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“The majority of this protective effect occurred at low levels of exercise and was observed regardless of [exercise] intensity,” wrote Samuel B. Harvey, Ph.D., an associate professor at the University of New South Wales, and colleagues. “Assuming there is no residual confounding in our final model and the observed relationship is causal, our results suggest that if all participants had exercised for at least one hour each week, 12% of the cases of depression at follow-up could have been prevented,” the authors wrote.
The study drew upon a prospective, Norwegian cohort (33,908 people), having no symptoms of common mental disorder or limiting physical health, to address three questions: 1) Does exercise provide protection against new-onset depression and anxiety? 2) If so, what intensity and total amount of exercise is required to gain protection? 3) What causal mechanisms underlie any association between exercise and later depression and anxiety?
In phase 1 of the Health Study of Nord-Trondelag County (HUNT 1), all residents of a rural county in Norway aged 20 years or older were invited to complete questionnaires about their lifestyles and medical histories and undergo a physical exam. As part of the baseline HUNT 1 assessment, participants were asked about depression and anxiety symptoms as well as how often they exercised and the intensity of this exercise.
Data from HUNT 1 were used to select a “healthy” cohort without any evidence of current physical illness or depressive or anxiety disorders at baseline that was followed up nine to 13 years later in HUNT 2. Of the 22,564 individuals successfully followed in HUNT 2, 1,578 (7%) developed case-level symptoms of depression and 1,972 (8.7%) developed case-level symptoms of anxiety (as measured by Hospital Anxiety and Depression Scale). The prevalence of case-level anxiety was similar regardless of the participants’ levels of baseline exercise; therefore, no association between baseline exercise levels and later case-level anxiety could be made.
Participants who reported undertaking no exercise at baseline had 44% increased odds of developing case-level depression compared with those who exercised one to two hours per week.
“Those who engaged in less exercise at baseline tended to have higher resting pulse, lower levels of perceived social support, and more sub-threshold symptoms of depression and anxiety, and they were more likely to develop new-onset physical illnesses over the course of the study,” noted the researchers. “Importantly, the majority of the protective effects of exercise against depression are realized within the first hour of exercise undertaken each week, which provides some clues regarding causation and has major implications for possible future public mental health campaigns.”
For related information, see the Psychiatric News article “Productivity, Exercise May Predict Antidepressant Response.”
(Image: iStock/leaf)