Mindfulness mediates depressive symptom improvement during heated yoga: A secondary analysis of a randomized controlled trial

Background. Despite growing evidence that lifestyle interventions reduce depressive symptoms, the psychological mechanisms underlying these effects remain poorly understood. This study examined whether mindfulness and rumination mediate the antidepressant effects of heated yoga (HY), a multicomponent intervention combining physical activity, attentional training, and thermoregulatory stress. Methods. This prespecified secondary mediation analysis builds on a randomized controlled trial in which 80 adults with moderate-to-severe depression (IDS-CR > 23) were randomized to 8 weeks of twice-weekly HY or waitlist control. Subsamples with complete mediator data contributed to rumination (n = 56) and mindfulness (n = 60) models. Causal mediation analyses with 10,000 bootstrap resamples estimated indirect effects on Week 8 depression severity via Week 4 mediator changes. Sensitivity analyses assessed unmeasured confounding required to nullify observed effects. ClinicalTrials.gov: NCT02607514. Results. As previously reported, HY produced significantly greater IDS-CR reductions at Week 8 versus controls (p < .001). HY was associated with decreased rumination (p < .01) and increased mindfulness (p < .001) at Week 4. Increased mindfulness was statistically consistent with mediating depressive symptom reductions (ACME: -2.71, 95% CI [-5.42, -0.99]), whereas decreased rumination was not (ACME: -2.41, 95% CI [-6.28, 0.43]). Results were resilient to sensitivity analyses. Conclusion. In this RCT of a behavioral lifestyle intervention, mindfulness but not rumination emerged as a statistically significant mediator of depressive symptom reductions, identifying mindfulness as a key candidate mechanism through which multicomponent lifestyle interventions may exert antidepressant effects and suggesting a target for optimizing behavioral treatments for depression.