The study found that children who were more physically active at 6 and 8 years old were less likely to have symptoms of depression 2 years later.
The researchers - from the Norwegian University of Science and Technology (NTNU) and NTNU Social Research, both in Trondheim, Norway - report their findings in the journal Pediatrics.
Symptoms of depression in children are not necessarily the same as those observed in adults.
Symptoms can include (but are not limited to):
- Frequent tearfulness
- Increased anger or hostility
- Loss of interest in activities once enjoyed
- Loss of appetite or change in eating habits
- Problems with relationships
- Poor communication
- Social isolation.
Researchers looked at data obtained from following hundreds of children over the course of 4 years in order to search for links between physical activity and symptoms of depression.
'A way to prevent or treat depression already in childhood'
The data covered almost 800 children aged 6 at the start of the study. From follow-up examinations, the team also obtained data on 700 of the children when they were aged 8 and 10.
Measures of physical activity came from accelerometers that the children were asked to wear around their waists for 24 hours a day for 7 consecutive days, and only remove them when bathing or showering.
Measures of the children's mental health were assessed from structured interviews conducted with parents.
The team found that children who were physically active at age 6 and 8 years were less likely to have symptoms of depression 2 years later.
"At both age 6 and 8 years, higher MVPA [moderate to vigorous physical activity] predicted fewer symptoms of major depressive disorders 2 years later," note the authors.
"This is important to know, because it may suggest that physical activity can be used to prevent and treat depression already in childhood," says study author Silje Steinsbekk, associate professor in the department of psychology at NTNU.
"We also studied whether children who have symptoms of depression are less physically active over time, but didn't find that to be the case," she adds.
Moderate to vigorous physical activity 'benefits'
Moderate to vigorous physical activity is defined as activity that is intense enough to make the person sweat and get out of breath.
Although the researchers did not examine the mechanisms through which moderate to vigorous physical activity might lower risk of depression, they cite a number of reasons that have been proposed in other investigations, including:
- Being physically active may serve as a distraction and reduce opportunities to ruminate about negative experiences
- In the form of sports, physical activity offers opportunities for enhancing self-esteem
- Physically active children may be more socially integrated than inactive children who do not engage in sports and peer events.
There is also evidence to suggest that "roughhousing" or "horseplay" can contribute to children's emotional and social development.
The authors note that while their study only found a small effect of physical activity on lower risk of depression, it was similar in size to "those obtained by psychosocial intervention programs in children and adolescents."
Given that nearly all children can be targeted by efforts to increase their amount of moderate to vigorous physical activity, "the gains at the population level might be substantial," they note.
Researchers also suggest that there is now a need for randomized studies to test these findings and show whether increasing physical activity protects against depression.
Such studies are more robust than observational studies that follow a group, because they compare the effect of increasing physical activity in one group against the effect of not increasing it in another matched group - rather like testing the effects of a drug in a clinical trial.
"Being active, getting sweaty and roughhousing offer more than just physical health benefits. They also protect against depression."
First author Tonje Zahl, NTNU