Doctors in Texas say physical fitness may be one of the best ways to prevent depression and may even be as effective in treatment as psychotherapy is.

Their conclusions are based on a study of middle-school-aged children in northern Texas who were tested and surveyed for fitness and depression. They found that sixth graders, especially girls, were slightly less likely to become depressed by seventh grade if they had strong cardio-respiratory health — that is, the ability to perform strenuous activity over a sustained period. The researchers took out the children's weight as a factor.

The doctors, however, did not advocate scrapping psychological treatments. "Promotion of cardiorespiratory fitness can be an important strategy for preventing depression in middle school adolescents," they wrote in their paper, which is being presented Thursday at American Psychological Association's 122nd Annual Convention. But exercise "needs to be coupled with interventions that more directly address symptom treatment."

For the study, the authors surveyed 437 students, a little more than half of them girls. They answered questions about depression symptoms and their physical fitness. They also ran laps as part of a standard test of cardio-respiratory endurance. The girls in the survey were more likely to be depressed, with 28 percent showing symptoms in sixth grade and 29 percent in seventh grade. Twenty-two percent of seventh-grade boys and 19 percent of eighth-grade boys were depressed.

The researchers discovered an association between levels of fitness and levels of depression. But this is where their conclusions about fitness as a preventive treatment get muddy. They only controled for one outside factor (weight), though plenty of others, including genetics and personality, may account for depression. The doctors also mentioned the "social milieu of fit students," which is probably code for "fit kids are often the athletic popular kids." This is middle school, after all.

"With these limitations in mind, the present study nevertheless suggests that CRF is an important factor for protecting students against depression in middle school, particularly female students," wrote the authors, led by Camilo J. Ruggero, a psychologist at the University of North Texas. "Although results were modest, they are arguably as large as other interventions."

Source: C.J. Ruggero, et al. Does cardiorespiratory fitness protect against depression during middle school? American Psychological Association. 2014.