Choi KW, et al. - Using a genetically informed method for evaluating potential causal inference, researchers analyzed bidirectional relationships between physical activity and depression. Genome-wide association studies (GWAS) summary data were available for 611,583 adults. In this two-sample Mendelian randomization study, robust evidence was found to support a protective relationship between physical activity and the risk of major depressive disorder (MDD). Findings point to the importance of the objective physical activity measurement in epidemiological mental health studies and support the assumption that improving physical activity can be an effective prevention strategy for depression.