The more fit a child is, the less likely he or she is to suffer from depression, a new study suggests. That's especially true for girls, according to researchers from the University of North Texas, who found a link among middle schoolers.

The findings, which have yet to appear in a peer-reviewed journal, came after researchers analyzed the fitness levels of 437 sixth-graders. Overall, those with better fitness were less likely to report symptoms of depression when they entered seventh grade, notes Science World Report.

It may have to do with "better self-esteem, healthier weight, or getting more positive reinforcements that go along with being active," the lead researcher tells WebMD.

However, "it could be more biological. We know certain proteins and hormones associated with less depression respond to increased exercise." Zeroing in on the issue is important, though, because depression at that age can lead to chronic problems later in life.

Researchers see it as one more piece of the puzzle for overall mental health in kids and suggest that school fitness programs could be key. (Meanwhile, a study finds depression can take years off your life.)

More From Newser