by Michael D. Anestis, M.S.
This morning, I came across a very interesting study published in the current issue of the Journal of Consulting and Clinical Psychology by Sarah Jerstad, Kerri Boutelle, Kirsten Ness, and Eric Stice (2010). Jerstad and her colleagues presented data examining the degree to which physical activity impacts depression vulnerability and depression impacts subsequent physical activity in female adolescents.
The authors noted that a number of previous studies have found a negative correlation between physical activity and depression, meaning that higher levels of depression are generally accompanied by lower levels of physical activity. Additionally, Jerstad et al (2010) pointed out that psychologists have put forth a number of theories as to why this is the case. O'Neal, Dunn, and Martinsen (2000) suggested that physical activity might cause an increase in neurotransmitters known as monoamines (e.g., serotonin, norepinephrine, dopamine), which then causes the individual to experience more positive emotions and a lower risk for depression. Just and Alloy (1997) suggested that physical activity might distract individuals from their negative emotions and thus prevent them from engaging in destructive thinking styles (e.g., rumination) that would otherwise increase depression risk. Ekeland, Heian, and Hagen (2005) argued that physical activity has been associated with higher levels of self-esteem, which could serve as a protective factor against depression. These theories are not mutually exclusive - each of them could contribute to the relationship simultaneously - but the data currently available cannot inform us as to which might have the greatest impact. What's more, cross-sectional data - data collected all at once instead of involving multiple measures over time - cannot even provide any insight into whether decreased physical activity causes higher levels of depression or whether increased depression causes a decrease in physical activity. Along those lines, certain symptoms of depression such as anhedonia, motor retardation, and decreased energy could theoretically cause individuals to stop engaging in physical activities.
In an effort to clarify these issues, the authors conducted a study in which 496 adolescent females were assessed on physical activity levels, depression symptoms, and a host of other important variables every year for six years. They chose to focus on females because, beginning in adolescence, females experience depression two to three times as often as men. They also noted that one earlier study conducted by Birkeland, Torsheim, and Wold (2009) failed to find any relationship between physical activity and depression levels in adolescents over a 10 year period; however, that study used a short self-report measure of depression, which decreases confidence in its findings. Jerstad et al (2010) hypothesized that low levels of physical activity would predict subsequent increases in depression, even when controlling for the impact of body dissatisfaction, bulimic symptoms, social support, and body mass. Additionally, they hypothesized that high levels of depression would predict subsequent decreases in physical activity controlling for those same covariates. Importantly, both depression symptoms and bulimic symptoms were assessed through the use of semi-structured diagnostic interviews.
Overall, 13% of their sample experienced major or minor depression. The more physically active an individual was at baseline, the less increase in depression symptoms she experienced over the course of the study. Specifically, for each additional activity that a participant engaged in 10 or more times in a year, risk of later depression symptoms decreased by 1%, risk for major or minor depression decreased by 8%, and risk for major depression decreased by 16%. This particular set of results supports the notion that physical activity can, in fact, reduce an individual's vulnerability to varying levels of depression in the future. The degree to which physical activity reduced depression risk was modest, meaning that being active is by no means a cure all, but significant mental health benefits are something to take seriously even when the effect size is not large.
Additionally, the authors found that participants who met criteria for major depression were 35% less likely to participate in an additional physical activity 10 or more times in the next year. Participants who met criteria for minor depression (5 symptoms of depression, at least one of which is at sub-clinical levels) were 18% less likely to participate in an additional physical activity 10 or more times in the next year. Increases in one depression symptom over a year diminished the likelihood of engaging in an additional physical activity in the subsequent year by 21%. These findings support the notion that becoming more depressed will impact physical activity in the following year.
Taken together, these findings present us with a scenario in which becoming less physically active makes people more vulnerable to depression and developing more depression makes an individual less likely to become more physically active. In this sense, a vicious cycle can develop in which one problem leads to the other which leads back to the first problem and so on. As always, this study is not perfect and readers should be careful not to jump to conclusions about the importance of physical activity in depression. That being said, the authors should be applauded for developing a methodologically strong study that provided us with data capable of actually addressing the issue of activity and depression. So what is the take home message? These data seem to indicate that keeping adolescent girls active is a healthy and potentially fun way to reduce vulnerability to depression. Physical activity in and of itself will not make an individual immune to depression nor will it serve as a stand alone treatment for those who become depressed, but it is an affordable way to help decrease the likelihood of a young girl struggling with depression.
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