Good performance in school may be linked to the amount of physical activity children get, according to a new report published Monday.

Researchers examined the results of 14 previous studies involving thousands of children in the U.S., Canada and South Africa. They were looking for evidence of a potential link between exercise and academic achievement. Followups were done at periods ranging from eight weeks to more than five years.

The authors of the Dutch-led study say their probe was prompted by concerns that pressure for higher marks was translating into more time for classroom instruction and less time for phys ed.

What the authors say they found was "strong evidence" that more exercise was associated with better academic achievement.

"The findings of one high-quality intervention study and one high-quality observational study suggest that being more physically active is positively related to improved academic performance in children," the authors write.

The article suggests several reasons for such a link:

• Exercise improves blood and oxygen flow to the brain, improving cognition.
• Exercise boosts levels of endorphins and norepinephrine, decreasing stress and improving mood.
• Exercise helps to create new nerve cells and supports synaptic plasticity.

But the authors say further studies are needed, pointing out that relatively few high-quality studies have explored the relationship between exercise and good marks. For instance, none of the studies they looked at used an objective measure of what was meant by "physical activity."

The report appears in the January issue of Archives of Pediatrics & Adolescent Medicine.