Introduction Few studies have examined the association between physical activity (PA), measured objectively, and depressive symptoms in adolescents. The aim of this study was to determine whether there is an association between objective measures of PA (total PA and time spent in moderate and vigorous PA (MVPA)) and adolescent depressive symptoms.

Methods Data were available on 2452 adolescents aged 14 years participating in the ALSPAC cohort. Depressive symptoms were measured using the short version of the Moods and Feelings Questionnaire (MFQ) (self-report). Total PA (counts per minute (cpm)) and time spent in MVPA (minutes) were available based on accelerometry. The association between PA and MFQ scores was modelled using ordinal regression.

Results Adolescents who were more physically active (total PA or minutes of MVPA) had a reduced odds of depression (adjusted OR per 100 cpm total PA: 0.92 (95% CI 0.88 to 0.97); adjusted OR per 15 min MVPA: 0.94 (95% CI 0.88 to 1.01)). In a multivariable model including both total PA and the percentage of time spent in MVPA, MVPA was not independently associated with depression (adjusted OR MVPA (tertiles) medium 1.06 (95% CI 0.88 to 1.28), high 0.99 (95% CI 0.82 to 1.20)).

Conclusion The total amount of PA undertaken was associated with adolescent depression but the amount of time spent in MVPA, once total PA was accounted for, was not. Thus, the relevance of the intensity of the activity may be different for different dimensions of health. This would have important implications for public health messages if confirmed in longitudinal studies and randomised controlled trials.