Dr. Aaron Cypess, staff physician at Joslin Diabetes Center, and colleagues say their study used PET imaging data to document children's amounts and activity of brown fat, which -- unlike white fat -- burns energy instead of storing it.
The researchers reviewed PET scans that had been conducted on 172 children ages 5-21 at Children's Hospital Boston.
Active brown fat was detected in 44 percent of the children -- about the same for girls and boys. Children age 13-15 had the highest percentage of detectable brown fat and the highest brown fat activity.
In addition, the study, published in The Journal of Pediatrics, found body mass index was correlated inversely with brown fat activity, meaning that the thinnest children had the highest brown fat activity.
However, it is not known whether the relationship between BMI and brown fat is that children have more brown fat because they are thin or if having more brown fat causes children to be thin, Cypess, the study's senior author says.
"That's the billion-dollar question," Cypess says in a statement. "But we do know that brown fat is a core component of pediatric and likely adult metabolism."
Notable deaths of 2014 [PHOTOS]