Importance: Associations have been documented between physical activity and depressive symptoms, but the direction of this association is unclear.
Objective: To examine whether depressive symptoms are concurrent with physical activity and to examine the direction of the relationship from 23 to 50 years of age.
Design, setting, and participants: Participants included members of the 1958 British Birth Cohort, a general population sample of all persons born in England, Scotland, and Wales in a single week in March 1958 who were followed up to 50 years of age (2008). We included approximately 11,000 cohort members with information on depressive symptoms or frequency of physical activity at 23, 33, 42, or 50 years of age.
Exposures: Depressive symptoms were measured using the Psychological subscale of the Malaise Inventory; frequency of physical activity, by questionnaire.
Main outcomes and measures: Number of depressive symptoms (on a scale of 0 to 15 items), depression (defined as being in the top 10% for symptoms at 23, 33, 42, or 50 years of age), and frequency of physical activity (times per week).
Results: At most ages, we found a trend of fewer depressive symptoms with more frequent activity; for example, per higher frequency of activity per week at 50 years of age, the mean number of symptoms was lower by 0.06 (95% CI, -0.09 to -0.04). In longitudinal analyses, activity was associated with fewer symptoms from 23 to 50 years of age (per higher frequency of activity per week, symptoms were lower by 0.06 [95% CI, -0.07 to -0.05]), and the magnitude of association did not vary with age (P=.21 for interaction). Those who were inactive at 23 years of age and remained inactive 5 years later showed no change in symptom level (mean difference, -0.01 [95% CI, -0.04 to 0.02]); those increasing activity to 3 times/wk had a lower mean number of symptoms (mean difference, -0.18 [95% CI, -0.22 to -0.15]). Such differences equate to estimated reductions in odds of depression by 19%. A longitudinal relationship observed between symptoms and activity weakened with age (P<.001 for interaction). Mean activity among those with no symptoms at 23 years of age and 5 years later was higher by 0.60 (95% CI, 0.57-0.64) times/wk; in those with 1 additional depressive symptom, 0.53 (95% CI, 0.49-0.56) times/wk. Activity frequency did not differ among those with no symptoms at 43 years of age who subsequently had 0 or 1 symptom at 48 years of age. Associations for depression were generally similar to those for the full symptom spectrum.
Conclusions and relevance: The relationship between activity and depressive symptoms was bidirectional, albeit more persistent during adult life in the direction from activity to depressive symptoms. Findings suggest that activity may alleviate depressive symptoms in the general population and, in turn, depressive symptoms in early adulthood may be a barrier to activity.