Background: Exercise improves cardiorespiratory fitness (CRF) and reduces depressive symptoms in people with depression. It is unclear if changes in CRF are a predictor of the antidepressant effect of exercise in people with depression.

Aims: To investigate whether an increase in CRF is a predictor of depression severity reduction after 12 weeks of exercise (trial registration: DRKS study ID, DRKS00008745).

Method: The present study includes participants who took part in vigorous (n = 33), moderate (n = 38) and light (n = 39) intensity exercise and had CRF information (as predicted maximal oxygen uptake, V̇O_2max) collected before and after the intervention. Depression severity was measured with the Montgomery-Åsberg Depression Rating Scale (MADRS). V̇O_2max (L/min) was assessed with the Åstrand-Rhyming submaximal cycle ergometry test. The main analysis was conducted pooling all exercise intensity groups together.

Results: All exercise intensities improved V̇O_2max in people with depression. Regardless of frequency and intensity of exercise, an increase in post-treatment V̇O_2max was significantly associated with reduced depression severity at follow-up (B = -3.52, 95% CI -6.08 to -0.96); adjusting for intensity of exercise, age and body mass index made the association stronger (B = -3.89, 95% CI -6.53 to -1.26). Similarly, increased V̇O_2max was associated with higher odds (odds ratio = 3.73, 95% CI 1.22-11.43) of exercise treatment response (≥50% reduction in MADRS score) at follow-up.

Conclusions: Our data suggest that improvements in V̇O_2max predict a greater reduction in depression severity among individuals who were clinically depressed. This finding indicates that improvements in V̇O_2max may be a marker for the underpinning biological pathways for the antidepressant effect of exercise.

Declaration of interest: None.