Physically fit middle-school girls are less likely to develop symptoms of depression.

Researchers found that sixth-grade girls who performed better on a cardiorespiratory fitness test were less likely to feel depressed when they were surveyed again in the seventh grade. There was a smaller but similar effect on boys' depression, HealthDay reported.

"We don't know exactly why there is a link [between fitness levels and depression], but it is probably a number of things," Camilo Ruggero, lead researcher of the study, told HealthDay. "It might be better self-esteem, healthier weight or getting more positive reinforcements that go along with being active, and/or it could be more biological. We know certain proteins and hormones associated with less depression respond to increased exercise."

For the study, researchers surveyed more than 400 students, 55 of whom were girls, at six middle schools in a metropolitan county in North Texas as part of a larger, county-wide program to assess physical fitness.

In sixth and seventh grades, participants answered questions about their symptoms of depression and fitness. They were also weighed and completed a shuttle-based run, which is a fitness testing procedure involving short bursts of speed.

"A student's physical activity level may change from week to week, whereas fitness is a result of more prolonged physical activity," Ruggero said. "Assessing the students' body mass index, how well they performed on a shuttle-run test and their own feelings of personal fitness helps to give us a more complete picture of each student's fitness level."

They found that 28 percent of the girls in sixth grade and 29 percent in seventh grade had elevated symptoms of depression. The most powerful predictor of depression in seventh grade was having had symptoms of depression in sixth grade. However, once researchers controlled for this, fitness was an important factor in curbing students' depression a year later.

Depression in adolescence is associated with a range of poor school and health outcomes later on and is the main cause of disability in this age group, Ruggero said, citing the World Health Organization.

"Depression that begins at this time can lead to chronic or recurring depression in later years," he said. "Fitness programs are one way to help prevent depression in middle-schoolers, but schools should also use other interventions, such as one-on-one or group therapy, that more directly address symptom treatment among depressed adolescents."

Ruggero said children and teens are more motivated to be physically active or to participate in sports when it's fun.