Children who get more exercise may have fewer symptoms of depression than their peers who are less active, a recent study suggests.
Researchers used activity trackers to see how much physical activity children got, then interviewed kids and their parents to assess whether kids had symptoms of depression.
When kids got more moderate to vigorous physical activity at ages 6 and 8, they were less likely to have symptoms of major depressive disorder two years later, the study found.
"Our results indicate that increasing physical activity in children may prevent depression," said study leader Dr. Tonje Zahl, of the Norwegian University of Science and Technology in Trondheim.
While plenty of research has documented the potential for exercise to boost mood and help with depression for adults, less is known about the ways activity may influence mental health in young children. Often, depression and other mental health disorders don't emerge until adolescence or early adulthood.
The study included a sample of children from Trondheim. Researchers surveyed parents to assess children's mental health at age 6, then did follow-up interviews with parents and kids at age 8 and 10 to determine whether they had symptoms of depression.
Altogether, they had data on 795 kids at age 6, 699 at age 8 and 702 at age 10.
At ages 6 and 8, children were asked to wear accelerometers to track their movements for one week, removing the devices only to bathe. The accelerometers showed that at age 6, kids typically got about 1.19 hours a day of moderate to vigorous physical activity and had 8.58 hours of sedentary time.
At age 8, active time was 1.18 hours a day on average with 9.22 hours of sedentary time.
Researchers assessed kids for nine different symptoms of depression and found on average, kids had 0.52 symptoms at ages 6 and 10, with 0.46 at age 8.
Higher levels of moderate-to-vigorous physical activity at 6 and 8 predicted fewer symptoms of major depressive disorder two years later, with every extra hour of such activity linked with a moderate decrease in symptoms.
The study didn't find an association between inactivity and symptoms of depression, however.
One limitation of the study is that symptoms of depression don't necessarily mean children would have a clinical diagnosis of depression, the authors note in Pediatrics.
In fact, only a few children had diagnoses of major depressive order: 0.3 percent of participants at age six, and 0.4 percent by age eight.
The study is also observational, and can't show how any amount of exercise might cause kids to become depressed.
Even so, there are many possible biological explanations for why exercise might make depression less likely in kids, said Dr. Gary Goldfield, a psychology researcher at the University of Ottawa in Canada who wasn't involved in the study.
For one thing, exercise releases several chemicals in the brain that can improve mood or reduce feelings of depression including endorphins, serotonin and dopamine, Goldfield said. Physical activity can also lead to lower levels of the stress hormone cortisol.
"Physical activity can also reduce depression through many psychosocial processes such as reduced stress, increased social interaction, improved cognition, learning and academic performance as well as increased self-esteem, body image and quality of life which can positively impact mood," Goldfield added by email.
The study results suggest that children, like adults, can get lasting benefits from regular exercise, said Dr. Larry Rosen, professor emeritus of psychology at California State University, Dominguez Hills.
"The message seems clear," Rosen, who wasn't involved in the study, said by email. "Exercise now keeps you feeling better later."