Bidirectional association between mental health and physical activity in older adults: Whitehall II prospective cohort study.

•We explored associations between mental health and physical activity in older adults over 10years.•Mental health and physical activity were correlated at baseline and moved together over time.•Greater physical activity was associated with better mental health in early old age, and vice versa.•Our findings suggest persistent positive longitudinal and bidirectional associations.•Neither variable had a ‘causal’ impact on the other.