Sedentary behavior during adolescence is a marker of depression in adolescence. A new study reported in The Lancet Psychiatry shows that walking for one hour daily, or doing chores for the same length of time, at the age of 12 years, can reduce the risk of depression at the age of 18 years.
On the other hand, being physically inactive for long periods in the teenage years is a risk factor for this condition. Researcher Aaron Kandola says, “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.”
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The researchers looked at over 4,000 adolescents who have been part of longitudinal studies from their birth as part of the Children of the 90s cohort study. These children were asked to wear accelerometers, devices that track body movements, for at least 10 hours a day for at least 3 days, at three-time points – 12, 14, and 16 years.
The devices showed what type of activity the child was doing over the observed time period – light, moderate to heavy, or sedentary. Examples of light activity include walking, playing an instrument, or painting. Moderate to heavy physical activity include running and cycling. Accelerometers were used because the data they provide is more trustworthy than self-reported physical activity.
The children were also assessed for depression using a clinical questionnaire, which included questions on symptoms like low mood, impaired concentration, and lack of pleasure during previously enjoyable occupations. The questionnaire used assesses the symptoms present and their severity on a spectrum of depressive mood rather than a Yes/No diagnosis of depression.
The researchers found that between the ages of 12 and 16 years, the children became progressively less active physically. This was seen as a decline from about 5.5 hours to just over 4 hours. In contrast, sedentary behavior increased from about 7 hours to just under 9 hours.
Correlating the physical activity with depressive symptoms, every additional hour of sedentary behavior per day at the age of 12, 14 and 16 years was linked to an increase in the total score of depressive symptoms by 11%, 8%, and 10.5% respectively by the time they entered early adulthood.
If a child recorded sedentary behavior at all three time points, the depression scores were about 30% higher in late adolescence.
If the amount of light activity was considered, each additional 60 minutes at these time points was linked to a lowering of the depression scores by 9.6%, 7.8%, and 11.1% lower, respectively.
Moreover, moderate-to-heavy activity in childhood was associated with lower depression scores, but the evidence for this is weaker because the average child spends only about 20 minutes a day doing this level of activity. In other words, they cannot differentiate between the beneficial effects of light and heavy or moderate activity from these results.
The observational nature of the study means that the exercise or activity can’t be said to be the cause of the reduction in depressive symptoms. However, in their analysis the researchers adjusted for the effects of other factors which could also cause an increase or decrease in depression, such as socioeconomic factors, a history of mental issues in the parents, and the amount of time for which the accelerometer is worn.
They also prevented the possibility of the cause-effect relationship occurring the other way around, that is, that the lesser physical activity was the effect and not the cause of depression, in their analysis. They did this by taking into consideration the presence of depressive symptoms at the beginning of the study.
The way out
Sitting too much is already known to be a risk factor for early death, overweight, diabetes, cardiovascular disease, and depression or anxiety in adults. One meta-analysis of 13 studies on this topic found that sitting for over 8 hours a day without any compensating physical activity brought the risk of death to a level comparable to that of smokers and obese people. To counter this, just one hour or 75 minutes of moderately heavy activity could reduce this risk.
Kandola says that inactivity has been on the rise among youngsters for many years, but not much reliable research has been done into the effect of this sedentary lifestyle on their mental health. This is despite the fact that the number of young people who have depressive symptoms is on the rise. He says, “Our study suggests that these two trends may be linked.”
Senior study author Joseph Hayes adds, “A lot of initiatives promote exercise in young people, but our findings suggest that light activity should be given more attention as well. 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.” He suggests standing or active school lessons, for instance, because, according to him, small environmental changes could make a more active life easier for everyone.
Depressive symptoms and objectively measured physical activity and sedentary behaviour throughout adolescence: a prospective cohort study Kandola, Aaron et al. The Lancet Psychiatry, https://www.thelancet.com/journals/lanpsy/article/PIIS2215-0366(20)30034-1/fulltext