential, but their brains are very vulnerable," Driscoll says. "We need to be more diligent now."
He and his colleagues aren't sure exactly how extreme obesity at an early age and IQ may be related.
Some children might have an unknown genetic tendency to become obese, Driscoll notes.
Hormonal and metabolic abnormalities linked to obesity might affect the developing brain, but, "We need to do more investigation," says Driscoll.
SOURCES: Miller, J. "The Journal of Pediatrics", August 2006; Vol. 149: pp. 192-198. Daniel J. Driscoll, M.D. Ph.D., professor of pediatrics, and John T. and Winifred M. Hayward professor of genetics research, University of Florida. News release, University of Florida.
By Miranda Hitti
Reviewed by Louise Chang, M.D