Exploring the long-term sustainability of lifestyle therapy for reducing depressive symptoms: A mixed methods study of the Curbing Anxiety and depression using Lifestyle Medicine (CALM) non-inferiority trial at 9-month follow-up

Lifestyle therapies targeting nutrition and exercise are emerging as effective treatments for a range of mental health conditions. However, less is known about their longer-term effects. The Curbing Anxiety and Depression Using Lifestyle Medicine (CALM) trial showed that remotely delivered lifestyle therapy (exercise and nutritional counselling with a dietitian and exercise physiologist) is as clinically and cost-effective as psychotherapy (with psychologists) of similar intensity and frequency for reducing depressive symptoms in Australian adults. We aimed to assess reductions in PHQ-9 (Patient Health Questionnaire-9) scores were maintained at 9-month follow-up (primary aim) and to identify which factors contribute to the maintenance of lifestyle therapy practices compared with psychotherapy (secondary aim). A mixed methods design was used. For the primary aim, a generalized equation model fit PHQ-9 scores across baseline, 8-weeks, and 9-months. Analyses were run using intention-to-treat (ITT; N=182), per protocol (PP; N=135), and complete case (CC; N=64). Verbatim responses to interview questions about barriers and facilitators of maintaining practices were analysed using a descriptive qualitative approach. Both treatment arms showed significant reductions in PHQ-9 scores between baseline and 9 months (CC: lifestyle therapy MD=-3.76, 95%CI-5.71,-1.80; psychotherapy MD=-3.24, 95%CI -4.63,-1.86). Non-inferiority was established for the PP and ITT samples. However, between arm differences were observed in the CC sample at 9-month follow-up, with the lifestyle arm showing higher scores than psychotherapy (β: 2.68; 95%CI:0.84,4.52, p =0.004). This possibly reflected the significantly lower baseline PHQ-9 scores in the psychotherapy CC sample, which was explored using post-hoc analysis. Facilitators for maintaining lifestyle practices included the ease of incorporating nutritional changes into everyday life. Barriers included physical and mental health challenges. If replicated within a larger sample, these findings suggest that lifestyle therapy has comparable longer-term effects on depressive symptoms as psychotherapy. • Lifestyle therapy showed similar long-term effects on depressive symptoms as CBT • Nutrition practices were well maintained at 9-month follow-up • Maintaining lifestyle practices was facilitated by familiarity and feasibility • Barriers to lifestyle practices were life stressors and physical ill health