By David Kiefer, MD

Clinical Assistant Professor, Department of Family Medicine, University of Wisconsin; Clinical Assistant Professor of Medicine, Arizona Center for Integrative Medicine, University of Arizona, Tucson

Dr. Kiefer reports no financial relationships relevant to this field of study.

SYNOPSIS: Lower cardiorespiratory fitness increases the risk of developing depression in adults.

SOURCE: Schuch FB, Vancampfort D, Sui X, et al. Are lower levels of cardiorespiratory fitness associated with incident depression? A systematic review of prospective cohort studies. Prev Med 2016;93:159-165.

Using a systematic review research methodology, Schuch et al saved clinicians the task of having to review individual clinical trials themselves. The authors justified exploring cardiorespiratory fitness (CRF), which they defined as “… the ability of the circulatory and respiratory systems to supply oxygen to working muscles during sustained physical activity, typically expressed as mL O2/kg-1/min-1 … ,” because of some prior work supporting its effect on human health as being as great or greater than the more generic variable, physical activity (the authors provided some references to this effect). In this review, the researchers found three CRF clinical trials, and used two of them to generate data for this analysis; the third study did not include a hazard ratio (HR), precluding its inclusion in the pooling of data. The adult study participants did not have a mental health condition diagnosed at baseline. In addition, the clinical trials were prospective in design, and had at least one year of follow-up. Studies also had to have the primary outcome as either the risk (odds) of developing depression, or “… the association between cardiorespiratory fitness and depression or depressive symptoms.”

Overall, the pooled analysis included data on 1,131,330 people. A higher risk (P < 0.001) of developing depression was demonstrated in those adults with low CRF (hazard ratio [HR], 1.76; 95% confidence interval [CI], 1.61-1.91) and medium CRF (HR, 1.29; 95% CI, 1.20-0.138). The authors interpret these results as evidence that “…interventions that specifically target CRF might also promote positive mental health outcomes.” However, there are caveats. This is merely an observation study, so causation can’t be implied; for example, we are not able to say that low CRF causes depression. Furthermore, few details were provided of the methods that CRF was measured, other than the fact that the studies did not use the “gold standard” VO2max. Future studies that standardize this measurement would do much to help clinicians know exactly what to tell patients about how long, what type, and how intense exercise would benefit their mental health. Obviously, more studies are needed (just two were included here), but the number of patients studied in this analysis was impressive. In addition, the authors did not include the results of the high CRF groups, so any implication that more exercise leads to a lower risk of depression would be merely conjecture. Going forward, there is little reason for clinicians not to help patients move (pun intended) away from being in the low and medium CRF groups, no matter how that is defined, and it may also benefit their mental health.