The primary objective is to conduct a randomised controlled trial (RCT) in N=250 depressed participants comparing aerobic exercise to a stretching/relaxation control condition, examining effects on a range of potential clinical and mechanistic factors: depressive symptoms; immune-metabolic function; activation in the brain's reward and effort processing circuitry using functional magnetic resonance imaging (fMRI); cognitive tasks, focusing on reward processing; and a subset (approximately one-third) of participants will complete L-6-\[18F\] fluoro-3,4-dihydroxyphenylalnine (18F-DOPA) positron emission tomography (PET).
The secondary objectives are to assess: (1) the degree to which changes in the mechanistic factors are related to changes in interest-activity symptoms of depression resulting from aerobic exercise; (2) whether baseline mechanistic or clinical factors are associated with symptomatic improvement measured by symptom questionnaires following the exercise intervention; (3) whether aerobic exercise-induced changes in the brain circuits underlying cognitive control overlap with those implicated in motivation.
The trial will use an RCT design, with depressed participants randomised to eight weeks of either 45 minutes aerobic exercise of moderate-to-vigorous intensity activity (experimental group: three times per week, N=125) or 45 minutes of non-aerobic stretching/guided relaxation (control group: three times per week, N=125). The target sample size following expected attrition is N\~105 per arm. Participants will complete the trial in staggered cohorts, with no more than six participants per class.
Blood and saliva samples will be taken before the intervention at baseline (between weeks -1 and 0), mid-intervention (week 3 and week 4), and post-intervention (week 9 to week 14) visits, to assess changes in immune-metabolic markers. Blood and saliva samples will also be collected at baseline and post-intervention from approximately 30 healthy controls.
Functional neuroimaging during effort-based decision-making and cognitive control will be taken at baseline and post-intervention. The same functional neuroimaging measures will also be collected at baseline and post-intervention from approximately 30 healthy controls.
Cognitive assessments will be completed online at baseline, every other week during the intervention (week 1, week 3, week 5, week 7), and post-intervention. The same cognitive assessments will also be collected at identical time-windows from approximately 30 healthy controls.
Questionnaire assessments will be completed online at baseline, every other week (week 2, week 4, week 6) and post-intervention (week 9 to 14). The same questionnaire assessments will be collected at baseline and post-intervention from approximately 30 healthy controls.
Accelerometers will measure physical activity continuously at baseline and during the intervention. Fitness testing will provide a measure of cardiovascular fitness at baseline and post-intervention visits. Daily depressive symptoms will be recorded using abbreviated scales using the Neureka smartphone app throughout the intervention. Three-monthly follow up of symptoms/cognition from baseline over six months will assess the durability of effects (week 21 and week 33).
In a subset of participants, approximately one-third of the participants will also complete a positron emission tomography (PET) scan pre-randomisation and at a visit during weeks 4-9 to 14, to assess dopamine synthesis capacity. The same PET scan will also be collected from approximately 30 healthy controls.
