Teens with metabolic syndrome — a set of health conditions linked with the development of heart disease and diabetes — perform worse in school than their healthier counterparts, according to a new study.

Researchers studied 111 adolescents, including 49 with metabolic syndrome and 62 without, and found that those with the condition performed 5 to 15 percent worse on tests of their academic abilities.

Additionally, brain scans showed kids with metabolic syndrome had smaller hippocampuses, the part of the brain involved in learning and forming memories.

While much research has linked metabolic syndrome to health problems later in life, "this paper demonstrates is there is something going on in their brains today, not 20 years from now," said study researcher Dr. Antonio Convit, a professor of psychiatry and medicine at the NYU School of Medicine.

"They're performing in the normal range, they're just not performing to their full potential," Convit said.

Metabolic syndrome is diagnosed in people who have at least three of these five criteria: high blood pressure, insulin resistance, high triglycerides, a large waist and low levels of HDL ("good") cholesterol.

Children in the study fell across a range, with some having none, one or two of the criteria. As the number of criteria that children met rose, their test scores declined, Convit said. While only a few differences were statistically strong, such as scores in math and spelling, the trend was clear.

"In none of those tests did the kids with metabolic syndrome score higher than the kids in the control group," he said.

While obesity has been a primary target for wellness programs, Convit said weight itself should not be the target, but the symptoms of metabolic syndrome, which can be assessed by a pediatrician and often improved by exercise. Extra weight alone isn't a problem in an otherwise active child, he said.

"You can have a kid who is quite fit who won't have these problems, even if they're carrying excess weight. It's the coach potato…who [has] the problem," he said.

Particular attention should be paid to kids with insulin resistance, Convit said. It is unclear how metabolic syndrome might lead to cognitive problems, but one theory is that insulin resistance makes it difficult for the brain to get sufficient glucose needed to fuel higher-level thinking and learning.

Dr. Jennifer Miller, a pediatric endocrinologist at the University of Florida who studies the connection between early-onset obesity and brain function, said that the causes of insulin resistance itself are unclear, but it is believed to have a strong genetic component.

Studies of elderly patients who are not physically fit have shown that cognitive function problems can be alleviated, to some degree, by exercise, Miller said.

The hope is that exercise could have a similar benefit in children.

"We know exercise improves insulin resistance, improves blood pressure, improves body weight, so it should help," Miller said. But a study in children has not been done.

Convit said he and his colleagues are beginning to explore that question in young adults.

Miller and Convit both expressed hope that research in this area would provide more incentive to parents and policymakers in promoting active lifestyles.

"I would hope that a study like this that shows an impact on cognition would have a much bigger impact on parents, in terms of their child's obesity," said Miller.

The study appears online today (Sept. 3) in the journal Pediatrics.

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