We aimed to establish the treatment effect of physical activity for depression in young people through meta-analysis. Four databases were searched to September 2016 for randomised controlled trials of physical activity interventions for adolescents and young adults, 12-25 years, experiencing a diagnosis or threshold symptoms of depression. Random-effects meta-analysis was used to estimate the standardised mean difference (SMD) between physical activity and control conditions. Subgroup analysis and meta-regression investigated potential treatment effect modifiers. Acceptability was estimated using dropout. Trials were assessed against risk of bias domains and overall quality of evidence was assessed using GRADE criteria. Seventeen trials were eligible and 16 provided data from 771 participants showing a large effect of physical activity on depression symptoms compared to controls (SMD = -0.82, 95% CI = -1.02 to -0.61, p < 0.05, I2 = 38%). The effect remained robust in trials with clinical samples (k = 5, SMD = -0.72, 95% CI = -1.15 to -0.30), and in trials using attention/activity placebo controls (k = 7, SMD = -0.82, 95% CI = -1.05 to -0.59). Dropout was 11% across physical activity arms and equivalent in controls (k = 12, RD = -0.01, 95% CI = -0.04 to 0.03, p = 0.70). However, the quality of RCT-level evidence contributing to the primary analysis was downgraded two levels to LOW (trial-level risk of bias, suspected publication bias), suggesting uncertainty in the size of effect and caution in its interpretation. While physical activity appears to be a promising and acceptable intervention for adolescents and young adults experiencing depression, robust clinical effectiveness trials that minimise risk of bias are required to increase confidence in the current finding. The specific intervention characteristics required to improve depression remain unclear, however best candidates given current evidence may include, but are not limited to, supervised, aerobic-based activity of moderate-to-vigorous intensity, engaged in multiple times per week over eight or more weeks. Further research is needed. (Registration: PROSPERO-CRD 42015024388).

Background: Depressive disorders are common and associated risks include the onset of secondary disorders, substance use disorders, impairment in social and occupational functioning, and an increase in suicidality. As the onset often occurs in youth, there is a clear imperative for early identification and intervention to ameliorate, if not prevent, associated distress. Methods: An extensive search of relevant databases and an ancestry search was undertaken. Results: There is a limited but growing body of literature on this topic that is discussed in relation to a clinical staging model, which may prove to be a useful framework for identifying where an individual lies along the continuum of the course of a depressive illness thus allowing interventions to be matched for that stage. The identification of a subsyndromal and prodromal stage of depressive disorders provides early intervention opportunities. Conclusions: It is argued that a clinical staging heuristic may increase the number of those treated early, which may in turn delay or prevent onset, reduce severity, or prevent progression in the course of depressive disorders.

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Preventive strategies can be divided into universal, selective and indicated prevention and early intervention. Universal interventions are directed to the general population. Selective approaches are targeted at people who have risk factors for an illness, but who do not show any current signs. Indicated approaches target high risk individuals with minimal signs or symptoms foreshadowing mental disorder, but who do not meet diagnostic levels at the current time. Early intervention involves treating those with already diagnosable disorder in a timely and optimal manner aiming to decrease the severity of the illness, and reduce secondary morbidity. Although universal and selective interventions are not yet viable strategies, indicated prevention and early intervention are now realistic possibilities in schizophrenia. Development of methods to identify those at risk of psychosis continues to evolve. Promising results in the prevention and delay of transition to psychotic disorder from high risk state have been found. Early intervention in schizophrenia, including promotion of early help-seeking, has been shown to reduce the duration of untreated psychosis, which is known to be associated with poor outcome in schizophrenia. Early intervention programmes which optimise the care of the first episode have been shown to produce better outcomes than routine management.