The study also found, however, that breaking up that sedentary behavior with even light physical activity — such as walking at a slow pace, doing chores around the house or playing a musical instrument — helps reduce the risk.
“Our findings show that young people who are inactive for large proportions of the day throughout adolescence face a greater risk of depression by age 18,” says Aaron Kandola, the study’s lead author and a doctoral student at University College London, in a released statement. “We found that it’s not just more intense forms of activity that are good for our mental health, but any degree of physical activity that can reduce the time we spend sitting down is likely to be beneficial.”
“We should be encouraging people of all ages to move more, and to sit less, as it’s good for both our physical and mental health,” he adds.
Why the study was done
Depression appears to be on the increase among adolescents. A U.S. study found, for example, that the prevalence of major episodes of depression among American teens has risen by 52 percent in recent years, from 8.7 percent in 2005 to 13.2 percent in 2017.
Depression that begins during adolescence often continues into adulthood, so identifying risk factors that are modifiable during the teen years may also help prevent episodes of major depression later in life. In adults, researchers have found a strong association between regular exercise and a reduced risk of depression, as well as a separate link between sedentary behavior and an increased risk of depression.
Findings from studies involving teens on this topic have been inconsistent, however — perhaps because almost all the studies had teens self-report their physical activity. As Kandola and his colleagues note, such reports are prone to inaccuracies.
The researchers decided to address this evidence gap by using more objective data than self-reports to examine the associations between physical activity (or lack of it) and symptoms of depression among teenagers. For that data, they turned to the University of Bristol’s Children of the 90s study, which, since 1991, has been following the health of 14,500 British women from pregnancy onward, as well as their children.
More than 4,200 of the children in that study have worn accelerometers to track their physical activity (light, moderate and vigorous) for a minimum of 10 hours over a three-day period at ages 12, 14 and 16. They have also filled out periodic questionnaires designed to measure symptoms of depression, such as low mood, loss of pleasure and poor concentration.
When Kandola and his colleagues analyzed all that data they found the amount of time the children spent being physically active fell significantly between the ages of 12 and 16. Most of the decline was due to a decrease in light activity and an increase in sedentary behavior.
Specifically, time spent doing light physical activity fell from a daily average of five hours, 26 minutes at age 12 to a daily average of four hours, five minutes at age 16. Meanwhile, sedentary behavior increased from an average of seven hours, 10 minutes to eight hours, 43 minutes.
The data also revealed that the adolescents who were the least active — and the most sedentary — tended to have the highest depression scores. Each additional hour of sitting per day at ages 12, 14 and 16 was associated with an 8 to 11 percent increase in depression scores by age 18, for example. And each additional hour of doing light physical activity at age 12, 14 and 16 was associated with an 8 to 11 percent decrease in depression scores by age 18.
The longer the behaviors went on for, the stronger the association. Young people who consistently spent high amounts of time being sedentary at the three earlier ages in the study had depression scores that were 28.2 percent higher at age 18 than their peers who were rarely sedentary. The teens who consistently engaged in high levels of light activity at all three ages had depression scores that were 19.6 percent lower at age 18 than their peers who spent little time doing such activities.
These findings held even after the researchers adjusted the data to account for young people who were depressed at the beginning of the study, as well as for factors known to be associated with depression during adolescence, such as socioeconomic status and parents’ history of mental health problems.
The study did found some links between moderate-to-vigorous activity at earlier ages and lower depressive scores at age 18, but because the young people spent such a relatively short amount of time engaged in activities of that intensity (an average of about 20 minutes per day), the data wasn’t strong enough for the researchers to draw any clear conclusions from it.
Limitations and implications
The study is observational, so it can’t prove a direct, causal relationship between physical activity and depression in adolescents. But there are scientifically plausible reasons to believe the relationship is real, as the study’s authors point out:
Physical activity might influence depressive symptoms through a variety of psychosocial and biological mechanism, such as stimulating neuroplasticity in brain regions implicated in depression, reducing inflammation, or promoting self-esteem. Most studies have shown these effects with moderate-to-vigorous physical activity, but light activity might act through similar pathways. Long periods of sedentary behaviour might negate these protective benefits, potentially increasing the risk of depressive symptoms.
“A lot of initiatives promote exercise in young people, but our findings suggest that light activity should be given more attention as well,” says Joseph Hayes, the study’s senior author and a psychiatric epidemiologist at University College London, in a released statement. “Light activity could be particularly useful because it doesn’t require much effort and it’s easy to fit into the daily routines of most young people. Schools could integrate light activity into their pupils’ days, such as with standing or active lessons.”
“Small changes to our environments could make it easier for all of us to be a little bit less sedentary,” he adds.
FMI: You’ll find the study on the Lancet Psychiatry website.