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Source: https://www.thelancet.com/journals/lanpsy/article/PIIS2215-0366(20)30050-X/fulltext

PIIS221503662030050X.pdf

 

Adolescence is a key window of vulnerability for depression, the leading cause of disability worldwide. The prevention of depression among young people is therefore of substantial public health importance. A growing body of large-scale data now supports the conclusion that physical activity is associated with a reduced risk of depression12 and thus serves as a promising prevention target. However, few studies have focused on adolescents and, of these, many are cross-sectional and have relied on self-report measures of physical activity that might poorly capture the full range of relevant activities over time.
 
In The Lancet Psychiatry, Kandola and colleagues3 draw on a prospective population-based cohort from which objective measures of physical activity were collected46+at three different timepoints (around 12 years, 14 years, and 16 years of age) to provide the most fine-grained characterisation to date of physical activity patterns across adolescence and their implications for depression. Between 12 years and 16 years of age, mean time spent in sedentary behaviour increased by more than 90 min/day—from about 7·2 h/day (SD 1·1) to 8·7 h/day (1·1). A 60 min/day increase in sedentary behaviour at any of the three timepoints was associated with an 8–11% higher depression score at 18 years of age. Notably, this risk appeared to be driven by the displacement of light physical activity: decreases in the amount of light activity directly mirrored increases in sedentary behaviour between 12 years and 16 years of age, and each 60 min/day increase in light activity was associated with a lower depression score at 18 years, with effects that were opposite and roughly equal in magnitude to those of sedentary behaviour. Findings persisted when adjusting for baseline depressive symptoms or when excluding individuals with elevated baseline symptoms, and even when considering depression as a binary outcome. Notably, moderate-to-vigorous physical activity, which has been the focus of most previous studies, had a less consistent association with depression compared with those of light activity or sedentary behaviour.
 
Adolescents are known to become less active over time,5 for multifactorial reasons. The good news from this study is the implication that light activity—which can include movements as simple as standing, stretching, or casual walking—might be an effective strategy for decreasing the burden of adolescent depression. While not able to establish causality, this study's fully prospective analyses and adjustment for a range of plausible confounders strengthen the validity of these findings, which, if supported by further research, could have important implications for expanding recommendations offered to young people. According to WHO physical activity guidelines, adolescents should aim for at least 60 min of moderate-to-vigorous activity each day.6 Absent from these guidelines, Kandola and colleagues note,3 are specific recommendations around lighter forms of activity, despite the fact that such activities occupy a substantial portion of waking hours and might be a more amenable starting point for individuals with high levels of sedentary behaviour.
 
To guide future recommendations, more data on the benefits of lower-intensity activities are needed. Objective measures, using accelerometers and other devices, are well suited to capture physical activity across the intensity spectrum. Evidence indicates that objective—but not self-report—measures of physical activity appear to be causally related to depression risk,2 perhaps because of their ability to detect broader forms of movement that are potentially relevant for mental health. There are foreseeable challenges, however, to translating these data into implementable recommendations. The very advantages of light physical activity (eg, easily accumulated in small ways throughout the day) make it harder to monitor and quantify reliably. Presently, there is also between-study heterogeneity in the definitions of light and moderate-to-vigorous activity based on accelerometer outputs. Thus, the question remains how we can pool estimates into comparable metrics to inform robust guidelines.
 
As exhortations to move more and sit less become more common, it is also worth noting that the optimal intensity (and perhaps even duration and types)7 of physical activity might depend on which outcome we are trying to target. More vigorous activity might indeed be most beneficial for physical outcomes such as bone health or adiposity,8 whereas, for depression, lighter activity might be sufficient or even preferable. One size likely does not fit all: physical activity guidelines and their implementation might require a precision approach.
 
The overall relationship between physical activity and mental health is intuitive and something the general public has perhaps implicitly understood. This study by Kandola and colleagues
3 adds to a now-robust evidence base pointing to physical activity as a potential primary and tertiary prevention strategy for depression. Evidence on the mental health benefits of lighter activity, especially in young people, is also accumulating.9 Changing behaviour to align with this evidence is the next frontier. Identifying major drivers of sedentary behaviour in adolescents, as well as potential sources of light activity in their daily lives, could enhance the design of targeted prevention strategies. For now, the data suggest that, when it comes to depression, a broad spectrum of physical movement—including light activity—is probably the right move.
Figure thumbnail fx1
 
KWC has received grant funding from the National Institute of Mental Health. JWS has received grant funding from the Demarest Lloyd Jr Foundation, is an unpaid member of the Bipolar/Depression Research Community Advisory Panel of 23andMe, and is a member of the Leon Levy Foundation Neuroscience Advisory Board.
 

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DOI: https://doi.org/10.1016/S2215-0366(20)30050-X

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