Does physical activity predict depression? Or does depression predict physical activity?: Findings from breast cancer survivors

As more and more individuals are surviving cancer, interventions to promote mental health and quality of life in cancer survivors are growing. It is well established that physical activity is an appropriate intervention for preventing and treating depression in cancer survivors, who are frequently at risk of developing multiple depressive episodes following treatment. Unfortunately, the interplay between depression and physical activity in cancer survivors is unknown. Specifically, it is unclear whether physical activity buffers against depression or whether depression leads to low physical activity. Perhaps both are true. By answering this question, researchers can help determine whether targeting depression before physical activity (or physical activity before depression) can benefit cancer survivors’ quality of life. Ultimately, understanding this phenomenon will enhance the effectiveness of mental and physical health interventions for this growing population.

In a longitudinal observational study involving 201 female breast cancer survivors, the women completed measures of depression and wore a physical activity monitor to measure daily physical activity at five time points. A few methodological strengths are apparent in this study, including the longitudinal design and the objectively measured physical activity level. Longitudinal designs are considered strong research designs because they allow researchers to determine the temporal order of associations between variables. In addition, using an activity monitor to measure physical activity instead of using self-reported activity levels allows researchers to avoid biases in participants’ recall of their own activity level.

Brunet et al. (2018) found unexpected results. Mainly, previous depression did not predict future physical activity, and previous physical activity did not predict future depression. In addition, these effects did not vary across the multiple time points. These findings are counter-intuitive because previous research has shown predictive associations between depression and physical activity. But given that the researchers did not necessarily expect any change in physical activity or depression over time (i.e., there was no intervention), it is possible that the study did not include a large enough sample to detect subtle changes. Greater statistical power (which is typically achieved by including more participants) would allow researchers to detect smaller effects.

Given that physical activity has been shown to reduce depression in community and cancer survivor samples, it will be important to determine why these associations have been found and in what ways these associations may be different in cancer survivors. In order to prevent and treat depression in cancer survivors, clarifying the barriers to physical activity that these individuals face during their transition to survivorship will inform intervention development. Ultimately, physical activity and depression interventions may need to be artfully combined in order to enhance quality of life for cancer survivors.

Reference: Brunet, J., O’Loughlin, J. L., Gunnell, K. E., & Sabiston, C. M. (2018). Physical activity and depressive symptoms after breast cancer: Cross-sectional and longitudinal relationships. Health Psychology, 37(1), 14-23.