The study had two important limitations.
First, only about 40% of the patients assigned to exercise were fully adherent to the program and actually exercised 90-120 minutes per week. And 40%-50% of the patients in the control reported that they had actually exercised to some degree during follow-up. Such unintended crossover might have adversely affected the study findings.
Second, the study could not rule out the possibility that patients with more severe depressive symptoms were less likely to exercise, "so it is not clear whether exercise resulted in less depression or if depression resulted in less exercise," the researchers said.